Lotus Elan

TC jack shaft bearings

PostPost by: rdssdi » Mon Sep 14, 2009 12:59 pm

Has anyone had difficulty with the jack shaft (i.e. old push rod cam) not fitting into new bearings?

I have been told that the bearings need scrapping to provide clearance. It is my impression that he "cam" bearings have a thin layer of babbit and that would easily be removed by scrapping? Should the "cam" journals be ground to size and polished instead?


Bob
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PostPost by: GrUmPyBoDgEr » Mon Sep 14, 2009 3:22 pm

Well that's a new one to me & I've rebuild a couple (but it was a long tome ago)
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Editor: On Sunday morning, February 8th 2015, Derek "John" Pelly AKA GrumpyBodger passed away genuinely peacefully at Weston Hospicecare, Weston Super Mare. He will be missed.
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PostPost by: handi_andi » Mon Sep 14, 2009 9:09 pm

As me in about a weeks time after I have fitted mine :lol:

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PostPost by: rgh0 » Mon Sep 14, 2009 10:31 pm

Generally a jackshaft fit is issue is one of two things

1. The bearing has been slightly burred on the end when pushed into place and requires a slight bevel to clean up the end
2. The line bore of the bearing tunnel is not straight or the jack shaft is not straight.

If your rebuilding a engine using a known good block and jackshaft then its probably a slight burr on the bearing end.

I would not turn down a jackshaft bearing to undersize or try to scrape the inner surface of the bearings (which is very thin and not designed to be scraped) until I had understood what the problem was by very careful measurement as straight standard components should fit.

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PostPost by: rdssdi » Mon Sep 14, 2009 11:14 pm

I did place a slight bevel at the end of the bearing and it did not remedy the problem. I did an internet search and found reference to cam bearing scrapping on several sights. The engine machine shop that did the work on the block said the cam bearings often need this. He said the outer layer, babbit material I presume, is over 10 thousands thick. From crude measurements with a caliper it appears I need around 3 thousands removed. He also said the bearings would have to be removed to scrape them and of course refit.

I am using ACL 3C711S bearings.

Also mentioned as a method to scrape the cam bearings is to take an old cam and cut a grove across the journals and turn it into the engine. The groove will allow he surplus material to be scrapped away.

Its always something.

Bob
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PostPost by: Lincoln62 » Wed Sep 16, 2009 2:09 am

Scraping white metal bearings is something from the 20's and 30's. Modern bearings shouldn't need scraping and aren't designed for it.

Maybe the bearings are undersize? Maybe they were marked incorrectly by the manufacturer?

I'd be doing some careful measuring.

Peter
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PostPost by: rgh0 » Wed Sep 16, 2009 8:07 am

In the end if your confident the shaft and tunnel is straight and the cam journals are standard diameter and the bearing fit in the tunnel is correct and the ID of the fitted bearings is to tight still then you have to decide what to do.

Turn down the jackshaft to a non standard undersize or increase the ID of the bearings to bring back to standard size. I would first try a new set of bearings preferably from a different manufacturer to ensure its not a bearing manufacturing tolerance issue.

Finally I would proabably have a go at reaming out the bearing in some way to open it up - if its only a thou or two then it can probably be done OK with care despite the bearings not being designed for that.

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