I am not with the bike, so I can't take pics now, but...

I took the barrel off my machine yesterday and was surprised to see some (slight but annoying) vertical scoring (less than 50 miles on it).

While looking at it, I'm wondering if I have a bent or twisted conrod... or maybe misaligned flywheels... or what?

My question: Can I use the line where the case halves meet to look at the conrod position? Should that line (where the cases meet) be exactly in the middle of the conrod? Or to put it another way--should the conrod be centered on the case halves? Mine is at least a couple mm toward the valve side of the motor... if this is confusing, I can take a pic later. The distance that it is off center--if it is--does not change much (or at all) as I turn the motor over.

Need pictures to understand my question? I can have some in a couple hours. Thanks.

Nick

email (option): nicktog at gmail com

Visually aligning the conrod with the crankcase joint is only a rough guide to crank position..

The crankshaft is positioned within the crankcases by tightening FULLY the engine shock absorber assembly nut, which pulls the crank over to the drive side of the motor and locks it against the outer drive side bearing...

The drive side of the crankshaft (the main shaft) has the oil flinger plate, the roller bearing inner and the main bearing spacer mounted on it...

These determine where the crankshaft is positioned within the cases once it is locked against the fixed outer bearing (which is fixed in place by a circlip on one side, and the crankcase wall on the other)...

The spacer should measure 1.000"-1.005" long..The bearing is a standard width...and the oil flinger plate shouldn't be left out unless it is replaced by a washer/spacer of the same thickness...It should also be flat and undamaged if fitted...

If these aspects are all correct the crank will be in the correct place...

The best way to check for a bent conrod is to remove the piston, then replace the gudgeon pin only..Next place two suitable parallel bars across the mouth of the crankcases either side of the conrod...Then turn the engine by hand until the pin is resting on the bars...If the rod is bent you will have contact on only one bar....

Is the scuffing of the bore/piston on one side only?...or is it on the thrust face? (the front or back of the piston)....Ian

email (option): ian@wright52.plus.com

Thanks Ian!

I will post some pictures too.

The scuffing is on the 'valves side' of the barrel. Which makes me think:

1. I need to take a look at the engine shock absorber nut... sounds like it might be a bit loose (hopefully).

2. I will try the test you recommend with the bars.

Thank you!

nt

email (option): nicktog at gmail com

The long and flexible M20 conrod (the same part in the M21) is particularly prone to twisting, or more commonly bending, along the axis of the crankshaft...In other words, if side loads are applied to it...

It can be the result of rough handling when the crank is out of the engine...when the barrel is off and the engine is out of the frame, as the result of a piston seizure or, most commonly probably, if the gudgeon pin is knocked in or out without supporting the conrod and particularly so if the piston is not pre heated before removal or replacement of the pin...

If the engine has an unknown history and has not been run for a long time it's always worth doing at least a simple check as described in my last post during the engine rebuild...

If the crank is apart for fitment of a new big end a more detailed check can be carried out...

As you have marking on the side of the piston the rod should definitely be checked along with the shock absorber...With the latter do the nut all the way up until it hits the sleeve that carries the engine sprocket and then knock it up tight, either with a soft punch and hammer or a suitable C spanner and hammer...

You should also consider the possibility that the marks may have been caused by some contamination left in the engine during assembly (if the marks are only light)....

It would certainly be useful to see a picture of the piston to better assess the nature and likely cause of the damage...Ian

email (option): ian@wright52.plus.com

Some photos:

Looking from the front: valves to the left, looks like it could go a bit to the right. There doesn't seem to be any sideways movement--the small end position doesn't move left or right.



Maybe you can make out the scuffing, it's not terrible, but it has very few miles, the piston shows similar :

email (option): nicktog at gmail com

The bore doesn't look too bad...I'd be more interested to see the piston..

May I ask you what prompted you to strip the engine if it was recently rebuilt?...Ian

email (option): ian@wright52.plus.com

Hi Ian, I will go do that test now. I don't have the piston right now, it's at a machine shop, I'm having a new gudgeon pin bushing made. The damage to the piston is as slight as on the cylinder wall--not much more than hashmarks.

Thanks for your patience and fast replies, Ian!

nt

email (option): nicktog at gmail com

Yes, why I am taking apart a motor with less than 50 miles on the top end:

Until recently I lived in China (while I am really from the states). Each year, I only had a week or two to work on it, so I would do what I could, but always reassemble before leaving again--I was afraid leaving it in boxes would mean I might forget. When I put it together last time, I was unhappy with the gudgeon pin fit in the bushing, just a bit too sloppy. So I reassembled it with the loose bushing, and now here I am a year later, taking it apart again to fix that.

nt

email (option): nicktog at gmail com

Hi again, okay I just did that test--looks absolutely straight to me... I'm thinking it must be the shock absorber nut... though we'll see. This bike came from Myanmar--looked wonderful on the outside, but every time I look inside it, I am deeply disappointed. Haven't split the cases yet.

Thanks again,
nt

email (option): nicktog at gmail com

Looking at the pix, it appears as if wear is at 90deg to the thrust face - which is not normal, unless the piston is a cheapy and wrongly machined - or as you say, the piston is running off center.

Ian's comment about checking if the rod is bent into a C shape needs to be done.
Have a look at wear/witness marks on the piston - hey may tell us something.

It is possible that the rod is bent into an S in which case the only method i know is to split the crank and measure the rod on a flat surface and check that there is the same distance from surface to big/small end.

usually there is enough clearance in the piston to allow the crank to run off center to the crank center-line - Have a look inside the piston to see if there are any witness marks on the piston gudgeon pin bosses - there should be no/minimal rubbing.

There are a few engines out there where the cod-rod is not central in the piston (my Douglas for example + the Velocette valiant) - and i am not aware of any side-load problems - not a particularly elegant bit of engineering design, but they work.

My sympathy for purchasing something 'restored' in the East - i've just suffered through my 3HW rebuild - which came via India !

Hi guys, thanks for the replies.

I measured as Ian suggested, and everything looked right, but it still didn't line up. So i split the cases:

It looks like the rod shifted out of place, or knowing the guys who built it, it just was never in the right place. I'm sick of cleaning up this bike from them, but at least with the motor, I can give it the full attention that they didn't.

So now that I'm in there, I am hoping to replace all the bearings.

Ian, I sent you an email asking about your sets, did you get that?

Or has anyone sourced them in the states?

Thanks,
nt

email (option): nicktog at gmail com

Hi Nick

Please can you post a picture of the piston.

Regards

Pat

email (option): sacombsashtrees@hotmail.com

Hi Pat et al,

Here is the piston:



I am going to start a new thread about my flywheel woes.

nt

email (option): nicktog at gmail com