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piston alignment

Last time I wrote, my piston was scraping the timing side of the cylinder. Well, I have worked out the issues with the crank and rod, and now find that the piston is scraping the drive side. No damage is done, I'm just fitting it together.

The spacer that rides between the two drive side bearings (and sets the position of the piston once the cush drive is tight) is a bit too short--it measures about .004" UNDER an inch, when it should be 1.000 to 1.005, if my understanding is correct.

The problem is, just looking at it, 1.005 is not going to be enough. I'm thinking more like 1.010 or 1.015

Has anyone had to do this? will it work? If you did, did you shim it, or make a new distance spacer?

Why would it happen? I have a decent looking flinger, the bearings are fine... could the cases be sloppy?

Anything else I'm not considering?

EDIT: Forgot to mention--I'm certain that the rod is straight and big end is sorted--but the rod is still riding too close to the drive side for me to take a chance on it.


email (option): nicktog at gmail

Re: piston alignment

The spacer between the main bearings should be 1.000"-1.005" long...

This does not position the piston, but positions the crank correctly within the crank cases when the engine shock absorber nut is fully tightened and the crank is locked against the fixed outer drive side bearing....

Regarding the piston position and nothing else, if the crank was .010" -.015" thou off centre it wouldn't make any practical difference to the piston/bore relationship as this would easily be accommodated by the large amount of clearance either side of the small end bush between the pistons gudgeon pin bosses...

If the spacer is the correct length, the oil flinger plate is fitted and is flat and undamaged, the correct main bearings have been fitted and the big end nuts have been tightened sufficiently to pull the flywheel cheeks against the shoulders of the big end crank pin then the crank will be in the correct position...

End float of the con rod between the flywheels should then be about .010"-.012"..

Correct positioning of the crank within the cases relates more to the correct meshing of the oil pump drive gears than it does the piston, due to the large piston to rod clearance previously mentioned...

It is important therefore that the spacer length is correct and the other conditions mentioned are attended to...

If all these are correct you shouldn't have a problem with the piston position unless the small end bush is too long...Ian

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Re: piston alignment

Hi Ian, I have assembled (with cush drive) and disassembled this motor so many times in the last few days... and had the guys who old-Harley guys take their bikes to look at it to. My only guess was that my previously wonky crank wore out where the bearing rides on the drive side. This is a bike from Burma, and absolutely everything has been put together horribly.

I understand what you are saying about the piston position and the space between the bush and piston--The small end bush and the piston are not rubbing--I put blue ink on inside of the piston where it would be rubbing, fully assembled the motor (except the head), turned it over a bunch of time and took it all apart again. On both sides, the ink was undisturbed and didn't transfer to the bushing, so I guess I will have the machine shop make a spacer at 1.005 and try not to worry about it, though it does seem to me that the rod should be perfectly centered....? Maybe not...?

If this repair doesn't work, I will be parting this bike out. I have a ridiculous number of hours of work into it, and not even one trouble-free hour. Wish me luck!

email (option): nicktog at gmail

Re: piston alignment

And, as always, thanks for your thorough and patient responses!

email (option): nicktog at gmail

Re: piston alignment

If the rod is straight and the small end bush is not touching the pin bosses inside the piston then the crank/rod assembly is not exerting any side force on the piston and the crank shaft position doesn't come into play, even if the crank was a little worn where the oil flinger plate fits...

The only result of that would be a mismatch in the mesh of the oil pump drive gears...

If the piston is pressing against the bore then something you think is right, such as the rod being straight....isn't...

Lining the con rod against the crank case joint visually can only be regarded as a rough check of crank position and should not be regarded as the definitive way of checking this...

Note also that although the crank case joint is in the centre of the crankcase the 'cut outs' machined into the cases adjacent to the joint are frequently not machined on the joint centre line...This can be misleading when you are looking at rod position...

Sight off the crank case joint, not off the sides of the cut outs...

Good Luck... ....Ian

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Re: piston alignment

Aloha Nick,

Perhaps you should upload some pictures just to make sure that its all the correct parts? Giving the bikes origin it may have been put together with what was at hand at the time. E.g: My bike from Indonesia had a piston in it that no "BSA-person" could identify.
Or maybe you have checked this already.

Like Ian writes if the rod i straight the extensive play between piston and rod should prevent the rod from pushing the piston to either side, but if the rod is not centered in the cases then that needs to be addressed anyway.

Is the piston new and has the cylinder been rebored? If not they both on these bikes as I understand it tend to "get worn out of round", and maybe you then turned the piston 180 degrees during some assembly?

I am just guessing here, to give some pointers. And Ian is without any doubt the more experienced and knowledgeable person here.

To part the bike out sound drastic to me, I'm sure this can be fixed.

But most importantly now I think is to know the status of the cylinder and the piston, new or used?


Re: piston alignment

p.s. I cannot recall having ever seen as dissembled motor with the cylinder and piston being evenly worn all the way round. What is the extent of the "scraping"? Are you the worrying kind?


