Standard Roller Bering Clearances

[Bob Culver]

Some time ago someone publshed here a 1929 crank blueprint which gave journal tolerances as front -.00025 and rear + .00025
Used with the bearing tolerance Tony quotes would help explain why so many rear mains worked loose. The tolerances i quoted also appear in Machinery Handbook 1996. Neither that book or NSK Manual lists specifically for imperial and as NSK make both presumably the same tplerance applies to all.
I dunno how Phoenix are finished but if more than +.00025 any C3 clearances would be reduced
C3 bearings are preferred where a lot of deflection occurs ie 3/4 and semi floating rear wheel bearings. (Contrary to preloading a/c where deflection occurs!)
NSK recommend interference does not exceed 7/10000 of shaft dia.

[jr2766]

Thanks for taking the time to respond Tony
I was hoping the community had some firm guidlines because I have heard a couple of recently rebuilt engines and while I understand there are a lot of variables I'd be very disappointed to achieve such an outcome.
Cheers
Jim
runcimans@westnet.com.au

[Chris KC]

In a perfect world yes Jim; but I think main bearing arrangements are an area where many A7 enthusiasts agree to disagree, you "pays yer money and takes yer choice" as they say...

[Tony Press]

Thanks for taking the time to respond Tony
I was hoping the community had some firm guidlines because I have heard a couple of recently rebuilt engines and while I understand there are a lot of variables I'd be very disappointed to achieve such an outcome.
Cheers
Jim
runcimans@westnet.com.au

As I always advise- only buy genuine bearings and not the cheaper Chinese copies, often readily available and in genuine looking packaging.  

From my experience there are still a few original stock SKF or R&M imperial bearings around - most likely with normal clearance. May be not the cheapest but well worth it.

If you do happen on a Three Dot or C3 it should be ok but I wouldn't use a Two Dot or C2.

I do not know what rolling bearing shaft tolerances Phoenix cranks manufacture to- engine builders should check to see if over tight.

[Bob Culver]

Hi Tony
not sure why you recommend avoiding any C2 bearing which might cahnce across?
All and Sundry
Those who regularly overhaul Sevens must adopt some guidelins for wear and new tolerance. There must be a lot of serviceable used bearings about.  With the oils of old wear was severe. Many Sevens crept about with severely retarded spark. Pre war Len Southward of auto musem note ran a business servicing Sevens. He told my father cars came in with the flywheels rubbing on the case!
The Companion refers to bearings of T0 tolerance. I dunno where that nomenclature comes from. Possibly same as single spot.
The Companion also suggests sorting through used bearings with a view to matching odd bits for a close clearance; such  legitimate tricks are dismissed by now wealthy owners.
Incidentally for those many original shafts worn undersize it seems practical to sleeve, turn housing and fit a readily available cheap metric bearing. However turning the housing to the precise size and concentricity is not easy.

[Bob Culver]

For any that may be interested, a bit more from the NSK manual:
 
For a rigid shaft or housing, the bearing  rings expand/contract 70% to 90% of any interfernce fit.

For angular contact ball races our sizes CN (normal) grade the unmounted axial cleance  is 40 to 60 microns, about .0015 to .0025 inch. This is decreased by any interference fit. 

I dunno how the original Seven ac bearings matched but in the interest of the lip I am a little sceptical of assembled preload. (Adopted in my car but had a steel housing!)