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cornishflyer

Sherpa T 250 Conrod sideplay

28 posts in this topic

Hi all,

I'm just starting to tear into the engine I got with my #158 Sherpa basket case project. There has obviously been a pretty catastrophic failure at one point, the magneto side is full of corrosion, I guess it was full of water for a long time. The cases where completely void of any oil. The crank didn't turn. When I removed the clutch side cover I found that the nut holding the flywheel on was completely undone and floating around, there was no lock wire on that nut, nor the clutch nuts. The area around the woodruff key on the crank is a bit mangled, but I should be able to clean that up. 

1) I have two questions. The primary side seal interfaces with what looks like a flanged collar that slides onto the crank. How does that come off? Is a small flywheel puller enough or do I need to use some heat?

2) What is the side play supposed to be on the con rod in the crank webs? Mine seems to be around 5mm which seems crazy big.

 

Cheers

IMG_20170923_175211.jpg

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That seal sleeve is normally a light push fit. I haven't had to heat one up to get it off.

Yes the big end should not have that much side play. Should be more like 0.5mm. Might have had the wrong conrod kit fitted.

Some Montesas do have the conrod kept central by the top end of the rod, but Bultacos like yours use the bottom end of the rod to guide the rod and depending on the piston used, also may have centralising spacers on the gudgeon pin.

Fairly standard Bultaco failure to shear the drive side key when something goes wrong. One Bultaco I bought recently had that nut welded to the crankshaft! Being a Sherpa T it was a slow process grinding the weld away inside the drive side flywheel recess.

It's hard to see how much clearance there is on the outsides of the crank wheels in your video, but is it possible that the crank wheels have moved apart to cause that side play at the big end?

Edited by feetupfun
thought of something else
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5 hours ago, feetupfun said:

It's hard to see how much clearance there is on the outsides of the crank wheels in your video, but is it possible that the crank wheels have moved apart to cause that side play at the big end?

Thanks feetupfun, once I split the cases I'll know, but I also fear that the crank webs have been pushed apart somehow. Either way I'm going to be needing new bearings, seals and con-rod kit at the very least.

Once I get this seal sleeve off I'll report back.... cheers.

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Took the crank out and used LOTS of heat on the seal sleeve and still can't get the thing off. I need to send this one for a crank rebuild anyway so hopefully they can get it off for me.

Who knows what has happened here, but I don't think much of the previous owners rebuild work, the cases were sealed with bathroom silicon.

 

silicon.PNG

21981869_10155613497875833_576158429_o.jpg

weld.PNG

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It looks like there is a washer missing off the ignition side of the crank pin. That might be what is causing so much play. They are supposed to have a washer on each side of the bigend pin.

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9 hours ago, bullylover said:

It looks like there is a washer missing off the ignition side of the crank pin. That might be what is causing so much play. They are supposed to have a washer on each side of the bigend pin.

Thanks bullylover, that might explain the slop. I thought the crank might have spread but when the pin is protrudes out of both sides. I can't see any markings on the conrod itself to say it's the correct part, but the pictures I can see on the net do look similar. 

Out of interest, why replace the whole conrod and not just the bearings? Surely the bearing would wear faster than the conrod? 

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15 minutes ago, cornishflyer said:

 

Out of interest, why replace the whole conrod and not just the bearings? Surely the bearing would wear faster than the conrod? 

The conrod can become oval, but worth checking.

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Mmmh beside getting oval the inner surfaces of the con-rod where the bearings are situated get worn, this happens to the upper and the lower roller bearings.

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If you are serious about saving money, the big end hole in the conrod can be honed out and oversize diameter (imperial sizing) rollers can be used to refurbish it.

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2 hours ago, feetupfun said:

If you are serious about saving money, the big end hole in the conrod can be honed out and oversize diameter (imperial sizing) rollers can be used to refurbish it.

Thanks. I keen on saving some money, a lot more since I've opened the cases and discovered what else needs to be done. Obviously not as keen as the skinflint that serviced the engine before me though.

I can't "feel" and up and down play in the big bearing when I pull on the conrod but I can see corrosion in the small bearing area so should just replace the whole thing to be safe.

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If you do go the honing route, there are imperial-sized big end rollers made for Harleys that are slightly bigger diameter than the Bultaco rollers.

I'm not surprised that there are a few things wrong with your motor because I have never encountered a second hand Bultaco engine that didn't need extensive internal attention before being re-birthed. One I bought recently the crank was so chewed up on the magneto flywheel taper, that half of the crank is unusable. The other side of the crank had the nut welded on, but should be OK to use. Another Bultaco I bought had the primary drive chain welded together. I figure someone couldn't work out how to get it apart so they ground the end off two chain pins, then welded the cover plate back on the chain afterwards. I haven't pulled either of those motors fully apart yet (saving the fun for a rainy day)

At least the price of Bultaco conrod kits has come down in the past few years. I remember doing up a 350 Alpina engine about 15 years ago and back then, the piston kit and conrod kit cost about twice as much as the same parts cost for a M49 Sherpa T motor I did up recently.

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Quick update.

I measured the play in the conrod and sideplay at the top is only 1mm. When pulling on the gudgeon pin I can't feel any play at all so I'm thinking the conrod might be quite new and is OK. 

What I would like to do is get some of the crank pin washers and just get the crank rebuilt. I'm not sure if I can get those without the conrod, or what the measurements/material is?

I also can see on the exploded diagram that there is supposed to be a washer that goes on the end of the kick starter shaft (that fits into the RHS engine case. Mine is missing. Any ideas what that washer thickness is?

Cheers,

Dylan

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Quick update. I received my thrust washers today., they are each 0.5mm thick which is thinner than I was expecting because it's not going to solve this particilar issue.

My conrod is 17mm wide, and there is 20mm between the crank webs so even after installing the thrust washers there is going to be 2mm axial play. This goes against what I have read on this forum and in the couple of general Bultaco books I have access to, all saying there should be around 0.25mm clearance.

I can't see what is wrong at the moment. The big end pin won't allow the crank webs to close that side play down even more. Unfortunately I don't have anything else to measure up against. Could it be that the #158 sherpa did in fact have a lot more side play? Or perhaps that someone has installed the wrong size big end pin?

Scratching head...

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When you say "The big end pin won't allow the crank webs to close that side play down even more", are you saying that the big end pin is stepped?

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