Square 4 Conrod Side Play

[allan.walker]

On stripping down my Square Four engine, it seems to me that there is quite a lot of side play on the conrods. I don't mean 'wag' at the little end, but side-to-side movement between the crank cheeks.
I cannot find any figures here or online for allowable side play...can anyone please help?

Allan.

[Paul_Linden]

There should be a small amount to allow for alignment, expansion, etc.

Have the big end journals just been reground? Sometimes the engineers will cut too much off the side.

[allan.walker]

Hi Paul. They have been reground prior to my buying the bike and it has since covered 1000 miles or thereabouts.
I realise that there needs to be some play, but mine looks to me to be excessive and whilst I'm 'in there,' I thought it best to try and find out what the recommended clearance should be.

[john.whiting]

You d have to check the crankshaft drawings ......its quite possible the journals have been ground overwidth.......have a careful check of the radii and finish on the sides .......with shell bearing rods,the radii must be clear of the inserts....wheras with metalled in rods , thrust faces are normally included in the bearings......The simple rule is with a 70 year old motorbike ,bodgers may have done anything........Anyhoo,back to the rods...about .012-.015 would seem to be a reasonable side clearance .....Ive seen engines with much more than this ......but according to legend,too much side clearance may cause excessive oil consumption.......The most I ve ever seen is with a breed of race engine where a 250 Falcon crank was installed in a Holden red motor.(or was it tuther way round?).......in this case side to side was of the order of 1/4",and the motors did not seem to suffer.

[allan.walker]

Thanks John,
That's really useful guidance. The shells are actually slightly narrower than the conrod eyes, so there's no chance of 'binding' on the radius.

Just to clarify, when you say .012" to .015" side clearance, do you mean at each side of the rod, or is this the total clearance if the rod is moved to one side and the gap at the other side measured?

Cheers,
Allan.

[john.whiting]

Its far more important that the fillet radius hasnt been damaged ,or reduced......a bad job may cause the crank to break....Forged steel cranks are far more prone to fatigue breakage than the nodular cast iron used in cars....in fact many car reconditioning shops dont worry about fillet radius ,because cranks dont break,which is bad news when the get an Ariel crank...........With a big wheel ,reforming the edge to a certain radius loses up to a 1/4" of a $500 wheel .....so you can see why so many cranks are ground badly.

[allan.walker]

I see. I'll check the fillets compared with some other used Square 4 crankshafts I have.

Back to the side play, does the .012" to .015" refer to the total clearance or should there be that amount at each side of the rod? (i.e. a total movement of .024" to .030")

[D.Ryan]

Hi All
My MK1 has .022” total clearance. The cranks were reground by a previous owner to .030” under and the radii of the journals and mains on the timing side were very good. The journal size was spot on for new shells. As you say the shells themselves are much narrower than the rods are and I figured they were “well in spec” so I buttoned the motor up on them. Prior to the strip down I done over 5000 mile I on the motor before a main timing side bearing broke up. Fortunately no damage and I was able to make new bushes and have them re-poured and then I line bored them. The worst bit was that the top end had to be disassembled and of course the head gaskets had to come off and be replaced.
Dennis

[allan.walker]

Hello Dennis,
It's useful to know that .022" is fine. I'm making sure that everything is as it should be before putting it all back together.