An Odd Big Bore Stroker [Archive]

wilkin250r

12-21-2004, 10:35 AM

I've never heard of it, but that doesn't mean it can't be done.

Stroking the crank 4mm involves moving the rod outwards 2mm on the crank. Well, this has other effects. It effectively moves the piston UP 2mm at TDC, so you either need a shorter piston or a shorter rod. But a shorter rod can have problems, because you also move the piston DOWN by 2mm at BDC, which can possibly cause the piston skirt to hit the crank.

Obviously all these factors need to be taken into account. Generally, a stroker crank also involves a custom rod, and possibly a custom piston. I don't know the exact details of the KTM300 engine, but it's entirely possible this rod-piston combo fits the bill nicely.

SuperChris

12-27-2004, 08:28 AM

So I found out a little more on the buildup. It's rod is 2mm longer than the long rod TRX, plus the 4mm stroker crank puts the piston 4mm higher in the bore at TDC, but the KTM300 piston has a pin location 4mm lower. The whole package will fit inside a stock appearing motor and need no spacer plate. I bought the parts and sent a cylinder up to have it big bore sleeved. Wish me luck.

wilkin250r

12-27-2004, 01:41 PM

That sounds cool. What is the weight of the KTM300 piston, as relates to the stock 250R piston. If it's significantly lighter, it will also keep vibration to a minimum.

JTRtrx250r

12-27-2004, 02:57 PM

Chris, that sounds wicked to me, keep us updated;)

Good Luck w/ it!!:macho

SuperChris

12-27-2004, 03:06 PM

I'm not sure about weight, is there a place I could look it up? Otherwise, should I try to balance the crank?

beerock

12-27-2004, 06:12 PM

I would say true and weld the crank once the rod is installed. just grab any trx piston you have and grab the ktm piston and weigh them.

I think i may go with this set up with my B+B cylinder. resleeve it, true and weld the crank.Either use a trx piston with a spacer or ktm piston.

couple questions, is the wrist pin on the ktm a larger size then the trx wristpin or could you use a trx piston as well?

SuperChris

12-30-2004, 04:48 PM

Not sure on the wristpin, I'll find out for you. Not sure on the weight, I'll find out for you. All my stuff is at the shop right now getting machined and trued.

wilkin250r

12-30-2004, 07:51 PM

Originally posted by SuperChris
I'm not sure about weight, is there a place I could look it up? Otherwise, should I try to balance the crank?

Although I know a thing or two about engines, I'll admit I don't know all the details of something so complex as balancing the crank against a larger piston, or a larger stroke. Or, in this case, BOTH a larger piston and larger stroke.

Balancing the crank should increase power in the upper RPMs. Vibration isn't a very big problem at low RPMs, but as you increase engine speed, vibration will quickly rob power from your engine. Plus, it will make you crank last longer, and even your frame, as excessive vibration can break welds and engine mounts. (I found out the hard way).

It would really depend on piston weight, but I would imagine the 300cc piston would be heavier than the stock 250cc piston. I may not be absolutely necessary, but it's probably a good idea. I'd get proffessional advice in this area.

beerock

12-31-2004, 06:54 AM

I am speaking from experience with building my race motor in my boat which is a 351w with a balanced and trued scat crank, h beam rods, trw forged pistons, for motorsport cam and valve train etc.I got schooled from a guy who has raced top fuel funny cars etc. balance is the most importance thing in these high performance motors.

Regardless of piston size the longer rod will put more inertia on the crank which will inherently give some vibration regardless of piston size. I've always thoguht about adding weight to the counter balancer to over come this but I'm not sure what the weight ratio would be. I guess you would have to calculate using a 250cc piston crank, rod and counter balancer. So you could figure out exactly how much weight would be needed to counter act the weight difference. I feel if this was done to the big bore motors as well reliability could be higher.

xct250r

01-07-2005, 11:58 PM

i would highly recommend subscribing to www.macdizzy.com those guys are highly expericed in large 250r engines. mostly for sand drags though. there has been lots of talk about CPI's sabertooth 500 cc PV topend. are you in SW Mo the cage cart thing is taking off around here. personally I'm building a 402cc Pv R engine using Skidoo MXZ 800 snowmobile cly. 4mm stroker w/kx500 rod. (as soon as I finish house construction) Have parts but not time. far from bolt-on but same stroke as stock sled engine so porting is close to stock (this is the most tricky part about strokers)!

