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cornishflyer

Piston slap confirmation

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Hi All,

I''ve almost completed the build of my M158, and took it for it's first spin today. We put the engine together fully expecting piston slap as the clearance was well over service limits and sure enough it sounds like it's got a bad case of it. It's going well enough, first time I've ridden a Sherpa 250, very different power to my old 350. It picks up quite nicely at higher revs when you give it the gas, and the engine is turbine smooth.

Anyway, I just wanted confirmation on the piston slap diagnosis from the experts on here. 

Cheers

 

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Well the bike looks good, some engines are quiet and eventually start to make noises. Some Bultaco engines were noisy from the start, luck of the draw on manufacturers accepted tolerances. As you identified over tolerance on the piston to cylinder bore clearance naturally it’s going to ‘rattle’. Your engine could go for some time before mechanical kindness kicks in, bear in mind that excess clearance will affect engine performance and it’ll not do the piston or rings any good in the long run. Better to remedy this sooner rather than later. If the barrel is on its original bore size then a reborn and piston to the next size or two up. Small increase in capacity will be an advantage but not by much. Try for a Mahle piston, not cheap but as original equipment OE depending on Bultaco Factory finances at the time your engine was made. 

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Ouch, i feel bad for the motor just ticking over, i dread to think what it sounds like when you rev it

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Think I'd get it rebored and a new piston rather than ride it,itll never go like you want it to.It looks superb

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12 minutes ago, faussy said:

Ouch, i feel bad for the motor just ticking over, i dread to think what it sounds like when you rev it

Funnily enough the slap goes get a lot better when I rev it, it's worse when it's idling. It's worse when the engine warms up as well.

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Thanks experts, looks like I'm at least looking for a new piston. I was hoping to get away with the one I had until I could afford to get a new one but it looks like it's behold that.

feetupfun- we checked the crank when we rebuilt it, so I really hope it isn't the big end, but anything is possible. You might remember that this is the engine with the crank from another model (my previous post - twin bearings, stuck seal bobbin) so anything is possible. That was part of the reason for posting this as I've been worried it could be something other than the piston slap. Guess there is only one way to find out.

Cheers

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Your cranks big end might have ‘felt’ alright on the bench but there are a few ways that the bearings can wear from barrelling to tapering depending on main bearings wear to! There was no detectable play in my crank but when taken apart there were some appreciable wear signs on the big end pin. Just replacing the piston will not solve the issue of piston slap, well it might but I doubt and it will be a costly mistake not to have a rebore and correct piston and rings. 🙂

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Big end can feel fine while being bad. This pin & bearing came out of a crank I didn't know the history of but felt good, would have failed fairly quickly if I has run the motor without fitting new parts

20170903_114950.jpg

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Hi all, 

Has anyone used, or heard anything about the coated italkit pistons from feked off? 

Cheers.

EDIT: Just saw there is another post here on the same topic, pls ignore.

Edited by cornishflyer

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Can you supply the piston bore clearence 3/4 of the stroke length up, on the inlet and exhaust side. The noise does not sound so much like piston slap, but more like the right side flywheel is loose on it's parallel shaft, or the primary chain is shot. It doesn't sound like a big end failure either. Bye, Peter B.

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1 hour ago, peterb said:

Can you supply the piston bore clearence 3/4 of the stroke length up, on the inlet and exhaust side. The noise does not sound so much like piston slap, but more like the right side flywheel is loose on it's parallel shaft, or the primary chain is shot. It doesn't sound like a big end failure either. Bye, Peter B.

From memory the piston clearance is 7 thou, baggy for sure but I was expecting some noise. I did think this sounds different though.

I did wonder about the primary chain as the bad sound comes and goes riding around at walking pace, sort of like the chain is getting caught up. You can feel a bit of chain slap rolling the throttle on and off as well. I thought the tensioner was stuffed and I took the cover off to have a look but it looked ok, as did the flywheel feel tight. I didn't try to tighten anything while I was in there though. 

The clutch basket is alloy which I don't think is a standard for the sherpa, perhaps Pursang?

Also, it has a flywheel on one side of the chain. I think I read somewhere they came with one each side?

 

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Edited by cornishflyer
Added video

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I have 2 Sherpa's, both have an aluminium basket. At this stage, I'd check the flywheel if the crankshaft nut is tight. I had a similar sounding problem on my bike, that was the result of wear on the RHS parallel crankshaft, fixed temporarily with a special loctite, I think it was silver Loctite, whatever it was it could accept a clearence of up to 20 thou or so. Even though the crank nut was tight, the flywheel could still rotate on the shaft and sounded like your bike. 7 thou is a lot but that sort of clearance would make a ring a ding ding noise, not a heavy clump. Check the flywheel. You may be able to hold the magneto flywheel and then try to see if you can rock the RHS flywheel. Odly enough, your bike hardly makes any ringing noises. Could also be the primary chain, they never lasted too long before needing replacement. Bye, Peter B.

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2 hours ago, peterb said:

Hey Tony27, you got to put oil in your Sherco fuel!.

Not out of my Sherco Peter, that's still going strong. Big end came out of a crank from the states that I bought to build a spare motor for my CZ380

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