mickey dunlap gasket mod

[400rednEX]

anyone know how much compression you end up with when you do his mod, 1 ply of head gasket & rtv-no base gasket. the piston is supposed to end up .040 in the hole if i remember right.

[Hondaluva]

It depends on what piston you are talking about. With a Wiseco 89mm (440) 11:1 you may end up sitting pretty close to .040" in the hole but it depends on the thickness of your base gasket.

Let me give you an example here for a 400EX (440):

Bore: 3.504
Stroke: 2.756
Head Volume: 37.2
Head Gasket Thickness: .027" (cometic stainless)
Deck Height: .033
Dome Vol.(+)/Valve Pocket(-): -.08
Base Gasket: .012" (cometic stainless)
Compression: 10.31:1

Now if you set the same machine up the Mickey way:

Bore: 3.504
Stroke:2.756
Head Volume: 37.2
Head Gask. Thick.: .09" (single layer with BBQ paint on both sides)
Deck Height: .019
Dome Vol.(+)/Valve Pocket(-): -.08
Base Gasket: RTV
Compression: 11.36:1

This is just an example. But you can see how just a little bit can greatly change the CR. Check out the CR calculator at www.rosspistons.com

It's easy to take all the measurements when you are in the top end, and as you can see worth it.

[400rednEX]

i was talking with the stock piston and gaskets, if it adds a full point with your example it might be worth it. i bought a hotcam stage 1 and am wondering if i could get a decent gain that way or shell out the money for a group buy ross. it runs great now and i would have to drive at least 1 1/2 hours to get my cylinder bored right, and i don't want much down time.

[hawiianpwr]

Just get in with us and buy the piston. You can't go wrong with the price:)

[brif]

j, just buy the piston. You'll probly end up with it later anyways so why not make it sooner.cough 416 cough

[400rednEX]

yeah i should, but if it's still tore down for silver lake i can ride your 450r right? plus i'm cheap:D

[nacs400ex]

Originally posted by 400rednEX
yeah i should, but if it's still tore down for silver lake i can ride your 450r right? plus i'm cheap:D

Keep in mind if you go with the 11:1 comp and you do that gasket mod thats almost 12:1-----> meaning race gas.

[brif]

You still got 6 weeks and its only 5hours laber and about aweek turn around on the bore. :blah:

[toby400ex]

Originally posted by nacs400ex
Keep in mind if you go with the 11:1 comp and you do that gasket mod thats almost 12:1-----> meaning race gas. Actually if you get an 11:1 400ex piston it isnt 11:1 because the piston is designed for an xr400. But since the gaskets on the xr are thinner than a 400ex gasket you have to use xr gaskets just to get 11:1, or no base gasket and 1 layer of the head gasket.

[400rednEX]

i would only do the gasket mod with the stock piston because i only want 10.8 max compression, maybe a little less. the ross looks pretty good if they could order soon. or je.

[Hondaluva]

i was talking with the stock piston and gaskets, if it adds a full point with your example it might be worth it. i bought a hotcam stage 1 and am wondering if i could get a decent gain that way or shell out the money for a group buy ross.

Like I said before, my example was just an example. But it could make a huge difference. It's worth checking out. If you are in the top end it's not hard to take the measurements with a set of calipers and then just go from there. But I would go with a Ross piston and get the bore if I were you. You won't be sorry!!!

[nasty88r]

I just bought my 440 and I don't know if they used the base gasket and 1 layer on the head gasket.. But if I was to take it apart could I just take the base gasket out and use the 1 layer head gasket without measuring anything?? I mean will I have to worry about the valves smacking the piston if I don't measure it?? I thought you could just do it and be OK. I know it has the Wiseco 440 piston in it 11.1. If I do the head gasket which part do I put back in does it matter which layer of the gasket I use??? And is it worth tearing down just for that, will I notice a diff in power?? Thanx..:confused:

[nacs400ex]

Originally posted by toby400ex
Actually if you get an 11:1 400ex piston it isnt 11:1 because the piston is designed for an xr400. But since the gaskets on the xr are thinner than a 400ex gasket you have to use xr gaskets just to get 11:1, or no base gasket and 1 layer of the head gasket.

