BMC 1.5 Has my crank been re-ground?

[SailorTom]

I am trying to find the original diameter of the bearings on my crankshaft.

I am re-building a Thornycroft 90 engine, based on a 1.5 BMC.  The pistons are stamped with oversize marks and the bores measure at 0.020" oversize, so I know that the engine has been reconditioned in the past.  But were the crankshaft bearings ground at the same time?  (I have the specification for the standard bore, but not the crankshaft bearings)

I measure the journal diameters for the main and big end bearings all at exactly 2" / 50.8mm.   The round number of 2 inches makes me think that is probably the standard size, but I am not sure.  I have the part numbers from the old shells, but I can't find any cross-reference for them.

Does anyone know the standard diameters for these bearings?

Thanks for any help
Tom

 

[RLWP]

That's annoying, the crankshaft isn't in the manual! https://the-norfolk-broads.co.uk/downloads/bmc1500L-diesel-workshop-manual.pdf

I can measure a couple tomorrow if that helps

Richard

[SailorTom]

Richard,

That is the same as the manual that I have (on paper).  I expected the spec to be in there too, but couldn't find it.

I would appreciate it if you could let me know what your measurements are.

Tom

[RLWP]

Will do - I can probably do better than that. I have a customer's crank that needs to go off to the machine shop, I'll ask them when I am there

I'm expecting the answer to be 2"

This engine also has +0.020" pistons, although the bores look horrible

Richard

[Troyboy]

Ive got a BMC parts book for the 1.5 diesel and the part number for standard con rod bearings is 8G 2324, .010'' undersize is 8G 232410 and .20'' undersize 8G 232420.

The standard main bearings part number is 8G 2323 or 8G 2335 . Like the con rods if undersize the 10 or 20 is added .  Do any of those numbers match your old bearings ? 

Trevor

[X Alan W]

Isn't the size stamped on the back of the BE/MB shells it used to be in +10+20 etc meaning the crank journals are 10/20 thou undersize

[bizzard]

3 hours ago, X Alan W said:

Isn't the size stamped on the back of the BE/MB shells it used to be in +10+20 etc meaning the crank journals are 10/20 thou undersize

This used to be the case. No undersize markings meant (standard) size.

[X Alan W]

Looking on the ASAP site at BMC1500BE shells a close look reveals shells for 10thou under size crank journals have +10 & pt # stamped on the back They didn't show a "piccie" of a set of Std size shells to see what markings there were

[RLWP]

Bother. I took a micrometer with me, then forgot to measure the journals

Richard

[MtB]

3 hours ago, RLWP said:

Bother. I took a micrometer with me, then forgot to measure the journals

Richard

 

You mean you can't tell by looking at them???!!! How disappointing....

I once worked with a toolmaker who could tell you if a piece of steel had been heat treated and hardened just by picking it up. 

[RLWP]

Just now, Mike the Boilerman said:

 

You mean you can't tell by looking at them???!!! How disappointing....

I once worked with a toolmaker who could tell you if a piece of steel had been heat treated and hardened just by picking it up. 

I can do that, I could guess the approximate size of a journal too. But if you want a proper answer, I need to measure it

Richard

[SailorTom]

For those interested...

I took a punt and bought all the bearing shells at standard size.  They seems to fit snugly, so it looks like my crankshaft was never reground.

So now I know what isn't in the workshop manual.  Standard size of the bearing journals is 2.00" for big ends and mains.

 

[Mrshifter]

Can't find this info in the manual. Can anyone guide me on the correct bearing clearance between crankshaft and bearings please.  Mine has been reground in the past but I would like to check the clearance with my new bearings fitted with plastigauge or measuring with inside/ outside micrometers.

 

BMC 1500D

 

Thanks

Edited July 7 by Mrshifter

[Tony Brooks]

16 minutes ago, Mrshifter said:

Can't find this info in the manual. Can anyone guide me on the correct bearing clearance between crankshaft and bearings please.  Mine has been reground in the past but I would like to check the clearance with my new bearings fitted with plastigauge or measuring with inside/ outside micrometers.

 

BMC 1500D

 

Thanks

 

That is because you do not have the full manual - look here http://www.boatservicehaarlem.nl/bmc_tempest.pdf

it is in the Technical Data section.

 

Was the oversize not marked on the back of the old shells?

Edited July 7 by Tony Brooks

[Mrshifter]

Cheers

 

The oversize is marked as 20 though, however I don't know when this was done so I want to make sure all is good before I fit the new shells.  To be fair the shells I took out looked OK so am expecting it to be good.

 

Thanks again.