Re: piston alignment

Just a thought. I have heard of a similar problem where someone had fitted an incorrect drive side main bearing. Ron

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Re: piston alignment

There's usually a good 2-3mm between con-rod and the piston wrist-pin bosses - as these engines have the con-rod centered by the rod - flywheel.
So there should be no reason for a straight rod to cause this problem - as someone said, does the rod line up with crank-case joint ?

Normally an engine with show wear on front/rear of the piston - the thrust-faces.
I suppose a S-V could show a wear mark on the hot - valve side of the cylinder, certainly the cylinder wears heavily in this area ?

I wonder if a miss matched pair of crank-cases have been used - causing the cylinder to tilt off vertical?

I've had it happen on miss-matched triumph twin crank-cases - in my case i removed the location dowels - clamped the cylinder to the c/cases (to alignt the cases to the cylinder) and found where the sump-plate fits, to be out by over 1 mm !!!!!!
God knows what the main bearing alignment was like - but as i used super-blend bearings, i ran this engine till the crank broke (again!)

Re: piston alignment

Mismatched crank cases are a possible cause..The two halves are numbered on one of the front bosses where the engine bolts pass through them...A part number on one side and two (matching hopefully) crank case half numbers...

Bearing misalignment isn't an issue with BSA cases if the 'register' machined into the two crankcase joint faces are a good fit...These are machined in the same operation as the main bearing housings so if the registers fit together well on 'odd' cases the main bearings will line up...

It is possible to get slight misalignment of the barrel faces and magneto platform faces on 'odd' pairs due to production tolerances...Generally the difference is small if there is one...

I remove everything from the crankcases, bolt them together and skim both the barrel and mag platforms on a milling machine using a fly cutter to achieve flat faces, removing the minimum material required...Ian

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Re: piston alignment

Hi Simon,

Normally I'm the opposite of the worrying kind! Some might say maddeningly so...

But, I'm an American, in California, trying to salvage a Burmese bike (most likely a POS)... and I've never ridden another M20... nor have I ever even SEEN another M20 running (besides youtube).

Hearing you say M20 pistons normally wear unevenly makes me feel a bit better.

And you're right we need pictures. I took many pictures:

Shot of the small end bush... and, am I reading it wrong, or doesn't it look like the conrod is closer to the drive side?

 photo thumb_IMG_3685_1024.jpg

both sides of the bush after 'blueing' the inside of the piston (and assembling and disassembling), some transfer while getting the wrist pin in, but clearly not touching while running...

 photo thumb_IMG_3683_1024.jpg

 photo thumb_IMG_3682_1024.jpg
new, spacer--nice work, Tony!

 photo thumb_IMG_3689_1024.jpg

drive side case...

 photo thumb_IMG_3686_1024.jpg

drive side of crank:

 photo thumb_IMG_3696_1024.jpg

flinger, i had another shot mic'ing the thickness, but don't have it now. the flinger doesn't look like it is damaged to me.

 photo thumb_IMG_3692_1024.jpg

timing side case

 photo thumb_IMG_3697_1024.jpg

oil pump spindle and driving gear position. dark... spins nicely

 photo thumb_IMG_3713_1024.jpg

valve timing (upside down)--the 'line' is worn off from mine, but i believe i have matched the maintenance manual correctly--dot to dot (exhaust), the line goes on the fourth tooth from the dot.

 photo thumb_IMG_3716_1024.jpg

other related posts:

Thank you!

email (option): nicktog at gmail

Re: piston alignment

Hi Ron, NigP, and Ian.

Took me so long to get those pictures up that I just saw your posts. Thanks for the replies. Wish I took better pics of my bearings now too! Oh, and yes, looking at the differences between the two halves, I'd say there is 0% chance of them being a matched pair. I measured though, and the conrod does appear to be running vertical...

I have the motor together again now... I am hopeful it will start tomorrow. Please look at the pics above, and make bets on my chances of success!!!

email (option): nicktog at gmail

Re: piston alignment

Good photos!
Someone more knowledgeable than me can now confirm that it is all BSA parts in there. But looking at your previous threads on setting up the crank and so on I think that whistle would have been blown already.

There is no doubt that the crank is not centered (not that I ever doubted you)! But that does not make for an explanation of uneven cylinder wear as has been discussed already. Now the cylinder wear in the picture (of an old thread) doesn't look half-bad to me.
BUT listen to the other folks on this, they now more about these bikes than I do. I am not a beginner to old bikes, but I am doing my first M21 build. And a garage-buddy of mine once joked about how the inscription on my tombstone would be my last words "Naah, it'll probably work ok for now..."

One thing to keep in mind I think is that this is not a high-output engine, it is merely 15 hp to thump you down the road.

Re: piston alignment

It's morning here and I'm about to get back to work, but I've been thinking about this for the last twelve hours pretty much.

I think Ian got it--mismatched cases may be the cause of ALL these problems... But I think that's just the tip of the iceberg.

