Piston slap confirmation

[cornishflyer]

3 hours ago, peterb said:

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.

You were spot on Peter, there was noticeable play of the RHS flywheel on the crank shaft. As soon as I put some force on it "that noise" was clear as a bell.

Now I have to work out why the bolt came loose, even though it was tight. The key way is quite enlarged from some previous owners mischief but when the engine was recently assembled it was tight and there was no play. 

One thing I have noticed is that when the seal bobbin is fully inserted there is 4mm of shaft before the shoulder,  however there is 6mm recessed into the flywheel. I wonder if the flywheel is supposed to sit hard against that shoulder of the shaft. When the bobbin expands and contracts it might be loosening the bolt. Or do I just need to get another woodruff key and jam a whole load loctite in there?

 

 

Edited July 30, 2018 by cornishflyer

[woody]

If the key isn't a good fit in the slot, it can easily ride up the back of the slot when fitting the flywheel and this isn't always apparent when assembling, meaning that the flywheel isn't fully home on the shaft when tightened. As you say, best bet is to get a new key, loctite it in, let it set and then fit the flywheel.  Most of the engines I've had have had some wear of the slot, so this is what I always do. These flywheels coming loose isn't uncommon if it isn't fitted perfectly

The single sided weight is standard on the later 250, from M158 onwards I think. Earlier 250 had a double sided weight, as do the 350 models

[peterb]

Thats good information woody, you've had Bultaco's for a while then. The keyway ideally needs a repair job though probably not what you want to do right now. After fitting/locking in a new key, you could try using a Nord Lock washer under the crank nut, if you can find one with the right dimensions. I've used these with great success on other bikes, it is like 2 interlocking washers that lock in two directions. These guys have them down your way: https://www.hi-tensilebolt.com.au/product-category/washers/nord-lock-washers/ Bye, Peter B

 

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[cornishflyer]

Thanks for the link Peter, I'll look into those washers. 

For the enlarged keyway, I'm going to look at Loctite 660, which from this video looks to be my best shot. However reading the forums the stuff looks difficult to remove, which will probably come back to bite me. Asking someone that has done this before, do think 660 is overkill?

 

[sherpa325]

The problem you have is that the shaft is parallel and not tapered, so the flywheel is driven by the key not a taper. The only way I could see around this is obviously a new key and if this is a sloppy fit in the shaft then you really are left with trying to hand make a key that fits neatly in the recess. I did this years ago on a model 49 that was continually shearing the key on the clutch basket [before they put splines on the shaft]  I also made it out of a much tougher steel than the original which was mild steel. If you have access to a lathe its not that that difficult - just make one a few millimeters oversize and file to fit.

Cheers Greg

[thai-ty]

JB Weld. Repair keyway. Get flywheel to fit nicely WITHOUT the key. Use new key and fit with normal bearing fit loctite. Let it all go off. refit flywheel. Torque nut to correct spec.

[feetupfun]

6 hours ago, cornishflyer said:

Thanks for the link Peter, I'll look into those washers. 

For the enlarged keyway, I'm going to look at Loctite 660, which from this video looks to be my best shot. However reading the forums the stuff looks difficult to remove, which will probably come back to bite me. Asking someone that has done this before, do think 660 is overkill?

 

I've used 660 a few times over the years in bigger stuff and not had problems getting things apart again as long as you can heat them up, which softens the loctite. Your application would be fine but keep the 666 confined to the parallel shaft and the key and keyway

[cornishflyer]

Just wrapping this topic off (I hope).

I installed the new wood-ruff with with Loctite 660 and did it up nice and tight and the engine sounds great now. Funnily enough the piston slap isn't that bad at all, even with 7 thou clearance (this is in my garage which always makes it sounds nosier than outside). Thank you to everyone who offered advice.

I also installed a new primary drive chain and there is a (not unpleasant) whirring sound from the RHS flywheel side. You can hear it in the video when the revs come down after I rev it. I'm assuming this it the new chain on the tension wheel as the chain had around 10mm freeplay before I put the tensioner on. Do they usually do that?

 

Edited August 7, 2018 by cornishflyer

[feetupfun]

10mm freeplay is about right for a new chain.

The whirring noise on your video clip as the motor returns to idle does sound a bit unusual.

 

[b40rt]

Did you check for 10mm play having rotated the crank in several positions ?

 

[scot taco]

It is a bit hard to tell on video rather than in person,but I had a similarly odd whirring sound in my Pursang.It turned out to be a bad wrist pin bearing.

Beautiful bike!

[cornishflyer]

9 hours ago, scot taco said:

It is a bit hard to tell on video rather than in person,but I had a similarly odd whirring sound in my Pursang.It turned out to be a bad wrist pin bearing.

Beautiful bike!

Thanks scot taco. The bearing is new but I'll certainly keep that in mind.

 

10 hours ago, b40rt said:

Did you check for 10mm play having rotated the crank in several positions ?

I checked in a couple of positions but agree it's worth checking again. I'm also concerned that the alloy clutch basket teeth might be worn enough to cause this as the chain I removed was really past it's best, with over 20mm play. I'll have a closer look but can't think of any obvious way to check the wear of the teeth, they aren't "sawtoothed" for example. 

[scot taco]

I should have put this in my last post,but my wrist pin bearing was also a new one.The problem was that I had an updated rod kit that I put in the year before with a 1mm larger wrist pin and bearing.When I did the top end I forgot about it and they sent a stock bearing.

I should have caught it,but I put it together about 10:30 pm the night before a race  and I just rushed it too much.Anyway,the next day in practice it had a pretty pronounced whirring,but it was worse as the revs climbed so probably not the cause of your noise.It also did not make it through practice before it grenaded 

[cornishflyer]

 

36 minutes ago, scot taco said:

I should have caught it,but I put it together about 10:30 pm the night before a race  and I just rushed it too much.Anyway,the next day in practice it had a pretty pronounced whirring,but it was worse as the revs climbed so probably not the cause of your noise.It also did not make it through practice before it grenaded 

Ouch, did you have to split the cases?

[scot taco]

No,I just flushed it out real good with premix.I did have to resort to a back up cylinder and piston that was on the last bore.It was a little loose so I sent the piston out to be ceramic coated to bring it closer to tolerance.I was a little skeptical about doing that but it is still running pretty good,although I haven,t raced much since then,maybe ten motos on it.