Standard Roller Bering Clearances

[jr2766]

I am currently building up a 2 bearing motor with a Phoenix splash fed shaft. Can anyone advise if the standard MRJ 11/4 bearing for the rear main (standard clearance is normally designated C3 in the engineering business)   always works well or should one be seeking a closer tolerance ie C1 or C2 unit for that application 
Jim Runciman
Perth
Australia

[Howard Wright]

Hi Jim

I’m no expert on bearings (or much else these days) but I’m guessing that even a Phoenix crank will whip slightly and holding the back end with too tight rollers may not be a great idea?

I’ll wait for a shower of criticisms from those who know better .

Cheers

Howard

[Bob Culver]

Those who advocate considerable preload for the front a/c bearings do not subscribe to that theory! personally i suspect it contributes to broken flanges.

[Chris KC]

In my understanding C3 is an extra clearance bearing (not standard) and is generally specified in cases where there is unusually high running temperature or (more precisely) differential temperature (e.g. inner warms up before the outer) - it has some extra room for thermal expansion.

Whether this makes it suitable for an A7 rear bearing? - I will leave others to share their experience.

[Bob Culver]

Bearings used to carry bright spots. One was close clearance and 3 greater than normal. An old article in the Companion recommends one spot. As I have recounted before my father was fastidious and  with oils of the time and short running used to replace the rear or all bearings every 2 or 3 years, about 10,000 miles. He was sold bearings of all grades and reckoned he could hear the difference.
From the NSK Manual. C3 is a grade more clearance than normal (unmarked). Running clearance is much influenced by the interference fits. Curiously the bearing tolerances are negative on both dias. To 12 and 15 microns, inner, outer.. A micron bit less than 1/2 a ten thousandth of inch.  Older bearings were to a slightly more loose spec. 
The Seven housings are seldom now still round. If somehow get less then no clearance difficult to assemble.
For normal crank deflections I suspect the light casting easily accomodates.
Has anyone had a main bearing fail by the classic rolling bearing process of spalling? Or all just by clearance increase (largely due to rust)

[jr2766]

Bob and Chris
Many thanks for taking the time to reply
I plan to use about 0,001" preload which is close to zero on the front and I guess I can check the clearance on the rear bearing by doing a lift check. I was just wondering if say a C2 bearing was available for the rear main.
Thanks again
Jim

[Parazine]

Bearings used to carry bright spots. One was close clearance and 3 greater than normal. 

Like this you mean?

   

Hoffmann rear main. I always thought 3 dots was best - must have remembered it wrongly. Modern bearings don't seem to be marked.

[Bob Culver]

That is how dots appear on my hoard of ancient inherited used bearings.
The NSK book uses Class and CLASS; these seem to have different meanings and apply to differet maker and national standards. NSK bearings are C2, C3 etc but normal CN, between the two for internal clearance,  is apparently not normally marked.
Running clearances entirely OK in other applications have an audible influence on Sevens. I dunno what the experts regard as tolerable for a used bearing

[Ian Williams]

I would suggest talking to Tony Press, he is a countryman of yours and probably know more about bearings than anyone else on here.

[Tony Press]

Thanks Ian,

From Austin works blueprint drawing 1 1/8” Crankshaft XL 145 from the A7CA archive (I assume this is BB1,  A1 55)

Front bearing seating 1.124” to  1.125 “
Rear bearing seating 1.124” to 1.125”

1 1/8” to minus 1 thou.

From the early SKF Manual the Bearing Bore tolerance for an 1 1/8” bore bearing is 
- 0.0003” to + 0.0002”

1.1247” to 1.1252” (Minus three tenths of a thou to plus two tenths of a thou)

This indicates Austin were using quite a broad tolerance range which would give from a 3 tenths interference fit to a 12 tenths clearance fit– a sliding fit.

(SKF recommended fit for an 1 1/8” bore Roller Bearing on a Crankshaft is K5i : + 0.0005 to + 0.0002 : 1.1255” to 1.252”)

I don’t know if the 1 ¼” rear bearing had the same fit on the Crankshaft but I would assume that it was similar.

Checking the bearing list from the A7CA Index Cards (C.B.K. 17th November 49) shows Crankshaft Roller Bearings R&M MRJ 1 1/8, MRJ 1 ¼,  Hoffman RMS 11, RMS 12  with no suffix such as 2 dot or 3 dot, indicating all these bearings would have normal internal clearance.  

The SKF equivalents are: CRM 9 and CRM 10. SKF uses suffix markings: C2 clearance less than normal, no marking clearance normal,  C3 clearance greater than normal.

As an aside:
I am not sure why the booklet ‘List of Production Changes’ does not mention the Crankshaft changes from BB 1 to BB 67 in October 1929 (from Light Shaft to Heavy Shaft) on Page 32.