Crank and Bearings and Gaskets.

[Mrapocalypse]

Hi,

I'm at the stage where I am putting the bearings onto the crank and putting the casings back together on my M158 Sherpa 250.
No dramas with the other bearings and gears etc....

This photo is well before I installed the bearings and gear sets. But these posts are much more fun with Pictures right!



But: 2 Questions for the Bultaco Brains Trust.

I have seen that some folks like to put the bearings onto the crank then then into the casings and others have put the bearings into the casings then warmed the bearings with a heat gun and put the crank in last.

Advice has been to install bearings onto the crank first then into the hot casings and I think this is the way I will go. Open to suggestions tho.
And I saw on Youtube a guy using a locktite product on the crank to hold the bearings on? Thoughts.
Do I need to put any gap between the bearing and the crank face? Or will it set itself?

AND:

It had no gasket between the casings, but the rebuild kit came with one and the destruction manual has a gasket.
I'm keen to use a gasket. Seems obvious but this bike has had some VERY dodgey work don't on it in the past.

With a gasket installed do I still use the oil seal retainers to set the spacing on the crank and just work from there?

And here's my beautifully rebored and bead blasted head and barrel. Just for Bultaco in bits porn. Its on its' final oversize.

Always interesting to get these clues into an old bike's past. Someone did lots of work on this, it obviously belonged to someone who was pretty serious about it's use.


Cheers

Ian.

[David Lahey]

Yes use the centre gasket
Yes shim both seal retainers to keep the crank in the right spot. Do allow a bit of clearance when you do the shims, maybe 0.5mm total to allow movement as the motor heats up and cools down and the bearings find their ideal spots. Depending on the fits, it might slide on one of the crankshaft fits or one of the crankcase fits. Be careful when doing the shims to avoid bending the crankshaft.
Yes fit both mains up against the crankwheels and by doing this first you can be sure they are all the way home
Check the fit of the mains on their seats on the crankshaft - the fit might be too loose if previous bearings have been dragged off cold or if the bearing has worked a lot on it's seat in service. If the fit is too loose then you should do something to prevent the new bearing slipping in service. Metal spray/regrind, new crank half etc.
These motors also sometimes suffer from the bearing outers being a loose fit in the casings, also caused from cold disassembly or possible poor fit when new. This allows the outer to slip. The best way to fix this is to sleeve the holes. There are a few rough ways to get around it too. I have heard of people copper plating the OD of the bearings for example.
If you decide to try using loctite anywhere, make sure that the crankshaft can move on at least one bearing seat to allow for differential expansion (or you might bend the crankshaft)
Assuming your crank is still standard width with the new conrod, you should aim to have similar thickness of shims on both sides, and this should give good primary chain alignment and the conrod location should be in the middle of the bore. Do check these before you go too far with reassembly in case there is something dodgy with the crankcase halves or the crank is wider or narrower than standard
I've only rebuilt three of that type of Bultaco motor in my life, and in that limited exposure I have found one with a very loose (worn down) shaft fit on the magneto side bearing, and on another motor, both sides crankcase halves had worn bearing seats. I found another crankshaft half to fix the first one, and had the crankcase holes sleeved with steel on the other one. The third motor was OK, but that was in 1976 and it was a 1974 model bike, so the motor hadn't had a huge amount of use.

[Mrapocalypse]

Hi David,

Thank you very much very that very in depth answer.

I really appreciate it.

I'm going top use a heat gun on the crank bearings. Do you think that will do the job without adversely affecting the bearings?

Thanks again.

Ian.

[David Lahey]

yes heat the bearings to get them on the crankshaft easily and heat the bearing seat areas of the cases to make the bearings slide in easier. You can even put the crankshaft in the freezer to help with fitting the bearings. An electric or propane heat gun is fine for the main bearing seats in the casings when you are putting the sides together and an oven or a electric frypan is less effort for heating bearings. Heat the aluminium to less than 120 degrees and try to avoid heating up the sealant on the gasket surfaces too much.
Fit the gearbox bearings first and for that you can heat the casings evenly in an oven or electric frypan, and cool the bearings in the freezer

[David Lahey]

you can heat those sort of bearings to about 150 degrees without hurting them unless you leave the rubber shields on (some people leave the shields on the main bearings in Bultaco motors). If you put bearings or the crankshaft in the freezer, spray them with WD40 as soon as you can after they are in position, to avoid the risk of causing corrosion from the water that condenses on them

[Mrapocalypse]

Hi,

Thanks for the info on the many black arts of Bultaco Bearings.

Forearmed and ready to go.

Thanks again.

Ian.

[BOGWHEEL]

Remove the covers from all the bearings Boil the crankshaft bearings in oil and freeze crank - they will just drop on.
Oven is best to heat cases. If you did a bit of bashing to get the crank out originally - check the run-in.
Put the seal retainers on the cases before setting the crank. I normally put everything in the left side first.

Important - Don't forget the washer between the end of the kick start shaft and the right case!!

[Mrapocalypse]

Hi.

I had the crank rebuilt by Don Newell. Mainly because I did bash the hell out of it during flywheel removal.

Hence my gently gently approach here,

Have heard the oil boil idea, may be a little logistically hard for me so will start with the heat gun I think.

Cheers

I.

[sherpa]

The engine will go together any number of ways, the issue is bending the crank which is quite easy to do. I put the bearings into the cases insert the crank and by using a pipe sleeve and washer pull the crank into the bearing by tightening up the flywheel nut onto the washer and sleeve, which is mounted on the outside of the case. Also make sure you have the correct clearance main bearings, C3 from memory.
Cheers Greg

[Mrapocalypse]

Hi,

It all went well thanks to the above advice. Ended up having to put one crank bearing into the case and the other seemed to go in OK when the two halves came back together. All spins nicely and there are the appropriate number of gears.

Thanks again.

Ian.