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Conrod play

Hi
Thought I would once again ask for you all to share your wisdom.

Does anybody know how much play, side to side, with the Conrod is ok?
I have my cylinders head off the engine and taken the piston off.

The engine itself turns like silk. There is a little movement left to right on the Conrod and very little amounts up and down.
I don’t want to disturb anything unnecessarily, so thought I’d ask if this was normal, before I begin to strip any further.

Thanks
Steve

email (option): Sglover1104@gmail.com

Re: Conrod play

I have to say that it sounds like a search-term which I wouldn't use on every website ! :thinking_face:

Re: Conrod play

The side clearance on the rod should be .010" to .015". The rod will also allow a little bit of tilt from side to side at the small end. Now as for up and down movement, you should not be able to feel any as the clearance is about .0005" on a new bearing. More than .001" will cause a knock when running.

Re: Conrod play

You must be very careful when checking the big end not to confuse 'tilt' of the conrod or 'end float' with vertical movement...

That said the figures quoted by Bruce for end float are correct...A degree of end float over the quoted figures will run OK...The main thing is that the end float is .010" minimum...Due to variations in crank pin dimensions and as the flywheel tapers become worn this figure can get smaller, as the flywheels are pulled closer together when the big end nuts are tightened...If there is a problem there the sides of the big end liner/conrod will need to be surface ground to restore clearance... This is generally only an issue when a new big end is being fitted and isn't really a factor with an engine that has run for a few miles...

'Tilt' will be present to a degree even in a brand new big end assembly and it will increase as the big end bearing wears...It can become very evident in a big end that has covered a lot of miles...However, it's not anything to worry about as long as there is NO vertical play in the bearing....

'Vertical play' should be zero, or rather, undetectable when testing by hand...If you can feel vertical play the bearing is beginning to fail and should really be replaced...In practice you may get some more miles from it if it's not too bad but it will get progressively worse..If the engine already has a 'knock' it should be replaced immediately....Once you have experienced a full big end failure you will never confuse it with anything else...It's like someone banging on the crank case with a hammer....:laughing: ....Ian

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

Re: Conrod play

Thank you all again. I’ll get it sat on my bench and take a look at it more closely.
I have a complete set of bearings for the engine, I was hoping to get away with it. Better play safe I think and change the bearings.

email (option): Sglover1104@gmail.com

Re: Conrod play

A short route to finding out if there is any play:
Hold the crank assembly by the conrod and hit the top of the small end with the other hand /fist.
If a 'clang' is heard, then replace the bigend.
Ehhh, I suppose you grasp the fact that this only works with the crank assembly out of the crankcase.
A bit hard to perform with the engine still mounted in the bike....
Ofcourse measuring with a dail is the proper way to do it.


Oh bytheway may I suggest to include a type/make/brand in the topic name ?
Just 'Foot peg' or 'conrod play' is a bit dimm.
Yes I am aware that this is the BSA WM20 forum, AKA the wandering of topic one.....:grinning:

email (option): vincent@vincentvanginneke.com

Re: Conrod play

It's the 'WD Motorcycle Forum' these days...which does nevertheless wander occasionally...:relaxed: ...Ian

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

Re: Conrod play

I would say, at least in theory, when the conrod has tilt it must also have vertical play, otherwise it could not tilt in the first place....

Re: Conrod play

Just a question about big ends.

My spare crank shaft has no vertical play.
But the small end can be moved from left to right - about 3/4 of an inch (or some 20 mm).
Is it OK..?
Any thoughts ?

Re: Conrod play

I got my M20 big end repaired by a retired BSA mechanic some years ago. The big end only had slight play and he advised me the crank pin and race in conrod were fine and fitted oversize rollers to remove the play. Still going strong after all those years. These days that might not be an option.

email (option): tknalder@iinet.net.au

Re: Conrod play

['I would say, at least in theory, when the conrod has tilt it must also have vertical play, otherwise it could not tilt in the first place....']

A new big end has some vertical play because it has a running clearance...It will also display some 'tilt' at the small end because the small degree of movement is amplified by the length of the rod...However, the vertical play is so small that whilst it can be measured it cannot be felt by manipulating the rod by hand...The acceptable running clearance covers a range of size and as wear takes the clearance towards the upper limit the 'tilt' is further amplified...

The dimensional variations we are talking about here are only 4-5 tenths of a thou...Ian

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

Re: Conrod play

[' My spare crank shaft has no vertical play.
But the small end can be moved from left to right - about 3/4 of an inch (or some 20 mm).
Is it OK..? ...']

I assume you are talking about 'tilt' of the conrod from side to side?...That seems to be an enormous amount of movement, are you sure your dimensions are correct?...Ian

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

Re: Conrod play

Thanks Ian, I understand.....But the vertical play on a big end is not easy to determine correctly as you have to find the exact middle of the tilt before moving the conrod up and down....

The tilt of the conrod and the vertical play of the bigend have a connection of course:..the more tilt the more vertical play...and a rod with hardly no tilt will have hardly no vertical play.....

When 'tilting' the conrod your actually using the amplifying length of the rod to say something about the vertical play... Or am I taking the inner corner now ??





Re: Conrod play

I'd say that is the case...You could use tilt to determine the degree of wear...Of course this comes down a lot to experience and the number of different big ends that have been examined...You have to have a knowledge of both good and bad big ends to make a judgement...Feeling for and accurately assessing vertical play is also very much about technique...

To be honest, I tend to do it without thinking about it much as I've worked these engines types for over 40 years...

One other thing it's important to remember is that feeling for vertical movement (or tilt) isn't a definitive method of determining the condition of the bearing...It's just an indicator of its likely condition...
For example, as the BSA big end is a double row of rollers it is possible that one side is worn or the bearing surface has deteriorated but the other side is OK...

The 'good side' prevents vertical movement which can mask the failure of the other side...In the end stripping the big end down for examination and measurement is the only guaranteed method of determining wear/condition...Ian

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

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