:p whoops, my bad. So is it a true 11:1 piston for the XR400 :confused:

[cals400ex]

Originally posted by nacs400ex
:p whoops, my bad. So is it a true 11:1 piston for the XR400 :confused:

i was wondering the same thing. i have a feeling it isn't from what i read.

[Hondaluva]

I just bought my 440 and I don't know if they used the base gasket and 1 layer on the head gasket.. But if I was to take it apart could I just take the base gasket out and use the 1 layer head gasket without measuring anything?? I mean will I have to worry about the valves smacking the piston if I don't measure it?? I thought you could just do it and be OK. I know it has the Wiseco 440 piston in it 11.1. If I do the head gasket which part do I put back in does it matter which layer of the gasket I use??? And is it worth tearing down just for that, will I notice a diff in power?? Thanx..


It's always a good idea to check your measurements just to be safe especially if you didn't set it up. But if you are certain that it is a Wiseco 11:1 you should be fine with the gasket mod. If they set it up with stock gaskets you could be close to 9.4:1 compression and the gasket mod could put you well over 10:1. So you would notice a difference. Before you tear into it, take a look engine, just above case and below the cylinder. If it's clean enough you should be able to see a layer of gasket if they used hondabond you should see some stuff squishing out...good luck:D And remember that you can't reuse the piston cir clips so you would have to pick up some new ones...You may be better off riding the crap out of this machine and then freashing up the top end in the off season...it would be more enjoyable.

[toby400ex]

Originally posted by nacs400ex
:p whoops, my bad. So is it a true 11:1 piston for the XR400 :confused: Im pretty sure it is a true 11:1 for the xr400

[nasty88r]

So you are saying if I do the gasket mod I will be at 10:1?? What else do I have to do to get to 11:1??

[Hondaluva]

So you are saying if I do the gasket mod I will be at 10:1?? What else do I have to do to get to 11:1??

You will never know unless you take the measurements and crunch the numbers in a formula calculator. I had planned to go to final set up with stock gaskets but after I took the measurements I realized I was going to end up with 9.4:1 compression. So I went with no base gasket and a .027" stainless Cometic head gasket and ended up with 10.6:1. Stock EX gaskets are around .020" for the base and .035" for the head gasket. So yes changing the gaskets from .055" total to .027" bumped my compression way up. Go to www.rosspistons.com and take a look and the CR calculator, it's a good one. As far as the XR gaskets go, I don't know the thickness. I can only speak for what I am comfortable with.

[toby400ex]

Originally posted by nasty88r
So you are saying if I do the gasket mod I will be at 10:1?? What else do I have to do to get to 11:1?? If you are talking about using a stock piston i dont think you can get 11:1 without deckin your cylinder or something else.

[Hondaluva]

If you are talking about using a stock piston i dont think you can get 11:1 without deckin your cylinder or something else.

Sorry, I keep forgetting if we are talking about a stock piston or a 440 bore. It seems that there are so many of these threads out there I get confused :huh Everything I have been talking about is the experience I have gained from building my 440 with a Wiseco 11:1 piston.

[toby400ex]

Originally posted by Hondaluva
You will never know unless you take the measurements and crunch the numbers in a formula calculator. I had planned to go to final set up with stock gaskets but after I took the measurements I realized I was going to end up with 9.4:1 compression. So I went with no base gasket and a .027" stainless Cometic head gasket and ended up with 10.6:1. Stock EX gaskets are around .020" for the base and .035" for the head gasket. So yes changing the gaskets from .055" total to .027" bumped my compression way up. Go to www.rosspistons.com and take a look and the CR calculator, it's a good one. As far as the XR gaskets go, I don't know the thickness. I can only speak for what I am comfortable with. What compression should i have with a je 11:1 compression with no base gasket and a stock thickness cometic head gasket?

