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DickyM

Crank centering

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Hi all

My Sherpa198a project is moving ahead again after a break for a few summer jobs. Engine stripped and cases split and I am now thinking ahead to assembly. I've read the workshop manual (which doesn't actually cover the 198a), studied the parts manual exploded view and trawled through similar questions/answers on the forum. I am therefore familiar with the two main methods of assembling the main bearings. Fit onto crank and then into crankcases or into crankcases first and then fit crank.

But I did notice that quite a few people on the forum talk about the importance of taking care to centralise the crankshaft in the cases as you bolt them up.

However although I haven't seen it mentioned anywhere or by anyone my bike has a round section circlip fitted between the two drive side bearings. This means that when the drive side crank nut is fully tightend the right side of the crank pulls up to a fixed position with the drive side double bearings pulled up tight either side of the circlip. I.E. The position of the crank is fixed, determined by the circlip, with no lateral movement for any adjustment.

I measured the crank position before I split the cases  and there is a gap of 0.7mm between the crankshaft face and the inside of the case on the drive side and a gap of 1.2mm on the alternator side.

If I want to centralise this the only way I can see to do it is to fit a 0.25mm shim between the crankshaft face and the inner bearing or between the two bearings. This should then give a gap of 0.95mm either side,

Has anyone done this successfully? Or is this a complete waste of time and best to just leave it just as it is?

I have a few other things `I need to clarify but I think I will post them separately. One thing at a time is perhaps best.

Regards to all and thanks in advance.

Circlip fitted.jpg

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Just because there is a circlip in (supposedly) the centre of the main bearing outers this will not stop the crank from moving in either direction when everything gets hot. Certainly it is good engineering practice to have the crank set in the middle of the crankcases but I don't think Bultaco mass production was that accurate. If there was no shims involved when you stripped the engine I would not be putting any shims in. More importantly its better to ensure the crank bearing housings in each case actually are in perfect alignment, or as close as damn it. The crank will move in the bearings (sideways) unless you weld everything in place. Some Bultaco engines have no spacer/shims in the little end to keep the conrod central, others have shims in place. I ended up making some alloy spacers as I was not happy that there was no mechanical control on aligment in the top of the engine (piston) as my engine originally had no shims for the little end and the gudgeon pin bearing (little end was free to slop about, thats how it was meant to be. The alignment was down to the con rod and the big end bearing shims to keep all aligned. If you are using C3 bearings these have a design intended amount of sideways movement to allow for heat expansion and contraction. Should you decide to use a shim to centralise the crank in the case mouth I would gently polish the crank shafts so that the main bearing inners are not such an interference fit. This makes assembly easier and if the crank needs to move when its hot the bearings may allow some slight movement sideways. I would assemble the barrel onto the crankcase without the piston fitted and check that the conrod is in fact central to the bore...such fun! You may well be surprised by what you find when doing this. Incidentally the barrel can move around on the studs an amount you may want to look at a more positive register for the barrel....giving away secrets here. I prefer to have the main bearings in the crankcase halves ( don’t forget the ‘o’ rings that go on the crankshaft ) and then slide the crank assembly (shaft lightly oiled) into the main bearings... only a slight tap required to achieve this. Check everything spins nicely before fitting the crank seals, I fit two to each side the inner goes in backwards and the outer fits in the right way round...one seal keeps crank pressure in and the a*** about face one stops air being drawn in through the seal...good eh! Thats nothing new and many engines are sealed this way. Happy spannering?

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With the circlip type cases simply heat the case and drop in a cold bearing.  Each one will fall into place and rest against the clip.  Temporarily install the seal retainer to hold in place.  Do the same on the mag side case.   Install the seal retainer to help with final bearing placement.  Make sure your crankshaft is ice cold.  Set it in the freezer for a half hour.  Once the cases are ready to go together I use a heat sink that I heat up generously and then let them sit on the bearings and the heat will transfer from the sink into the bearings.   This is a nice way to do it instead of direct heat from a blow lamp or the like.  Once sufficiently hot take out your ice cold crank and drop into place in the mag side case.  Working quickly with the other case drop it onto the crank and the whole assembly if done right will just fall together,  torque case nuts to 5ft/lbs and let cool.  Once cool eyeball where the crank is sitting.  It may be that it’s too far to the left or right.  Simply use a dead blow hammer and swat the end of the crank in the direction it needs to go to be centered.  I can hear the curses now.  “ you should never hammer a crank”.  And true you shouldn’t but just a light swat will move it enough to center it and allow it to turn freely.  Once done continue with the rest of the assembly 

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16 hours ago, section swept said:

Just because there is a circlip in (supposedly) the centre of the main bearing outers this will not stop the crank from moving in either direction when everything gets hot. 