genoapb

01-09-2005, 02:59 AM

Ive seen this motor combo many times but the ktm piston is something new! I love the idea of not having to use the spacer plate, it will keep the crankcase comperession high. Do you know the exact piston numbers? Also do you know what overbores that piston is available? I dont really think balancing is huge a concern on this combo but if you have the extra cash it will definately help. We had our 425 PSI crank rebalanced by crankworks.com and cost us about 160 bucks so yours should be a bit cheaper. I will warn you about one thing if your having your cylinder resleeved with a big bore sleeve make sure you use a shop that has experience with this cause at least 1 out of every 3 cylinders Ive seen done like this had head gasket leaking problems! Hope some of this helps

beerock

01-09-2005, 10:14 AM

Originally posted by genoapb
Ive seen this motor combo many times but the ktm piston is something new! I love the idea of not having to use the spacer plate, it will keep the crankcase comperession high. Do you know the exact piston numbers? Also do you know what overbores that piston is available? I dont really think balancing is huge a concern on this combo but if you have the extra cash it will definately help. We had our 425 PSI crank rebalanced by crankworks.com and cost us about 160 bucks so yours should be a bit cheaper. I will warn you about one thing if your having your cylinder resleeved with a big bore sleeve make sure you use a shop that has experience with this cause at least 1 out of every 3 cylinders Ive seen done like this had head gasket leaking problems! Hope some of this helps
\
if youve seen this combo before then you would know if the trx piston works? does it?

Ive heard all about the head problems, im going with a custom ringed head for a 270 sleeve.
I forget the guy on macdizzy who makes them. what was his name?

SuperChris

01-09-2005, 03:43 PM

There are a couple of them over there. Are you the guy that posts under the name Broque? I just got my stuff back from the sleeve company, they ground the flange on the head studs to clear the sleeve instead of grinding the sleeve for stud clearance. That's what I hear people did wrong when they complain about gasket leaks. I'm gonna try just using the big bore gasket'n see if it works. I'll get that piston on the scale in a few days. The wristpin is the same size and the TRX big bore piston will work with a spacer plate.

genoapb

01-09-2005, 04:16 PM

Yes the trx piston will work for this combo, but as stated you will need a spacer if you do it this way. If you do it this way also keep in mind that if your cylinder is already ported the stroker is going to add duration to your exhaust and transfers and if they are already pretty high like say 195 deg on the exhaust you may want to use a different cylinder. You will need a degree wheel to be sure. You could also try using an extra thick base gasket like .060" and then cut the remaining .120" out of the head if your duration is to high with the 4mm base spacer. The guys name on macdizzy site that does the stainless steel o-ring is Rick Kermal the owner of the site, im not sure if I spelled his name right.

beerock

01-09-2005, 10:32 PM

ya thats me chris. thats cool if you can weigh them.

genoa, Its not rick that makes the heads im talkin about, one of the mebers makes some custom billit heads for them. with domes shaped to match the porting.

I thought the skinnier the cylinder spacer the more bottom end you get? if you use a thicker base gasket it will have more top end but be peaky like a drag motor. I plan on playing with spacer plates with this motor, I have a 5mm spacer, 4mm and 3mm. Im going to dial in the power band with this motors port job and pipe.

well im excited about this because im going to resleeve mine soon. I just took my motor apart and will have pics of the nasty piston skirt failure i had my last day in glamis. check out the pics when i post em.

Chef

01-09-2005, 10:52 PM

The crank in my 425 is balanced...and its lasted quite a while with no problems. About midway through the season it quit wanting to idle...I think I might have had a reed break or something but I didnt have gaskets to pull it apart, and I'm not sure I can get them from PSI anymore. The counterbalancer may have been re-done to account for the huge piston, but it sure doesnt feel like it. It has some pretty good vibes. It might be more noticable because of my shaved down seat foam, but it will put your feet to sleep on the pegs at certain RPMs. Genoa, any idea where I could find another set of gaskets and o-rings for this thing? And how about a part number for a piston from Wiseco? Thanks man.