[nasty88r]

I am talking about a 440 11:1 piston. It looks like there is a base gasket on it and I think it has the cometic head gasket. So therefore it probably does not have 11:1 compression. So you are saying to lose the base gasket and use 1 layer of the head gasket and I will be close to 11:1?? If I use 1 layer for the head gasket do I have to use any kind of sealant on it??

[marty]

You don't do this for compression!!!! The reason you do the gasket mod is to get your squish correct. This will give more power and a cooler piston thought combustion efficiency. Higher compression is a side effect. Call the piston suppler and ask for the deck hight. I plan to do a 1mm over bore and called wisco, je, and ross. For 85mm piston wisco was 1.0208" (same as stock I believe) ross I think was 1.025" and je was 1.035". I don't have my notes in front of me but this is very close. I don't know if it is the same for different bores. So every piston needs a different gasket combination to get the proper squish. Even with these numbers you still have to check all your measurements if you want it right. Its very important that the 400ex is as fast and reliable as possible so we can all brag. Good luck Marty

[Hondaluva]

What compression should i have with a je 11:1 compression with no base gasket and a stock thickness cometic head gasket?

Again, you will never know unless you take the measurements. You have to load the piston and measure the deck height. Then crunch the numbers. I can't stress enought how important it is to check the measurements yourself or have your engine builder check them for you.


So you are saying to lose the base gasket and use 1 layer of the head gasket and I will be close to 11:1?? If I use 1 layer for the head gasket do I have to use any kind of sealant on it??

Again, you will never know unless you take the measurements. Losing the base gasket and using Hondabond will help for sure. But just because it's a Cometic head gasket doesn't mean it's the thinest they make. Besides if you are getting into the top end you should replace the head gasket while you are at it. It's only $20-$30. If you do the one layer you have to paint both sides with a high temp. paint...like a BBQ grill paint...just to add a little more bite to the gasket and to help seal the gasket.


You don't do this for compression!!!!

I disagree.


The reason you do the gasket mod is to get your squish correct. This will give more power and a cooler piston thought combustion efficiency.

I agree that it is important to have the proper squish. The goal is around .040" for the 400EX. But how else are you going to get the compression right if you don't change the gasket thickness. I guess you could deck the cylinder...but why if you can pull a couple of engine builder tricks and do it that much cheaper.

[marty]

If you have your squish right and your compression is not what you want decking the cylinder will not help. If you take any off the cylinder you will have to use thicker gaskets to get your squish right again.
If you want higher compression you have to mill the head. If you want lower compression I think you can cut the piston dome down. As hard as it is you have to get specks for all the pistons and do math.
This is where we can all help each other. I have the cc#s you will remove when you put in thiner gaskets or mill your cylinder. (I'll post at the end) I have seen the cc#s for combustion chambers, and valve reliefs on this site. Valve reliefs are probably different on ever different kind of piston, as well as dome volume.

THE PISTON MAKERS NEED TO PUT THIS INFO WHERE CUSTOMERS CAN SEE IT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!

It would be cool to have a special "MATH" place here where we can post these merriments and some formulas. This would let us pick the right parts the first time. Well here is some math I hope it helps.

This is how many CCs you will add or remove to your combustion area with a .010" change to your gaskets, or cylinder decking.

BORE SIZE:

85mm = 1.44 cc

86mm = 1.47 cc

87mm = 1.5 cc

88mm = 1.54 cc

89mm = 1.58 cc

[da_catt]

what is the deck height with the stock base gasket? and what is it without a base gasket? I am getting ready to put my je 416 piston in and it would save some time if someone all ready knew the deck height.

