The symptoms of a 3 bearing crank failure?

[Tony Press]

Our Club Secretary had a comprehensive engine failure with a Club three bearing crank when returning from a Club run earlier this year. The faiilure is through the rear web.

He has now has a new engine with a two bearing crank.

Isnt that a 2 bearing crank in a 3 bearing case with the middle bearing hacked out?

The original three bearing crankcase was a little far gone to 'hack out the middle bearing' so he replaced it with a two bearing crankcase and new crank.

 

He had a two bearing crank before it broke

You are correct of course- I was confusing two failures. 

The one under discussion was a two bearing ADI process crank- he wanted to build a three bearing engine with new crankshaft but the only three bearing crank available was another ADI shaft and he wouldn't use this.
He now has a replacement two bearing crankcase with a Phoenix crank.

[fatcatvera]

So with the kind help of Arthur from the MA7C I now have what I hope is a usable replacement 3 bearing crank - for a start it rings nicely compared to another that didn't (thanks also to Robert Leigh for the offer of cranks/short engines). I've also crack detected this crank it along it's whole length with dye penetrant - see results below. No obvious linear cracks around the centre main webs where it seems the 3 bearing cranks typically fail.

Big end journals are measuring around 1.2935/1.2950" (with a vernier - which is the best I have for the moment) - 19thou / 17.5thou u/s.

Centre main measures 1.7085" (again with a vernier) - 41.5thou u/s.

Plan is to give the bearing areas a bit of a polish and see how they look, measure them more accurately with a vernier for size & ovality, then decide whether to get the crank ground. I have an existing set of 30thou u/s big ends that suited the failed crank that are in very good condition. Existing c/m is 40thou as well and in usable condition so could re-use that too if plastigauge tells me the clearance is ok.

I'd prefer to save the crank at it's current bearing sizes if I can to extend it's life as much as possible.

Then I also have the challenge of trying to check the c/m is in line with the end mains and do something about it if I need to...


                       

[Chris KC]

Then I also have the challenge of trying to check the c/m is in line with the end mains and do something about it if I need to...

Ray, on this last point I will e-mail you

[fatcatvera]

I now have a nicely machined replacement crank. Crank case measures within 1thou for c/m alignment (with block&head bolted on). So the engine is now going back together.

I have a concern over using an oil pump cover plate gasket though. There was one fitted originally, but it was torn by the rotation of the pump and it's vanes. This leads to excessive clearance and high likelyhood of gasket material being pumped through the engine - which is far from ideal and will also mean high internal pump leakage and low pressure/flow output. I consider it probably better to not fit a gasket here as the surface and are good and very flat and will seal well. What's the collected view on leaving the gasket out?

Ray

["Slack Alice" Simon]

I don't think there is such a gasket.

There is a gasket between the pump body and the crankcase, and in the parts list I use this gasket is drawn as if it goes between base and body, but it does not.

Simon

[Ruairidh Dunford]

Simon is correct - the oil pump cover does not have a gasket, although I have dismantled a number of engines where one has been fitted as you describe.

Ideally you also want to face the cover plate so that it is perfectly flat, they often have a circular wear mark...

[Tony Press]

Simon is correct - the oil pump cover does not have a gasket, although I have dismantled a number of engines where one has been fitted as you describe.

Ideally you also want to face the cover plate so that it is perfectly flat, they often have a circular wear mark...

The Spare Parts Lists all misleadingly show the Gasket as though is to be fitted between the Cover and the Pump Body instead of between the Pump Body and the Crankcase !

[Chris KC]

Yes, what they said!

[fatcatvera]

Thanks. Didn't seem correct that a gasket had been fitted there previously....

[Speedex750]

Hi Mr f,

I would suggest before putting the crank into the case you assemble the centre main onto it and make sure it turns OK. Although it seems blindingly obvious now, I've recently discovered the hard way, that individual bearing housings are not directly interchangeable. If your pair come from a previously running engine you should be OK but if not better to know before the engine's too far built? You can bolt them together roughly with two 3/8BSF x 4" set screws, the shells centralise the two halves together. There would be no harm in also trial assembling the big ends to check they're free as well, which leads onto doing a rod straightness check with a king pin through two adjacent little ends 1 to 2 and 3 to 4.

Dave