Wiseco piston clearances

[Bully fanatic]

Hello all. I am after some information for my Sherpa T. It is a 199B frame with a 151 engine. I just got a Wiseco piston for it and I couldn`t get a refund from Bultaco. It is still on the standard bore which was done in 1975 and the piston rattles a fair bit as it goes up and down. I thought they would last longer than that! I now have a brand new first over size Wiseco piston to go in it. What I need is the bore clearance I will need for the Wiseco piston. I know the Wiseco`s expand more that the original Mahle`s but I don`t know how much more. I have looked up the Wiseco website and it didn`t have the information I needed on it and I couldn`t find a contact method on there. I also asked Tim Weaver which is where I bought the piston from and he didn`t say anything. So can anyone help as I want to get it rebored next week but I need to know what clearance I have to have for the piston. So can anyone help me. Graham.

[BOGWHEEL]

Graham - This is the guru to ask. Ron Russ, Wishart, PH 3343 2502.
He will talk you into a small clearance for a trials motor. 1.5thou for the Mahle, but i've heard anything from 2 to 3.5thou for the Wiseco, as the expansion is a bit f@#k nose.
My M180 has 6.2thou gap, Don said to hone and new ring it, and see how it goes? But taking it to Ronnie next week for his opinion, lets know if i can do anything while i'm there.

[TriCub]

Graham.
If it's a new generation Wiseco the recomended clearance would be around 2.5 thou. Years ago most forged pistons were made out of a very high expansion rate alloy but the newer Wiseco and Wossners are being made from a much lower expansion rate alloy and generally have clearances similar to cast pistons. Most pistons are made to go into the specified bore size, that is if the bore is 70mm plus .5mm oversize = 70.5mm bore then the piston is made around 70.44 to give .06mm clearance.
A trials bike if only ever idled around could get away with a slightly smaller clearence than the piston manufacturer intended but caution needs to be taken. Motocross machines quite often get .5 thou more if the rider is known to be a bit lazy when running in or if the bike is running alcohol fuel.
Also the rings supplied are sized for the specifed bore and if the bore is reduced to give less clearance then the end gap would need to be checked and filed to give the correct gap. You should allways check the end gap with new rings anyway.
Whoever does your bore job should be able to measure your piston and work out the bore size that Wiseco intended it to be used in. Check to make sure your engine shop know what to do with a 2 stroke when it comes to port chamfering, I saw one the other day with chamfer that big that the port bridges had almost disappeared. Also before you assemble you need to hot scrub the bore to remove any residual hone stone.

Bogwheel.
Ron Russ is quite ill and I believe he has stopped doing any work for good.

[Bully fanatic]

Thank you both for your replies. Tricub that is the information I was after. I was going to take it to Ron Russ but it looks like I`ll have to find someone else now. Thank you. Bogwheel I`ll see you on Sunday. Cheers Graham.

[BOGWHEEL]

Sorry to hear re: Ron. Always pleasant and his vast knowledge will be truly missed.

[sherpa]

My experience with the large Bully motors is that they don't need much clearance before they become noisy, maybe 5-6 thou. My advice would be to bore as tight as possible [1.5 - 2 thou] and run in carefully as pistons are becoming rarer and more expensive.

[TriCub]

My experience with the large Bully motors is that they don't need much clearance before they become noisy, maybe 5-6 thou. My advice would be to bore as tight as possible [1.5 - 2 thou] and run in carefully as pistons are becoming rarer and more expensive.

I'd advise caution in reducing the clearance to much as a seizure instantly kills one of those expensive pistons and even if it doesn't seize the reduced clearance causes a much greater heat build up in the piston during run in. This in turn speeds up the rate of wear during the run in until it gets to the clearance the motor is happy with.

[Bully fanatic]

Thanks Sherpa and Tricub. I know that the Mahle pistons can get away with 1.5 thou of clearance but they are a very high quality piston. The Wiseco is much cheaper and a lot lighter and a lot less expensive. we got two Wiseco`s for the price of one Mahle. I just have to find someone around Brisbane / Ipswich area who can rebore it properly. I know someone down around Greenbank who I know is good so I might chase him up. Graham.

[sherpa]

One of the overlooked problems in this area is trying to find a good machinist who is capable of working to close tolerances. I you decide to go with the smaller tolerance careful running in is essential. Heat the engine up to operating temperature and then let it cool down. This should be done quite a few times, also add a little extra oil to your premix during this run in period. This topic was discussed on Trials Central some time ago.
Cheers Greg
http://www.trialscentral.com/forums/top ... hl__piston

[BOGWHEEL]

Yes - that was my post. Ron did 1.5 with the Mahle. I let my daughter ride it around the paddock on and off for a day to bed it in. Two years later and still a very quiet motor. (pig to ride)