Drilling holes in the piston?

[jski]

When using Wiseco pistons it states that you have to drill the 3 holes in the piston to provide adequate lubrication to the exhaust bridge. I recently experienced a meltdown on a 1987 CR 500 that is setup for hill racing and has had some series port work done for improved top end. I bought the 500 last year and put maybe 3 hours on it before it lost compression and at first I thought it had a bad crank seal but the piston was a Wiseco and was not drilled when the motor was built for the previous owner. The shop where I took the cylinder to be bored said it had been sleeved and was out 15 thousands on the exhaust side. My question is this " Can not drilling the holes in the piston cause a meltdown where the exhaust side of the piston is melted and the cylinder wall on the exhaust side shows excessive wear or do I have other problems that may have to do with the porting that was done on the cylinder?"
Thanks,
Jim

 

[EricGorr]

The excessive wear on the bridge was probably not wear but an attempt to dish the bridge. Thats a common task performed after boring and honing a cylinder. The proper spec is only .004 inch more than the bore size.
The Wiseco pistons are turned for a specific shape that enables them to become round when at operating temperature. Its not totally neccessary to drill the holes if the bridge is relieved correctly, but its a good safety margin.
Regarding the meltdown, its not likely from the bridge work, its probably related to jetting and load. That engine doesn't like to rev, but if its pushed hard jetting has to be compensated (bigger main jet).
Good luck

 

[KXKen]

Originally posted by jski
I bought the 500 last year and put maybe 3 hours on it before it lost compression and at first I thought it had a bad crank seal but the piston was a Wiseco and was not drilled when the motor was built for the previous owner. The shop where I took the cylinder to be bored said it had been sleeved

I had the exact same problem with a 94 CR125. Same piston, no holes drilled, sleeved cylinder. Seized on the exhaust side of the piston after about 3 hours. This was after proper break in. When I pulled the plug it was black. It was my first bike in like 20 years so I wasn't all that familiar with it. The guy that sold me the bike said that he wasn't able to get it running properly after getting the cylinder sleeved (he said it ran fine with the plated cylinder). I messed with the jetting and also made the mistake of taking advice from my local dealer (ended up spending a whole lot of money replacing parts that where OK in the first place). I never did get the bike running right (ended up selling it). Looking back at it I'm guessing that (at least part) of the problem was a combination of the holes not being drilled and the bridge not being properly releaved. I wish I had the advice of DRN back then. It would've saved me a whole lot of money.

 

[Layton]

KXKen,

The holes are drilled in the piston to provide a blast of cooler gasses to the bridge in the cylinder. This is done to keep the piston from sticking in that area as is dishing the bridge. Neither would have affected the proper running of the engine in any way that you would have noticed.

More likely the power valve was improperly installed or because it was sleeved, maybe the porting was totally messed up.

I guess I don't have to close with "good luck" as you have sold the bike.;)

 

[jski]

Thanks for the replies guys but when checking the plug condition when it was running it always indicated being rich and not lean. I guess maybe it could have been experiancing blow-by on the exhaust side which if I am not mistaken would make the motor run hot on that side thus causing more damage to the piston over time. Eric regarding the carb and jetting question the bike runs a 48mm carb and I'll have to check the jetting.

Thanks again

 

[KXKen]

Originally posted by Layton
KXKen,

The holes are drilled in the piston to provide a blast of cooler gasses to the bridge in the cylinder. This is done to keep the piston from sticking in that area as is dishing the bridge. Neither would have affected the proper running of the engine in any way that you would have noticed.

More likely the power valve was improperly installed or because it was sleeved, maybe the porting was totally messed up.

I guess I don't have to close with "good luck" as you have sold the bike.;)

Layton, Thanks for the input. The bike ran good when it was cold but once it warmed up it had a bog in the low end. It wasn't an air leak, jetting, power valve setup or the stator I compensated on the jetting (larger pilot and lower needle clip position) to get the thing to run the best I could but it never really ran right after it warmed up. I like your comment about the porting though this is the most likely cause of it not running properly. Maybe this also contributed to the other problems? I still think that the relieve on the bridge and the holes not being in the piston contributed to the ring seizures I was having (it always seized directly above where the holes where suppose to be). I wish I had the bike back to fix just to satisfy my curiosity

 

[Layton]

KXKen

I still think that the relieve on the bridge and the holes not being in the piston contributed to the ring seizures I was having (it always seized directly above where the holes where suppose to be).

You are right on with the above statement. That is exactly why the piston should have the holes drilled in them.

When a new sleeve is pressed into a cylinder there is always some matching of the ports that has to be done. It sounds like someone may have gotten a little carried away in your case.