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+--- Thread: Rear main bearing tolerances (/showthread.php?tid=4765)
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Rear main bearing tolerances - jamesheath - 05-08-2020
Hi all,
After a long time sitting disassembled, I've plucked up some courage, and am starting to sort out my engine (never having done one before).
The first thing that needs attention I suspect is the rear main bearing. There's enough slop in it that I can feel it, and something was very noisy in the engine prior to dis-assembly.
Mounting a digital caliper to measure the vertical movement I can measure 0.12 to 0.13mm, depending on the rotation:
https://photos.app.goo.gl/eeqEfBmHLLUZtw71A
The only figure I've found re. play was that Jack French thought that 0.0013" clearance was "quite acceptable" - in this case does "clearance" equate to play?
I am pretty sure that it needs changing, but figured it wise to check first!
On the assumption that it is shot, what are the replacement recommendations? Can one get a sealed bearing replacement?
The engine is a '34, two bearing.
Thanks!
James
RE: Rear main bearing tolerances - Robert Leigh - 05-08-2020
Converting your measurement to Imperial like the Jack French figure you have about .005" which is too much in my book. I would not look for a sealed bearing in this application, but I would fit a lipseal to replace the original oil return scroll. If you want to improve bearing life I suggest the standard full flow filter conversion. Our cherished suppliers can supply everything you will need, generally of a quality which may be better than you will get from a local bearing supplier who might offer a cheaper chinese or similar substitute.
RE: Rear main bearing tolerances - Chris KC - 05-08-2020
At the very least I'd visually inspect the inner and outer races for any signs of fretting / brinelling/ corrosion; pretty much any mark that won't wipe off. I'd also check it is reasonably tight on the shaft. The question of how much slop is OK is not easy to answer, if you can genuinely measure it it's probably too much, I wonder how accurate that set-up really is though. One of our 'cherished suppliers' can sort you out with a new bearing if required, preferably one from a well known and respected maker.
My suggestion would be try and figure out what was making all the noise before going much further - if you can describe the noise and the conditions under which it occurred I'm sure many folks here will venture educated guesses.
RE: Rear main bearing tolerances - Dave Wortley - 05-08-2020
James, looking at your photo you are measuring the vertical movement between crankshaft and the rest of the engine. The A7 rear main set up is notorious for developing slackness between the bearing inner race and the crankshaft journal.The inner race wears away the crankshaft journal. So as Chris says that's the first place to check for play. It should be an interference fit here and then originally the rear main internal clearance ( total clearance between inner and outer races/ rollers) was a tight "0" fit. I think that is the SKF system of bearing clearance. Tony Press will be able to put a figure on that. My bearing books are at home and I am away at the moment. If the crankshaft journal is worn considerably then you may need to look at finding a different crankshaft.
Good luck,
Dave
RE: Rear main bearing tolerances - Robert Leigh - 05-08-2020
If you do have wear on the crank bearing journal it can be metal sprayed and ground to an interference fit for a good quality new bearing. It is not worth spending money on repairing the crank to fit the old bearing however good the races are.
RE: Rear main bearing tolerances - Dave Wortley - 05-08-2020
If you remove the crank for building up the crank journal it would be worth getting it crack tested. No point in doing work on it if it's going to let go! ( And the damage sustained on the rest of the engine can be expensive) I have used loctite 638 recently for worn down rear main journals but not had opportunity to test it yet. The best fit and forget solution would be a new crankshaft but of course this is not available for everyone's budget. Crack testing kits (Magnetic particle type are available from our cherished suppliers).They are inexpensive and easy to use.
Dave.
RE: Rear main bearing tolerances - David Stepney - 05-08-2020
When I replaced my rear main bearing in February last year, See my thread on this entitled 'Rear Main Bearings' 19.02.19) I used Loctite bearing fit to secure my new rear main bearing as it was at best a good push fit on the end of the crank. Nearly 8K miles later - so far so good.
RE: Rear main bearing tolerances - Bob Culver - 05-08-2020
The 2 brg engines are very sensitve to clearance in the mains, esp rear. Clearance entirely satisfactory for other rolling bearings leads to harshness and if really worn a cyclic vibration. Made much worse by any generous spark advance. Wear was/is not primarily abrasive but more corrosive from cold runnng, esp with the oils of yesteryear..
Bearings should be not less than standard clearance tolerance, NOS one or two rings tolerance. Current unmarked, not C3.
To crack test crank start from extreme rear. The rear journal must not be very heated and should not have been. A tight fit conveniently reduces clearance. Welded, brazed journals tend to soon develop cracks.
If almost any step on the outer track is visble/ feelable then worn. Measuring internal clearance loose is dubious but my pile of replaced bearings by a fussy owner will not freely roll over .003". Dont know what guideline the experts use for replacement. In the good old days when seven owners were not all rich those with piles of parts used to mix for a tight fit.
The flywheel must not abut the bearing.
The vital thing is to not damage the front lip when removing bearings.
RE: Rear main bearing tolerances - jamesheath - 06-08-2020
Thank you all for your very useful replies.
The point about where the play I'm measuring actually is is a very good one.... I've checked today, and it is within the bearing as opposed to the bearing to the crank.
So I think I'm replacing the bearing - I'll start hunting out supply.
Apologies for my lack of experience but I'm not clear what "lipseal to replace the original oil return scroll" means - I'll get researching!
David, I had started reading your previous post - I'll go and read the rest now!
Best, James
RE: Rear main bearing tolerances - David Stepney - 06-08-2020
(06-08-2020, 09:50 PM)jamesheath Wrote: Thank you all for your very useful replies.
The point about where the play I'm measuring actually is is a very good one.... I've checked today, and it is within the bearing as opposed to the bearing to the crank.
So I think I'm replacing the bearing - I'll start hunting out supply.
Apologies for my lack of experience but I'm not clear what "lipseal to replace the original oil return scroll" means - I'll get researching!
David, I had started reading your previous post - I'll go and read the rest now!
Best, James
James,
I found the comments on my thread most useful. Incidentally, an uncaged roller rear main seems to work very well. 7564 miles since it was fitted and all is well.
Regards,
David.