It didnt fall off on HP anytime though...thats for sure!

Chef

01-09-2005, 10:54 PM

Theres a video of this floating around somewhere...just dont know who has it. This is off the line in the same run. It was 200 foot asphalt drags.

genoapb

01-09-2005, 10:59 PM

We just bought gaskets and orings from psi not to long ago. Who balanced yours? Honestly you cant even notice the vibration on this 425.

beerock

01-09-2005, 11:17 PM

yo chef find me that vid? and im me?

SuperChris

01-10-2005, 10:00 AM

Is it just me, or are your back wheels off the ground?

wilkin250r

01-10-2005, 10:40 AM

Originally posted by beerock
I thought the skinnier the cylinder spacer the more bottom end you get? if you use a thicker base gasket it will have more top end but be peaky like a drag motor. I plan on playing with spacer plates with this motor, I have a 5mm spacer, 4mm and 3mm. Im going to dial in the power band with this motors port job and pipe.

This doesn't make sense.

I'll fully admit, you probably have more experience with different setups than I do, but there's no doubt that I know my physics.

If you have a long-rod crank with an 86 piston, you need a spacer plate. If you stroke the crank, generally you need a spacer plate, unless you have a special piston (like perhaps the KTM piston). The whole point of a space plate is because you are physically moving the TDC height of the piston. You need a spacer plate to keep the piston from physically hitting the head. If you move the piston crown up 5mm at TDC, you need a 5mm spacer to keep it from hitting the head. Granted, this will also change your port timing, but that's not really the function of a spacer plate, just a consequence. The function of the spacer is proper piston-head clearance.

Typical squish-band clearance for a high performance 250r is somewhere around 0.035 (35 thousands of an inch). 1mm is approximatly 0.040. So if you have a 5mm spacer plate and a 0.035 squish, and you change to a 4mm without any other modifications, your piston will hit your cylinder head.

Likewise, if you have a 4mm spacer, and you go to a 5mm spacer, your squish band is now 0.075, way too big, not to mention you would have a severe drop in compression.

You could possibly get around this by extensive machining of the piston head, but that seems to be more trouble than it's worth. It would seem to me to be much easier to simply modify your port timing to the desired characteristics rather than playing with the spacer plate thickness.

xct250r

01-10-2005, 05:13 PM

sometimes on strokers you have to space the bottom of the cyl. up w/ a plate and turn the top of the cyl back down or mill cyl to even get timing in the ball park. the less crankcase volume you have the more responsive the engine will be (no spacers, stuffed stock style crank). adding spacers (crank case volume) generally makes the engine rev more freely on topend (more overrev).

Chef

01-10-2005, 05:35 PM

Originally posted by SuperChris
Is it just me, or are your back wheels off the ground?

Nope, they're really off the ground. :cool:

beerock

01-10-2005, 09:44 PM

Originally posted by wilkin250r
This doesn't make sense.

I'll fully admit, you probably have more experience with different setups than I do, but there's no doubt that I know my physics.

If you have a long-rod crank with an 86 piston, you need a spacer plate. If you stroke the crank, generally you need a spacer plate, unless you have a special piston (like perhaps the KTM piston). The whole point of a space plate is because you are physically moving the TDC height of the piston. You need a spacer plate to keep the piston from physically hitting the head. If you move the piston crown up 5mm at TDC, you need a 5mm spacer to keep it from hitting the head. Granted, this will also change your port timing, but that's not really the function of a spacer plate, just a consequence. The function of the spacer is proper piston-head clearance.

Typical squish-band clearance for a high performance 250r is somewhere around 0.035 (35 thousands of an inch). 1mm is approximatly 0.040. So if you have a 5mm spacer plate and a 0.035 squish, and you change to a 4mm without any other modifications, your piston will hit your cylinder head.

Likewise, if you have a 4mm spacer, and you go to a 5mm spacer, your squish band is now 0.075, way too big, not to mention you would have a severe drop in compression.