[marty]

If the 87mm 416 je piston is the same as the 86mm je piston the distance from the wrist pin center to the top of the flat part of the piston (not the dome) is .0142" higher than the stock piston. I have read here that the stock piston is .043 down the hole with stock base gasket. so .043-.014=.029. If you use a .010 base you will be at .019 + .025 head gasket = .044 squish witch is very close. If you go much under .040 the piston can hit the head at high rpms. Call je 714-898-9763 and ask them the deck hight of piston. The 86mm I used in this example is 1.035. I hope this helps. FLY RED Marty

[da_catt]

thanks, that saves me some time. my 87mm piston is the same, 1.035

[Hondaluva]

da_catt ...that's fine if you want take Marty's word for it. And I am not saying that Marty is wrong. But I still can't stress enough the fact that if you are going to be in the top end you should just get some calipers and measure all this stuff yourself to make double sure. It's not much to do it and the piece of mind you will get should make you feel warm and fuzzy inside.

[marty]

I agree with Hondaluva 1000% ! You should check every thing several times. I Got the .043" off this site. The base gasket may not be perfect. Je may be wrong. If you install the base gasket and your over .020" you might want to use hondabond on the base and a xr head gasket. Check both sides of the piston on the wrist pin axis after you torque the cylinder. Then decide what to do. It will be worth the time and extra gaskets to get your engine right, and to feel warm and fuzzy.

[airheadedduner]

The reason you do the gasket mod is to get your squish correct. This will give more power and a cooler piston thought combustion efficiency.
After reading on the squish is used in a 2 stroke I don't think it can be applied to a thumper.

[da_catt]

thanks guys. i was trying to have as little down time as i could. It looks like i will have the cylinder bored and honed then put it together and measure it. then find the gaskets i need and order them.

[400rednEX]

squish is critical to get charge turbulance and and a good single fronted flame travel for 4 strokes too. same principle with automotive fast burn heads.

[airheadedduner]

Originally posted by 400rednEX
squish is critical to get charge turbulance and and a good single fronted flame travel for 4 strokes too. same principle with automotive fast burn heads.
Could you explain how proper squish is set up and why then?

[marty]

To put it simply the combustion chamber (pocket in head where the valves and spark plug are) is smaller than your piston. The flat surface of the head and the flat surface on the piston (around the dome or the bowl on a stock piston) create the squish zone. when the piston comes up it "squishes" the air fuel mixture into the combustion chamber. This causes the mixture to swirl and mix up good. It also sends the mixture closer to the spark plug. I think it keeps heat from the edge of the piston.

To set it up properly you need to measure how far down your piston sits at T.D.C. with the cylinder torqued down. Then add the compressed thickness or the head gasket. On a 400ex .040 is about as tight as you should go. On the exhaust stroke there is no downward force on the piston and at high rpm its upward inertia will stretch the rod and take up free play in the bearings. This will let the piston get very close to the head. So don't go under .040

[400rednEX]

good call marty. you still need a minimum clearance after the gasket(s) is compressed, i think it's .040 but don't quote me.

[airheadedduner]

I understand what it is but I still am not seeing how it is being used on a thmper. I can see a small squish band on my EX head but it is only like 7-10% bore size. To me that is just not large enough to be effective. When CT did the squish on my R head they set the squish band at nearly 60% bore size. The purpose of this is to concentrate the fuel charge is a small area, usually directly under the plug. Thats so the charge is burned quickly before it has time to detonate. It also sheilds much of the piston from the heat of combustion. So after fuel is burned only a small portion of the piston is exposed to heat and the heat is pulled away from the exposed area by the unexposed area dissipating it better. With only around 10% being used as a "heat sink" I can't see that having enough effect either. It also does another very importent thing. By burning the charge as quickly as possible you are creating a more efficeint burn. If the charge is burned to slowly then some of it may not be burned till after TDC making it useless and only contributing to more heat.

Anyways, thats why I am not seeming on how it is aplicable:confused:

[marty]

First on a 2 stroke there are no valves in the combustion chamber so it can be any shape or size you want.
Second the pent type head on the ex has a lot of good features: centralized plug, low surface area to volume, four valves, and hemispherical chamber.
Third it would take years to learn every thing about combustion chamber design.
Fourth I'm only a mechanic not an engineer.