But wasn't  that was the very reason the circlip was introduced on later engines..?

 

17 hours ago, DickyM said:

 

my bike has a round section circlip fitted between the two drive side bearings. This means that when the drive side crank nut is fully tightend the right side of the crank pulls up to a fixed position with the drive side double bearings pulled up tight either side of the circlip. I.E. The position of the crank is fixed, determined by the circlip, with no lateral movement for any adjustment.

 

Well............ there you have it......

What are you hoping to achieve by "centre-ing" ?

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Thank you all for your replies. Some useful thoughts there.

Lorenzo. There are several people on this forum that suggest getting the crank central is important to better running. Not having done a Bultaco before I don't really know. However whilst it's all apart I might as well do the best job I can and might as well centralise the crank, as long as there is no downside.

All. I spent time this morning measuring up the parts from my dismantled engine and made a drawing of the drive side crank assembly, just to get it clear in my head. See below. As far as I can see there is little chance of lateral movement whether hot or cold, especially if the crank nut is tightened to 90 lb ft as specified in the manual. That is very tight. Also with this in mind I cannot for the life of me understand why the inner bearing races need to be such a tight fit on the shaft on the drive side. A light friction fit is all that's needed as when all bolted up that is not going to move. Easier assembly too as Section Swept says. Any movement due to heat expansion will happen on the alternator side or be taken up by the C3 bearings. So the way it looks to me is that if I do want to centralise the crank I will have to fit a shim between bearing and crankshaft thrust face, if indeed it is worth doing.

I was very interested in Section Swept's comments about the crank/piston assembly. As I understand it you had spacers on the big end and none on the small end and you made up spacers yourself for the small end. So you have spacers on both big and little ends of the con rod. My crank piston assembly is the opposite. Spacers on the small end and none on the big end with side to side play of 3.1mm. (There are spacers in fact but the OD is small enough for the conrod to go over them.) See photo's. Sorry they're a bit blurry. This arrangement relies on only the small end to keep things in line. This all seems original. Bultaco stamped on conrod and original piston. Never been rebored. Replacement conrods available from In Motion or Feked have big end spacers and a wide bearing at the small end and no spacers so that the conrod can float. Neither of these suppliers have a conrod that matches mine either. 24mm crankpin but only 16mm gudgeon pin. I need a new piston as well probably 72mm as the 71mm range does not seem to be available. Not sure what diameter gudgeon pin this would have. Any advice on the way forward with this would be much appreciated. The current bearing seems ok but I'd prefer to replace a 41 year old bearing set up.

Finally there is a small hole (approx 2.5mm dia) behind the kickstart mechanism in the gearbox which comes out above the kickstart return spring on the outside. See photo. What's this for? Doesn't it drip all the time? Oil the spring maybe although enough oil gets flung off the rear chain to do that I would have thought. There is an adequate breather system so I don't get what this hole is for. Can anyone enlighten me?

Thanks again everyone.

 

Driveside crank.jpeg

Big End 2.jpg

Big End.jpg

Mystery Hole.jpg

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13 hours ago, worldtrialchamp said:

Polish the crank to allow it to float on the bearing inners? Oh lawd.....

 

worldtrialchamp....Thats not what I wrote, if you read again you will see I suggest reducing the interference fit....not allow the crank to slide about in the inner sleeve of the bearing. 

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 The big end is normally shimmed on the Bultaco, at least the ones I have done are.    But you never know whats gone on before either at the factory or by previous owners. The hole is a breather in my opinion, most people that know about them seal them up and vent elsewhere with some plastic tube to run up out of the way of water etc. As many of the different trials engines were originally intended for road bikes it is likely that the hole you have identified would be perfectly ok for road use but not so good for wading etc and more likely to become clogged up in competition use.