You could possibly get around this by extensive machining of the piston head, but that seems to be more trouble than it's worth. It would seem to me to be much easier to simply modify your port timing to the desired characteristics rather than playing with the spacer plate thickness.

my jug with a spacer plate has about 2-2.5mm from the top of the cyl sleeve, this is with a 88 rod/86piston. so if i go to a 4mm it would be 1-1.5mm and 3mm spacer would be 0-.5mm. I am pretty sure there alot of racers that use 3-4mm spacer plates for more bottom end. You can also use the crapmetic(cometic) gaskets that are thinner and some even in copper that are really thin to change the port height timing. I only use oem gaskets so i use diff thickness plates.

so yea i know my squish band is huge I plan on dialing that in with the spacer

beerock

01-10-2005, 10:02 PM

Originally posted by Chef
Nope, they're really off the ground. :cool: looks like he hit the wheelie bar and it popped the bike up in the air

Chef

01-10-2005, 10:12 PM

Originally posted by beerock
looks like he hit the wheelie bar and it popped the bike up in the air

Whatever it did, it did it fast. Real fast....haha

genoapb

01-11-2005, 12:39 AM

Beerock, I just want to be sure you understand what I was saying about the stroker. Lets forget about spacers and squish for a moment although what you said is true. Lets say you take a motor that has say 190 deg. exh. duration and 130 trans. with a stock crank(72mm) and a .020" thick base gasket. Now you take that exact same motor and base gasket but now you use a 4mil stroker, you will see increased exh. and trans. duration and they wont increase exactly the same number either so your blowdown will also change. So what im saying is to know for sure what your doing you will need to slap a degree wheel on there. Just a little info incase some of the readers didnt know that. Im not saying you cant just try different spacers and see what happens, thats fine too as long as you make sure squish it right. Now on the head matter I thought you were talking about that special stainless steel o-ring that rick adds to aid in blowing o-rings out of the head. You might be talking about RKtech though they custom build domes to match your type of porting.

genoapb

01-11-2005, 12:41 AM

Chef, have you ever ran that 425 bike on a 300ft track? If so what kind of times did it run and what all is done to the bike and motor? Were expecting to run in the 4.5-4.6 range this coming season on ours!

beerock

01-11-2005, 10:21 AM

Originally posted by genoapb
Beerock, I just want to be sure you understand what I was saying about the stroker. Lets forget about spacers and squish for a moment although what you said is true. Lets say you take a motor that has say 190 deg. exh. duration and 130 trans. with a stock crank(72mm) and a .020" thick base gasket. Now you take that exact same motor and base gasket but now you use a 4mil stroker, you will see increased exh. and trans. duration and they wont increase exactly the same number either so your blowdown will also change. So what im saying is to know for sure what your doing you will need to slap a degree wheel on there. Just a little info incase some of the readers didnt know that. Im not saying you cant just try different spacers and see what happens, thats fine too as long as you make sure squish it right. Now on the head matter I thought you were talking about that special stainless steel o-ring that rick adds to aid in blowing o-rings out of the head. You might be talking about RKtech though they custom build domes to match your type of porting.

yes i understand the set up your talking about, in esscence the same motor with a 4mil stroker would need a spacer plate to keep everythingaround the same as it was before exhaust duration etc..

wilkin250r

01-11-2005, 12:49 PM

I think we might be talking apples and oranges, here.

If you're cylinder with a 5mm spacer plate gives you a deck height of 2.5mm, then you have a problem. Obviously you need to decrease your spacer plate thickness to give you proper deck height (which, along with head gasket thickness, will determine your squish)

And that is my whole point. Your spacer plate thickness should be determined by your desired deck height and squish, NOT by your desired port timing.

If you start with a spacer plate that gives you proper squish, a thinner spacer plate (for more bottom end) would be such a radical change to your deck height and squish as to make it impractical. Such a change would require extensive machining of the cylinder head to bring your dimensions back to normal. It would be far easier to just modify your port timing by directly machining your ports than playing with your spacer plate.

beerock

01-11-2005, 01:17 PM

Originally posted by wilkin250r
I suppose it could work, if you used a combination of thin/thick base gaskets and head gaskets.

