help on top end!!

[livetoride249]

I got my new top end installed, and everythings back together, but when i try to turn it over by hand, as soon as the piston hits tdc it won't go any further, I'm not sure if it's the compression or if somehow something doesn't fit right. I only changed my top end, and it's identical to my old piston that I pulled out. I'm just afraid to really reef on the thing to see if it'll make a complete cycle. Let me know what you guys think!

[fearlessfred]

pull the spark plug and try,but do not force

[livetoride249]

Originally posted by fearlessfred
pull the spark plug and try,but do not force

Plugs not in it. I just have the motor on my bench, The cylinder and head bolts aren't torqued yet either, they're just snugged.

[gojk]

What did you replace? Sounds like you have a long rod crank and a short rod piston. Pull the head and turn it over. Does the top of the piston protrude out of the cylinder?

[livetoride249]

Originally posted by gojk
What did you replace? Sounds like you have a long rod crank and a short rod piston. Pull the head and turn it over. Does the top of the piston protrude out of the cylinder?

Yes it does protrude out of the cylinder. I bought the ATV used, and the guy said it was stock. except for the pipe and reeds. The old piston is fine, there are some scratches from where I pulled the circlips out, but I don't know if that'll affect the longevity of the motor. I should be able to use the new rings on the piston I removed right? They're both made by wiseco

[fearlessfred]

does it turn over with the head off? basicly what your say is that you instaledl new rings on an old piston that was in a running motor?

[livetoride249]

Originally posted by fearlessfred
does it turn over with the head off? basicly what your say is that you instaledl new rings on an old piston that was in a running motor?

I ordered a new top end from wiseco, came with a new piston and rings, gaskets etc. I took the part number off of the old piston, which had very little use on it. Used that part number to order my new piston. Installed the new piston and with the head on, it seems to bind up at tdc and won't make a full cycle. With the head off it turns over without any issues. I brought up my old piston because I believe that I can use the new rings and everything from the new top end kit on the old piston.

[JoePA]

Sounds like you are missing the spacer plate under the cylinder. Pics will help answer a lot of questions. Did the quad run with the old piston???

[livetoride249]

Originally posted by JoePA
Sounds like you are missing the spacer plate under the cylinder. Pics will help sole alot of questions. Did the quad run with the old piston???

Yeah it ran perfectly. And what? spacer plate? I just thought you had the gasket that went between the cylinder and cases.

[fearlessfred]

85 ,86 motors had the short rod crank and 87 thru 89 motors had the long rod motor. the only difference is approx.5 mm center to center length in the rod,the stroke of the crank is the same.the wrist pin height was changed on the piston to acount for the longer rod.the 85,86 piston has no window cut on the intake side of the piston. The bottom of the 85,86 has a c shape. The 87 thru 89 has a window cut in the piston.Now if you have a long rod crank with a short rod piston, the piston will be sticking out of the cylinder by approx.5 mm and will hit the head. the good news is that a spacer plate will fix your problem if that is the case.

[JoePA]

if it's a long rod crank with an 86 style cylinder and piston then you need to run a spacer plate under the cylinder. Something isn't adding up. What is the number from the piston?

[fearlessfred]

sorry missed 2 posts while typing ( slow typing)

[livetoride249]

http://www.ebay.com/itm/TRX250R-ATC250R-TRX-ATC-250R-CYLINDER-SPACER-PLATE-KIT-/400220129693#ht_500wt_1127


Is this what I'm going to need? I looked at the piston and the one I ordered is 85-86 it does have the C shaped cut, and the old one has the window. Potentially, if I didn't want to buy this spcer kit, i could use the old piston, it's still in perfect shape, with the rings that came with the 85-86 top end kit right?

Side not- I got the wheeler for $500 off of a friend and he said he replaced the motor with an ATC motor because he cracked the case on the old one. the ORIGINAL model was an '88. So I'm thinking it the motor wasn't actually from an ATC, since this one definitely has the long rod because the piston does sit about .5mm out of the top of the cylinder. I will upload pics in the next post.

[livetoride249]

http://i1126.photobucket.com/albums/l601/mx249kx125/0212121839.jpg


That's with the head gasket on as well.

[fearlessfred]

i would be guessing if the rings would fit on the old piston.sounds like the cheapest way to go,even if you had to buy new rings for the old piston

[Grande Huevos]

you can get the spacer plate you need for about $50 if you have the money to get it i would. Then you kno everything is right and solid with a brand new piston which in theory should run stronger then the old piston

[fearlessfred]

if he has a 86 cylinder that would work,but if he has a 87 tru 89 than it shouldnt be done because the ring locaters on the piston are in a
different location and would hang up in the ports. as far as i know the piston years and cylinder years need to match.he has allready confirmed that it came with an 87 thru 89 piston ,but needs to make sure that that the cylinder is the same.

[livetoride249]

Originally posted by fearlessfred
if he has a 86 cylinder that would work,but if he has a 87 tru 89 than it shouldnt be done because the ring locaters on the piston are in a
different location and would hang up in the ports. as far as i know the piston years and cylinder years need to match.he has allready confirmed that it came with an 87 thru 89 piston ,but needs to make sure that that the cylinder is the same.

I rode the ATV all summer, with the 87-89 piston, so that makes me think that the new rings from the 85-86 piston will work, since the ring locaters will be in the exact same spot. You still have to properly break in the new top end though with new rings, correct?

[mx91a]

Originally posted by fearlessfred
if he has a 86 cylinder that would work,but if he has a 87 tru 89 than it shouldnt be done because the ring locaters on the piston are in a
different location and would hang up in the ports. as far as i know the piston years and cylinder years need to match.he has allready confirmed that it came with an 87 thru 89 piston ,but needs to make sure that that the cylinder is the same.

Pistons of different years can be mixed and matched with cylinders of different years probably the most common setup is an 85/86 piston in an 87-89 jug with a spacer plate if using the long rod. Ring locator pins should be the same all years, located on the rear of the piston between the rear transfer ports and boost port but I can check when I get home. The YZ250 piston has the locator pins slightly closer together so when using that piston an eye needs to be kept on them if the boost port has been widened.

[fearlessfred]

Originally posted by mx91a
Pistons of different years can be mixed and matched with cylinders of different years probably the most common setup is an 85/86 piston in an 87-89 jug with a spacer plate if using the long rod. Ring locator pins should be the same all years, located on the rear of the piston between the rear transfer ports and boost port but I can check when I get home. The YZ250 piston has the locator pins slightly closer together so when using that piston an eye needs to be kept on them if the boost port has been widened. in other posts you have stated this and others have said that the pins are in a different location. please do check .why would anybody intentionaly mismatch piston design with port design

[mx91a]

http://i97.photobucket.com/albums/l209/mx91a/My%20Documents/pistons.jpg

Ok here is what I had laying around, a brand new OEM 87-89 66.00mm piston on the left and Wiseco 85-86 67.50mm piston on the right. To get the the most accurate measurement between ring locator pins on each piston I took a piece of fine wire and layed it on the piston between ring grooves, trimmed it until each end went from center to center of the locator pins and then straightened it out and measured the length. For the 87-89 piston the length of the arc between pins is 29.65mm and for the 85-86 piston is 30.35mm. Now naturally as the piston diameter grows so will the distance between locator pins but the one dimension that remains constant is the angle between the two pins relative to the vertical center axis of the piston. To make an accurate comparison between the two pistons I have we need to take into account the diameter of each piston and calculate the angle between ring locating pins respectively to see if they are indeed the same.

The formula to find this angle is:

arc angle = (arc length)*360/PI/D

The diameter of the 87-89 piston between ring grooves is 65.65mm

This gives an angle between ring locating pins of: 29.65*360/PI/65.65 = 51.75 degrees

For the 85-86 piston the diameter between ring grooves is 67.08mm so the angle between ring locator pins is: 30.35*360/PI/67.08 = 51.85 degrees

For 0.1 degrees difference between the two style pistons which can easily be attributed to measuring error it is safe to say the ring locator pins between the 85-86 and 87-89 pistons are the same.


Originally posted by fearlessfred
others have said that the pins are in a different location

Can you post a link to this I must have missed that post. I would like to see the data for this possibly an aftermarket company is producing them incorrectly that we should know about?


Originally posted by fearlessfred
why would anybody intentionaly mismatch piston design with port design

This one is simple, the 87-89 piston is only made in oversizes up to 2.00mm (0.080") over while the 85-86 piston is made up to 3.50mm (0.140") over stock. When your 87-89 cylinder is at a 68.00mm piston and needs a re-bore, instead of re-sleeving you can run the 85-86 piston with a spacer plate if you have the 87-89 long rod.

Now if you are good with hand tools check this out: windowed intake 85-86 piston, the best of both worlds!

http://i97.photobucket.com/albums/l209/mx91a/My%20Documents/IMG_0196.jpg

[fearlessfred]

thanks,that was a lot of effort on your part .i will look for the old post i was talking about and post it on here.
we were drilling the extra hole in the piston skirt back in 85 but did not elonggate the hole.i did not learn this on my own.I was lucky to have a mechanic freind who was a honda guru and he taught me a lot and saved my *** a few times.no internet forum to go to back then. I doesnt seem that there would be any disadvantage to running a modifyed 86 piston.
live to ride249 has asked a couple of times,can he use his new rings on his old piston. maybe you know, i dont.

[livetoride249]

Originally posted by fearlessfred
thanks,that was a lot of effort on your part .i will look for the old post i was talking about and post it on here.
we were drilling the extra hole in the piston skirt back in 85 but did not elonggate the hole.i did not learn this on my own.I was lucky to have a mechanic freind who was a honda guru and he taught me a lot and saved my *** a few times.no internet forum to go to back then. I doesnt seem that there would be any disadvantage to running a modifyed 86 piston.
live to ride249 has asked a couple of times,can he use his new rings on his old piston. maybe you know, i dont.

I greatly appreciate all the effort and information everybody has contributed. I broke in the new rings today, the rings were off a 86 piston and i put them on the 87-89 piston, and nothing broke... yet. I believe it will be fine since the ring locaters didn't change because i reused the 87-89 piston and the rings from the old and new piston are the exact same size.

[fearlessfred]

right on,hope she runs a long time.thanks to mx91a for teaching an old guy something,iust hope i dont forget it.