Rear Main Bearings

[Zetomagneto]

P            
Which one shall I use, all NOS in my parts bin

[Chris KC]

Brass cages!

[Zetomagneto]

Well 2 of them.Hoffman RMS12V3 is steel and has no clearance between rollers and tracks

[Bob Culver]

The less clearance the less harsh. Assuming crank has not been built up such that cannot assemble. Some of the old brands indicated tolerance with small bright circlrs, one to three.

[Tony Press]

Lovely to see ! (even though I am pro SKF)

SKF used ring markings-

Unmarked normal internal clearance.

C2 clearance less than normal,

C3 clearance greater than normal

If I remember Hoffman used a dot (or circle) marking.

If none or very small internal clearance (hard to measure) I would be careful if the bearing has the correct (tight) fit on the crankshaft, reducing the internal clearance when mounted.

[Ruairidh Dunford]

Tony,

I have been told that C3 bearings are best suited to use in higher temperature applications. Are there pitfalls to using such in our cars please?

[Chris KC]

Only an opinion Ruairidh but I think you are right, C3 are used where there is a risk that thermal expansion will close up the bearing clearance. I don't think this applies to A7's by any stretch of the imagination.

Possible arguments for using one?
Unusually tight fit on the shaft can close up bearing clearance slightly - but in that case you'd do better to rectify the shaft if needed (in the case of A7 rear main I believe a tight fit is a good thing).
Crankshaft whip - more slack in the bearing may accommodate a bit of deflection. But this is loading the rollers the wrong way and doesn't seem a good plan to me.
Or that you simply can't get anything else - in which case I'd use one!

[Dave Wortley]

I thought A7s rear main was supposed to have a smaller than normal internal clearance ie C2 (SKF). It probably didn’t have smaller than normal internal clearance after a few hundred miles use.

[Bob Culver]

If as much as.004 preload is advocated for a.c races surely nil or less clearance for the rear roller ditto is similarly ideal? And as previous, on non racers is spalling or measurable wear of roller ends experienced? Does it worsen with a close fit?
 
The table in The Companion p119 seems to advocate close tolerance single circle bearings (if obtainable)
 
As I have stated before my fastidious father replaced main bearings at very frequent intervals. He reckoned he could detect the difference in the various spot ratings, and claimed that the wider clearance were little better than the replaced. Tolerance ranges differed from current.
 
I have a box full of the old replaced bearings. I have a few C3 bearings inch and metric picked up for a pound or less at swap meets. Seem to be able to roll freely over.003 piece of shim, much the same as the old used bearings.
 
The difference in tolerance ranges is very small, but Sevens are very sensitive. Modern standard tolerance ranges may differ from those of yore, as do the range for od, id etc. Unmounted clearance Normal 25-50 microns, C3  45 -70. A micron is a millionth of a metre, less than ½ a ten thousandth of inch!
 
Operating clearance is directly influenced by housing/shaft fits.
 
It is currently normal to spend up large on Sevens and to sneer at many of the old practices. But I find sorting thru old bearings and mixing parts very close fits can be achieved.
 
I have asked before but wonder what the experts consider OK for reuse? There must be many situations where wear is negligible, esp with modern oils, garaged cars etc.
 
I dunno at what clearance use becomes unacceptable as distinct from just traditional bit rough and rumbly.

[David Stepney]

I am fascinated that my original post at the start of this thread has engendered such an erudite and learned discussion about rear main bearings. Not being an engineer, some of it has been a little over my head.

Thank you Chris KC for your words of wise caution on the use of Loctite. However, given that the engine generally appears to be in good fettle, I don't intend to have it to bits again in the foreseeable future and I shall cross each bridge as I come to it.

In fact, I put the engine back into the car this weekend and it was running by 5 o'clock this evening. Having left it to idle for 15 or 20 mins to make sure there were no immediate problems, I have just returned from a 15 mile trip down to my favourite pub and back. The car has behaved itself perfectly and is now much sweeter running, doing between 40 and 45 mph down the A 494 from Bala to the Llandderfel turn off and running at 30 - 35 through the lanes.

The only problem I noticed on my return home is that, for some reason, the carburetor has started to drip fuel from the air intake once the engine has been stopped. The carburation seems fine, with a nice even tickover, no hesitation and plenty of power ( for a standard Box saloon engine). I checked the usual things, The float is OK, as is the needle valve. I wonder if the float level is too high?  It might, of course, been doing it before and I simply hadn't noticed.

But to return to the point, the rear main appears fine. I have made a note of the mileage so we shall see how things go!