I'll admit, I've never played with spacer plate thickness. But lets imagine you start with a motor with proper piston/head clearance and a squish of 0.035. Decreasing the spacer plate thickness will effectively bring the cylinder down, which will decrease duration on all ports, and generally increase bottom-end horsepower. But along with the cylinder, you would effectively also bring the head down, so you would need a thicker head gasket to bring the cylinder head back to it's proper height for your squish and clearance.

Am I wrong on any of this?

no your not wrong, If your setup is like that(squish.035) you dont have any room to use a thinner spacer plate. YOU COULD use thinner base gaskets to change the port height. the squish clearance would even be lower then .035. and .035 sounds pretty damn tight. Using a thicker head gasket will change the squish but you dont want the piston to go any higher then the sleeve on the sides,if u used a special thick head gasket and allowed the piston to come up out of the cylinder you could possibly stick a ring. the dome itself could be raised to furthermore increase compression. I have done this with stock heads as well as many others ive used the middle of a trx gasket and ive used the thing part of the trx gasket and also the cr250 gasket. The best way togo through is with a custom billit head and dome with orings this way you can dial in the porting of the motor with the head.

now im not really sure if my b+B cylinder has a spacer plater welded on or not, but with a spacer i have about 4mm of spcae between the piston top and cylinder sleeve. which is why i am able to run thinner spacer plates. I havent seen another stock jug with a spacer and 86 pisotn in a while though so i cant say they are all ike that, liek i said i could have a custom spacer plate welded onto my cylinder base.:confused:

obviously the compression would increase with a thinnner spacer/gasket so be sure your running race gas.

just for more specs on my little 265 motor here they are.

88 rod
86 piston
5mm spacer
2-2.5mm space between piston top and sleeve top.
thin part of the trx gasket
86 head (has a large dome) 230 PSI(this is with the 2-2.5mm of space!!
im not sure if my b+b head was cusotm domed or not but heres a pic to show the differences in heads. someone on mac dizzy said it looked like a cr250 head. The power band on the b+b head was way more on off due to the small dome, the head concentrated its power around mid top to mid with that head with the 86 its a more relaxed power band with a higher over rev because of less compression.

beerock

01-11-2005, 01:25 PM

Originally posted by wilkin250r
I think we might be talking apples and oranges, here.

If you're cylinder with a 5mm spacer plate gives you a deck height of 2.5mm, then you have a problem. Obviously you need to decrease your spacer plate thickness to give you proper deck height (which, along with head gasket thickness, will determine your squish)

And that is my whole point. Your spacer plate thickness should be determined by your desired deck height and squish, NOT by your desired port timing.

If you start with a spacer plate that gives you proper squish, a thinner spacer plate (for more bottom end) would be such a radical change to your deck height and squish as to make it impractical. Such a change would require extensive machining of the cylinder head to bring your dimensions back to normal. It would be far easier to just modify your port timing by directly machining your ports than playing with your spacer plate.

like i said I always play with spacer plates I wanted the motor to have alot of top end so it could run with bigger motors. the bottom end was not bad at all either, just a little pipey from 1/2 on up. it woudl all of a sudden just scream. ive run the mtoor like that for over 400 hours(new top ends etc) i put over 200 hours on this setup and then the counterbalancer let go and destroyed the cases.

wilkin250r

01-11-2005, 01:47 PM

Let me get this straight? You're piston is sitting 2.5mm deep inside your cylinder, and you still have a compression of 230psi?

That doesn't sound right at all. Either your measurement techinques are way off (which I admit is unlikely, knowing your reputation) or your engine is beyond my level of knowledge.

beerock

01-11-2005, 02:22 PM

Originally posted by wilkin250r
Let me get this straight? You're piston is sitting 2.5mm deep inside your cylinder, and you still have a compression of 230psi?

That doesn't sound right at all. Either your measurement techinques are way off (which I admit is unlikely, knowing your reputation) or your engine is beyond my level of knowledge.

yes anywhere from 2-2.5mm in the cylinder. i didn thtink it was right either but this motor screams with this set up.
I don't know if the measurements being off was a dig on me or not, either way its all good. I doubt ill ever be abel to measure my deck height since im completely changing this motor(ktm rod/ piston) I guess i could take the head off the blown motor and measure it accurately but im positive its in the 2 mil range.

