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Old 10-19-2021, 01:42 PM   #1
ntzman28
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Default J38 Piston Arrow

Hello all,

Am re-ringing a Yamaha J38 out of an early 90s G9. I can tell as I'm taking it apart that someone in the past has probably done the same job: screws on the shrouds are marred, scratches on the crankcase side cover where it was removed before, use of only grey RTV on the cover and no gasket, etc. When I got to the piston, the arrow on the top was pointed towards the back/exhaust side of the head. Everything I've found online says the arrow should be pointed towards the front/cam side of the engine. As I lap the valves, hone the cylinder (looked good BTW), etc. while I await the replacement rings, I'm wondering if I should turn the piston around the right way, or if there is any harm or foul in doing so. Piston measures within tolerances, as does bore as best I can tell with what I've got to measure (dial caliper). I've heard that pistons are actually a little bit egg-shaped, but can't see that having it in with the arrow towards the back of the engine did any harm to the piston or cylinder. Should I take the wrist pin out and turn it around? I even thought about just turning the whole connecting rod around, but worry about uneven bearing wear at the crank. Was it a common thing for high-volume golf cart repair shops to turn pistons and con-rods around, and if so, what was the advantage in doing so? Thanks in advance for any replies. Am enjoying learning from everyone's prior posts. there is also a letter on the connecting rod bearing cap, and it was oriented towards the back of the engine as well.

Thanks,

ntzman28
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Old 10-20-2021, 01:01 PM   #2
smallblock450sl
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Default Re: J38 Piston Arrow

Yes the piston arrow should be pointing forward. The rod has a "Y" and should face the case cover side. Also here's a timing mark picture. Good lock
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Old 10-24-2021, 07:21 PM   #3
ntzman28
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Default Re: J38 Piston Arrow

Thank you! Found it in the service manual as well. Rod was facing the right way, so whoever did it before just put the piston on backwards. Very much appreciate the reply.
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g9a gas, j38 engine, yamaha
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