The original problem was that I knew the small end bushing was sloppy. In replacing that, I discovered (what I believe was) unacceptably uneven piston and cylinder wear after very few miles. I may have been too hasty there, but we'll see. That's when I wrote the the 'bent conrod...?' post. Thought it was that the conrod was screwed up--then the machine shop found the flywheels weren't set up within tolerance. Aha, I thought, the flywheels messed up the conrod!

It only occurred to me yesterday that the cases were from different motors because they had been polished pretty well, but I didn't realize that was a problem until Ian said it.

I measured the small bush distance from the cylinder wall a few times in different positions, and it appears to be running vertical, but that doesn't mean it's running true in the other directions. And for some reason the top of the piston still sits closer to the drive side, no matter what position the rings are in, and no matter how I assemble it. It's not much, but it's not right.

SO I think I just have a number of really mismatched old parts, and maybe my best efforts won't get them to go together.

I have a ton of new parts in it--full top end, bottom end work, full primary and clutch, parts to build a wartime gearbox...

So I'm going to try to run it, see if it's finally happy with all the work and money i've thrown into it... if not, lots of parts to help other bikes live on, coming soon!

email (option): nicktog at gmail

Re: piston alignment

Nick, please don't give up on it. As you mentioned, it's almost unique in the area where you're living.

There are actually quite a few WM20s in the US (spread over a large area of course) and there are people with spare engines etc.

In the UK, a £350 engine would give you all the parts that you need to build a decent one and it will enable you to use the parts that you've already reconditioned. In your position and if funds permitted, I think that I'd be advertising for one

The concept of 'throwing good money after bad' really doesn't apply to old English motorcycles ! You're building up a real relationship with the thing which will make it all the more enjoyable when it's sorted :-)

Re: piston alignment

Hi Rik,
Thanks for the words of encouragement. You're right, I have looked for local sources, but I haven't advertised. I should do that.
I had to tend to some work out of town. Bike is mostly back together, compression feels very strong to me. Will hopefully be back Wednesday. I hope to post a video...

email (option): nicktog at gmail


Hello to everyone
I would say Elly everyone Happy Easter holiday and I hope that all the people in all the world be happy

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Just finally getting back to this after more than a year...(!)

So I plan to follow Ian's advice--Have the barrel and mag/dyno surfaces machined on my mis-matched cases.

My question is--If those external faces are off, is there a chance the internals are similarly off, and is there a way to check?

I'm thinking of where the crank bearings run in the cases--should I check them for alignment? Is there a way?


Re: Re:

Just to add some more uncertancy are you 100% sure that the barrel bore is true ?
Most rebore shops index car cylinders off the head face and not off crank axis.
Some shops not as familiar with motorcycles will do the same and bore the barrel upside down using the head face as the reference point and not the crankcaes mouth as that requires making up special mounting plates.
It is very common to find barrels bored at an angle or off center both front back & left right.

To check your rod you need to put a pair of test rods through the big & little ends at least 6 " long then measure the ends of the rods in all planes.
I used to get a mate at a metrology lab to measure mine.
The first one he did he asked me what were the allowable out of alignment figures.
I looked at him stunned and said ( with ignorant confidence ) none.
HE replied OK you mean < .005".
Apparently that is around how far std car rods are off axis.
They used to measure rods for production race cars and nearly all got rejected because the big & little ends were not true to each other or to the rod axis within 0,0001"

I an yet to see a Pommie bike with a rod running dead true to the center of the bore, most are steel rule center but very few end up being vernier center let alone micrometer center.

Double check that the piston can not move left right at the top of the bore.
To cause the piston to rub on the side of the barrel the rod would need to be in the order of 0.1" off center because that is the amount of play in the rod/ journal + pin/piston side play.
I would put money on a twisted rod or small end machines off alignment every day of the week over an off center or sloping bore.
And then a sloping bore would be more previlant than a mal aligned crank.
There is a lot of tollerance using a ball & roller crank that you do not have using a bushed crank and they can take a lot of mal alignment.

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Re: piston alignment

Hi, after seeing your photos, re read Ian Wright's post about small end bush clearance. It looks a bit long.

Re: piston alignment

Thanks guys,

After a year plus, I am getting hazy on the details... I know that both the machinist and I felt good about the conrod, but to tell the truth, I can't remember how we checked.

Maybe it is the bore... the guy who did it mainly works on snowmobile engines.

I am hoping to have a workshop soon, where I can tear it apart again, post a bunch of pics here, and maybe finally get it right.

Once it's in pieces, I'll get them up...


Re: piston alignment

Two things: your conrod is not on the centre line of the bore ie the middle of the conrod should be were the crankcase halves meet. Also the small end bush should not stick out of the end of the conrod like yours does. these need to be addressed first.

Re: piston alignment

Keith H
Two things: your conrod is not on the centre line of the bore ie the middle of the conrod should be were the crankcase halves meet. Also the small end bush should not stick out of the end of the conrod like yours does. these need to be addressed first.

Amazing what you can not see, when it is staring you in the face.
Of course the con rod bush should be flush with the side of the rod eye and without that running clearence, the piston can not align itself with the bore so naturally if the con rod is off center and the piston can not float side to side it too will be off center

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