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DickyM it's the centreing of the conrod in the bore that is important rather than equalising the side clearances on the crankwheels. You may not need a shim to achieve this.

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Another thing that can help with this sort of work is to modify the ID or OD of the old main bearings enough so that after fitting the new conrod, you can do any test-fitting of the crankshaft with ease at room temperature

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DickyM - 

The small hole pictured is the gearbox breather, the only breather in standard form. It needs to be kept clear - if it becomes blocked it can lead to oil leaks and/or g/box sprocket seal failure. Attempts to provide alternative breathing through the filler plug have had mixed results, according to previous posts on the subject.

Over the years Bultaco appear to have experimented with either two or three main bearings / crankpin (and crankwhee)l diameters / conrod location by big-end washers and small-end spacers / location by small-end spacers only. Your conrod/big end bearing with narrow thrust washers is the original type of set up for your engine.

Edited by lorenzo
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DickyM - 

Whatever you decide with your engine, I urge you not to make any non-reversible changes - reducing the interference fit of main bearings "to make assembly" easier would be one such change............you could regret this when your main bearing inner races start to spin on the crankshaft........

Never assume that Bultaco didn't know what they were doing .....

Your con rod assembly looks like :-

702113166_Bultaco250originalconrodassy.-24mm.crankpin.thumb.jpg.a6063d82f00901fc047ce2d19d792bf8.jpg 

As far as I can tell replacements ARE available - this is one bought in January and now fitted to one of my cranks:_

1129374919_Bultacocrankshaftrebuilt-24mm.crankpin.thumb.jpg.3c34607f246167a74483f48869f5939e.jpg

(I was quite surprised to find this crank in a mod. 124 engine, which I believe would have had an 18mm. crankpin originally..! )

Also have o/size Mahle 71mm. pistons, although many have said these are "unobtainium".

"Seek and ye shall find" ?

The availability (or otherwise) of parts is influenced by how soon they are needed, and the depth of your pockets. I have several engines undergoing "the treatment " at the moment and I've not found one that's worked out cheaply so far........

 

Edited by lorenzo

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Well thanks everyone for your advice.

Centering the crank. Decided. Centralise the crank to the centre of the bore.

Crank assembly. Many and varied varieties out there. There doesn't seem to be a preference for one version over another so will use what's available.

Piston. I spent 3 hours last night looking for NOS 71.25mm, etc pistons and there are non easily available that I can find. Need a rebore anyway so decided to go for 72mm. Readily available.

Small hole is a breather. Will leave alone.

Thanks Lorenzo for the pictures. That does indeed look like my crank and by the way no intention of carrying out any irreversable mods. Wouldn't dream of assuming Bultaco didn't know what they were doing but they sure chopped and changed with their crank designs.

Good idea from feet up fun to modify old bearings so room temperature test assemblies can be made. Difficult for me to modify hard bearings so turned up a couple of dummy bearings in aluminium that allow the same thing although cant turn the crankshaft with these though.

Thanks again

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Thats the best method to ensure no adverse side loading to a piston running up the bore and down obviously. The main reason that there seems to be so many variables with cranks, rods and spacers was that Bultaco wanted to reduce any friction losses without too many dimensional challenges....a loose engine in other words. You will most probably find that your crank will not be exactly in the centre of the cases when the piston is in the centre of the bore. When you have finished at least you will have an engine that has been assembled properly and not just put together. A few passes through the old main bearings with an abrasive flap or Dremel attachment will give the desired effect, you will need to lock the inner to prevent it spinning, a ‘g’ clamp or mole grips can be used if sited carefully. Every factory trainer that has to show others how to do an assembly job eases the bearings so they never ‘struggle’ with a demo.

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On 7/11/2020 at 5:06 AM, feetupfun said:

Another thing that can help with this sort of work is to modify the ID or OD of the old main bearings enough so that after fitting the new conrod, you can do any test-fitting of the crankshaft with ease at room temperature

This.

+1.

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