A long time ago I set my timing with my pvl ignition with a nickel that measures 1.5 mil on vanier calipers. I put a pencil in the spark plug hole(rubber side down so it wouldnt slide) marked TDC (top of nickel)and turned the crank back till the mark was at the bottom of the nickel. the bike started up right away and later on I purchased a dial bore indicator and it was right on 1.5mm btdc. AN ODD WAY TO MEASURE IT but i had to work with what i had.

so knowing nickels are around 1.5-1.7mm ( i measured a couple and noticed a difference)I can accurately say the deck height of my piston is around 2-2.5mm. I will take a pic of the motor with the head off ..

wilkin250r

01-11-2005, 02:56 PM

No, the measurement thing wasn't a dig, that's why I added the comment in parenthesis. If I'm going to insult you, it will by much more witty than that. :blah:

beerock

01-11-2005, 04:34 PM

;)

Chef

01-11-2005, 05:02 PM

Originally posted by genoapb
Chef, have you ever ran that 425 bike on a 300ft track? If so what kind of times did it run and what all is done to the bike and motor? Were expecting to run in the 4.5-4.6 range this coming season on ours!

Nope...never have. Its a TT bike through and through. TT Portwork and exhaust, TT swingarm, etc. If you look at those pics we didnt even take the sway bar off. Just backed off both sides until there wasnt any preload in the bar and tried it. I was originally planning a dedicated drag setup for it, but never did it. Is your crank stock stroke and such, or is it even bigger than the 425?

beerock

01-11-2005, 05:11 PM

Originally posted by Chef
Nope...never have. Its a TT bike through and through. TT Portwork and exhaust, TT swingarm, etc. If you look at those pics we didnt even take the sway bar off. Just backed off both sides until there wasnt any preload in the bar and tried it. I was originally planning a dedicated drag setup for it, but never did it. Is your crank stock stroke and such, or is it even bigger than the 425?
I helped chef piece together this motor when he first started the project. :cool:

xct250r

01-12-2005, 06:24 PM

as long as there is not a ring ridge worn in the top of the bore there is nothing wrong with letting the top of the piston coming out of the bore. as long as the rings stay in a little. I don't do the machine work on my big quads, but regularly have my sons 90 cyl. and head in the lathe trying different combustion cahmbers and squish clearances. I've run the piston in that engine as much as .018 out of the hole. just cut the dome deeper in the head. anyother place to check for promblems in a stroker or just odd piston combinations is that the piston skirt at TDC does not uncover the any of the exauhst ports. I'd care what spacer plates or squish or big stroker crank you have if the port timing is not in the right range (exauhst or transfer) it WILL run like a dog Turd! :eek2:

SuperChris

11-08-2006, 02:52 PM

Hey Beerock, Did you ever get this motor put together?

beerock

11-08-2006, 03:15 PM

Originally posted by SuperChris
Hey Beerock, Did you ever get this motor put together?

ehehe no its still sitting on my desk bown from glamis 2 years ago, did you ever get yours going? hows it runnign if so?

SuperChris

11-08-2006, 11:33 PM

it runs like a raped ape. All sorts of pissed off. It's finiky, and you have to stay on top of it to keep it healthy. Make sure you have good port timings. The KTM piston gives up a lot of power because it had two exhaust bridges stock. The Wiesco part has holes drilled for them and they short circuit into the crankcase.

beerock

11-09-2006, 06:43 PM

Originally posted by SuperChris
it runs like a raped ape. All sorts of pissed off. It's finiky, and you have to stay on top of it to keep it healthy. Make sure you have good port timings. The KTM piston gives up a lot of power because it had two exhaust bridges stock. The Wiesco part has holes drilled for them and they short circuit into the crankcase.

ahh so it sounds like some modification tot he piston woudl help IE welding the holes shut then redrilling for the R exh bridge.

btw you never got those mesaure ments i asked for i think?

sounds good though