I'll try to cover a little more information. Burn speed is very important to avoid detonation. The faster it burns the less time there is for heat and pressure to act on the the end gasses. Also the less time it takes to burn the less lead time you need on the spark. The later you can hold off on the spark the less time the piston has to fight against the expanding gasses. Less lead tine also reduces detonation.

The valve configuration is also very important. The incoming charge is swirled differently depending on valve placement and intake design. Squish also contributes to burn speed by swirling the mixture. The proper squish also lets the piston get close enough to the head so heat transfer can occur. Squish also can keep the flame front from reaching the edge of the piston. (I'm not positive about that but I thing the combustion event is over before the piston heads back down.) But I also know peak pressure should be reached at 14 degrees A.T.D.C. on any engine. So I'm up in the air about no flame by the edge of the piston.

The squish band on the ex dose cause enough turbulence to create a faster burn rate. If you want a bigger squish band run a flat top or dished piston and mill the head to get the compression you want. This will only make the squish band a LITTLE bigger. This however is the ideal way to set up an engine. Pistons domes get in the way of the flame front, slow down burn rate, and cause detonation.

Again domed pistons are not as good as flat top. You can get more power from a flat top, run higher compression with less risk of detonation, get better flame travel, and a better burn rate.
A flat top will yield about 10:1 with no head machining. I tired to find a flat top 1mm over but could not. I don't know why no one makes flat tops? JE had the smallest dome and 10.6:1. I know I got a little off subject but I felt it was important.

Back on subject almost all engines use squish I even think some flat heads might. So this dose DEFINITELY apply. I'm still not sure if all the fuel is burnt before tdc. Like I said before peak pressure should occur at 14 degrees a.t.d.c. for every engine.

As long as I have the mike DO NOT port your head it will give you a much narrower power band which is not a good trade for a tiny bit more peak power.

Maby Mickey Dunlap or someone from C&D will chime in and give us some more info on squish and combustions chambers and stuff.

400rednEX .040 is striate from a Micky Dunlap post.

[woprs50]

ok i have a question im working on putting together my motor right now its a Powroll 440 kit the piston is a Ross 11.2 i put the cylinder on today and after reading this i just went out and measured my piston is .030in from the top of my sleeve at TDC and the head gasket they gave me is .025 so that gives me .055 do you think i should unbolt the cylinder and get rid of the base gasket ? or maybe get rid of one layer of head gasket im guessing that would get me somewhere around .046 with 2 layers im not sure what getting rid of the base gasket would do



and just curious but if proper squish helps keep heat from the edge of the piston wouldnt it be safe to say it also might help keep the head gasket from blowing as easily

[marty]

woprs50 how thick is your base gasket? Try this remove your base gasket, tighten your cylinder down again and take a new measurement. Then you can make a decision on a thiner base or no base and the right head gasket. I can't tell you how thick hondabond will be but I don't think it will be more than .002-.003. Cometic makes a .010 base and a .025 and a nice .027 head gasket, then the xr is .030. If you do use homdabond please take a dry merriment before you use the hondabond then a new measurement after and post how thick the hondabond is.

[airheadedduner]

From what I have read when the cylinder is filled with the fuel charge it is never evenly distributed to perfection and the edge of the piston is where you will most likely have lean spots. The squish is used to push the charge into a more concentrated spot. It does keep the flame away from the edge, but only if the squish is at the right height, and the fuel is burned quick enough.

I am not doubting your info, I just wanted more explaination. To me the size of the squish on most thumpers is just not large enought to be effective at increasing burn rate. It does, but not to a large degree. I guess this is mostly due to the fact that I am not as familiar with thumpers as with 2 strokes.

[Aallron]

The one thing that I am curious about but have never had the guts to try ... is installing the piston backwards... In the olden days of auto drag racing some of the guys would do this for a little extra zip out of the engine... not too good for longevity and loud to boot... anyone else remmember or has heard of this? Like I said just curious.

[airheadedduner]

I have never heard of that before:confused: