Piston from and old ATC?!

[davidninja]

So I've been having major problems with my '86 trx 250r. It hasn't run right since I bought it. It would backfire really bad so I adjusted the jetting and got it to idle nice. But, it still wouldn't hit power band, it would just break up and not pull at all. It's got an older style CT 310 head on it without interchangeable domes. Went through all the electrical system, new stator, new cdi, checked the ground, ohm tested everything. Next, I checked the compression with a good lisle gauge and it read between 75 and 90 PSI!! Pulled the head and at TDC the piston is about 1/4 inch lower than the deck. The piston has an arrow pointing towards the exhaust and is stamped 0039H 431m07200. The only pistons I can find with these numbers is from an '81-'84 ATC250r or a '78-'80 CR250r! So that would mean this is piston has 64.4mm stroke not the stock 72mm. Anyone ever hear of a setup that would need this piston? The crank is a stock '86 crank. I'm about to order the correct piston, but I just want to see what you guys think first.

[deathman53]

Do you have a spacer plate under the cylinder? Something doesn't sound right. You are right, you have a 80 over 81-84 atc250r piston, stock bore is 70mm and 72mm is 80 over. I think all had the piston pin in the same location.

The other thing I can think of is you had a slevvy 200cc motor, somebody didn't know what they had and tried to put a regular cylinder on. Not realizing that the slevvy motor was de-stroked and cylinder was cut-down nearly 1/4". That is a really really rare possibility.

[davidninja]

Thanks Deathman. No spacer plate. I'll have to measure the stroke before I go any further.... Not sure how to do this?? Just the distance the piston travels right? If it's less than 72mm I think I might be in trouble! I'll check it out tomorrow...

[deathman53]

I'm pretty sure that all piston heights for 81-86 are the same pin. In 87-89 it was changed for a different rod(longer). People do use 330 big bore 86 pistons for their big bore 300cc 81-84 r's, so I'm pretty sure pin heights are the same. I don't know how to measure the stroke. Pull the cylinder and see what marks are on the rod and try to get a good picture of the crank. A stroked/de-stroked crank will be easy to tell. I believe the Slevvy cranks used a shorter rod than 85/86. If I remember correctly, the 200cc motor combo's were almost a square stroke and bore. They used the stock piston and a stroke that was very close to the piston diameter to make a wide powerband.

[davidninja]

Okay, I did some more research. The '81-'84 ATC250R had an air cooled big bore (70mm) short stroke (64.4mm) motor. I'm fairly sure that the previous owner didn't know any better and put the wrong piston in thinking it would work. I can't tell if the crank on my motor is stock, but the rod is a KA4 (stock '86). I'm assuming the pin height is different on the air cooled '84ATC piston than the '86TRX piston. And i'll bet the air cooled head on the ATC was about 7.6mm shorter. Can anyone confirm before I order a new piston?? Does anyone know what the pin height is on a stock '86 TRX piston?

[86 Quad R]

naw but i have one of each piston at the shop that i can compare. i do know right off hand that the pin diameters are the same aswell as the rod lengths being equal.

[deathman53]

there actaully was 3 big bores for air-cooled, 300cc, 350cc and 360cc. Some of them were mugen and hondaline. The 350cc is a very rare Japanese prototype. The 360 is nearly impossible to find complete and replacement parts are even harder. The 300 is hard to find and parts can be adpated from 250r 330 pistons and fl350 to fit. I don't know the spec for the big bores, but stock cylinder was 70-72mm. The rod you have is a stock 86 rod.

[86 Quad R]

sorry for the delay......... but i got around to measuring the compression hieghts of the two pistons last night.

oem wc/85-86 piston measures - 38.0746mm/1.449"
oem ac/81-84 piston measures - 32.334mm/1.273"
with a difference of - 5.7406/0.2260079"

which is about the 1/4", that you measured your piston falling below the deck.

[davidninja]

Thanks 86 Quad R, just as I suspected.... I ordered a new forged piston from L.A. Sleeve and hope to have it in before the weekend!