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I know that the 86 has the shorter rod than the 87-89 trx motors which is the difference in the compression ratio. But I was looking in my repair manual and it shows the atc motors have even higher compression ratio of 8.0 to 1. So whats in those motors to make the difference just curious cause I bought an 86 atc motor for a race motor for my trx. Thanks
 

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if i remember correctly, the ATC motors had a thinner head gasket.
i think that was the only thing that made the difference in the compression.
 

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yep, thats the only thing that makes the difference between a 86 atc vs trx motor. The 85 atc motor had some differences as well, cases(thicker), kicker gears(weaker), something with the clutch(clutch basket bushing/bearing it rides on), cylinder(more radical porting) and clutch cover(color). In 87 it was almost the same as 86 except for crank and 5th/6th gearing. In 88 and 89 is where more changes happened.
 

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yep, thats the only thing that makes the difference between a 86 atc vs trx motor. The 85 atc motor had some differences as well, cases(thicker), kicker gears(weaker), something with the clutch(clutch basket bushing/bearing it rides on), cylinder(more radical porting) and clutch cover(color). In 87 it was almost the same as 86 except for crank and 5th/6th gearing. In 88 and 89 is where more changes happened.
And the A & B model cylinders, just a slite port angle change.
Neil
 
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