Engine Stripdown

[PhilM]

I'm now well into the engine stripdown of my 1936 "Two seater"
When I  removed the two front crankshaft bearings I found that they were mounted with the two large faces of the inner facing the rear of the engine as in the picture, from what I understand these should be mounted "back to back". neither face on either bearing has the word trust on it, I presume the two large inner faces should be facing eachother??
If mounted like this the inners would be touching but there is a small gap, maybe 0.030" (not yet measured) between the outers
Can somebody please confirm this would be correct??



Also there is a small amount of damage on the bearing retaining lip, this was pre-existing, which I don't think will be an issue but again, as a beginner I'd welcome another opinion on this.



Finally, I'm having a problem removing the timing gear from the camshaft. I've removed the nut and thrower but the gear is reluctant to part with the shaft, I've tried a 2 legged puller and a litle heat but I'm reluctant to use too much force  for fear of breaking something.

Many thanks,    ..... Phil.

[Colin Wilks]

Hi Phil
I am about to send off my crankcase to Ruairidh Dunford for him to install a new bearing lip for me. Not a lot of money and a well proven method of rectifying things. The pic is one he sent me to show me what the thing looks like. He drills through front to back and sandwiches the original bearing housing using four through bolts.

When I stripped my camshaft last year I supported the timing gear on blocks, left the nut on to protect the thread and tapped it with a heavy wooden hammer. I assume you've tried giving the end of the puller a thump whilst it's nipped up? It's the short sharp shock that breaks the taper.

[Chris KC]

Phil, the A/C front main bearings should be mounted 'back to back'. This means the sides on which the OUTER races have the smallest INNER diameter (i.e. the outer races are widest) should both be in contact with each other.

When assembled there should be 0.004" pre-load between them. Clamp the two bearings together firmly and check if a feeler gauge will go between the outer races (n.b. one each side so it doesn't skew over). You need a shim the same thickness as the feeler gauge +0.004" between the outer races.

It's worth checking your bearings are the correct depth for the housing, as there were two variations. They should protrude from the casing by 0.015" so that they are firmly clamped by the cover plate.

The bearing lip is a known weak point often damaged by careless assembly / disassembly. They have been known to fail in service. Yours doesn't look quite terminal to me but depends how solid the rest of it is. I'm sure many members would say have it replaced then you need not worry about it.

Timing gear - replace the nut loosely so when the gear does come off it doesn't fly too far. Wind up the puller as tight as you dare. Yes, a little heat on the gear. Wind up the puller a tad more. Sharp smack with a drift and hammer. Don't do this in the crankcase.

[PhilM]

Thank you both for your replies, it sounds like I need to consider the replacement of the bearing lip, once I get the crankcase cleaned up a bit more I’ll have a close look at that area.

The camshaft is out of the crankcase complete with timing gear, I did try giving the puller a bit of a tap when pulled up tight and after warming up but I suspect I was erring on the safe side and was a bit too gentle, I’ll give it another go in the morning.

Chris, many thanks for the explanation of the bearing situation, I’ve just nipped out to the garage to make sure I understood your description, when they are back to back as you described the inner races are overall wider than the outers by around 0.030” and i can barely get a 0.002" feeler betwwen the two outers, they haven't been washed out properly yet, does that sound right?

Thanks again,     ..... Phil.

[Bob Culver]

The bearings seem in good order and provided the tracks are free of indentations, rust or (unlikely) fatigue pits fine for reuse. Can shim for no clearance but I am curious as to basis for the .004 preload (and this before assembly). From catalogues for normal off the shelf ac races no mention of preload, seem to have a clearance. Preload seems to apply to special precision bearings for machine tools etc I am unsure if great rigidity is desirable, although the strong lip helpful. Other non rigid applications such as gearbox mainshafts and rear axles often stipulate a bearing more loose than standard.
I wonder if preloaded ac races contribute to broken lips.

The saga of the ac bearing and crankcase variations is covered in Woodrow pA2-45, but unfortunately the diagram A-41 lower depicts the inner rings as 1 9/16 instead of 1 5/8!

Many “mechanics” and others have limited grasp of ac bearings and cock ups common. Wrong bearing can alter crank location slightly. Conrods should be in centre of piston apertures etc.Ideally the cam gear needs to be supported near the centre. Not all amateurs accumulate lengths of heavy pipe etc for such applications, but with ingenuity all manner of devices can be utilised to provide an aperture and support. The thread on the camshaft is hardened and difficult to restore if damaged.

As far as i know the bearings do not fail from conventional fatigue but, esp with engines used for short runs with the oils of long ago clearance increases due corrosion wear and almost any clearance contribute to harsh running. Should be virtually no depth of wear on the rear roller track.

[Chris KC]

The question of A/C front bearing pre-load is covered in a little depth in the A7 Companion, albeit scattered over a page or two in typical Companion style. I think Jack French originally suggested 0.002 - 0.008" but most nowadays agree 0.008" is rather a lot.

Also depending on what bearings you end up with it's possible there may be no gap between the outer races but you still need to arrange the same amount of pre-load.

I'd agree with Bob if the old bearings are still sound and haven't been bashed about on disassembly then there's no real need to replace them.

The A7 Companion is a very worthwhile investment Phil, although do be prepared to read sometimes conflicting views and make up your own mind!

[Bob Culver]

If assembled as described, and assuming correct crankcase, bearings would have been preloaded, the rear one backwards and thus near collapse! Presumably the front cover plate  was not over tight! Examine races carefully for damage. Balls and tracks should all be very smooth. The lower edges of the  tracks  of one bearing may not look too good, but not normally in contact. 

NSK: For unselected ac ball races the axial clearance of a twin assembly unmounted ac bearing of Normal tolerance is given as 20 to 40 microns (.0008-.0016). This is same as Normal rollers but more than for ball races. Definitely no preload.
I dunno what the supplied Seven a.c bearings in the past or now provide but unless especially ordered would expect similar.  

Incidentally, to avoid confusion, the NSK catalogue refers to the inner and outer assemblies as rings.

[Colin Wilks]

As a humble Mechanic (Third Class or lower) I am left wondering how to mount my shiny new Hoffman bearings. The reading I have done seems to end up advising the user to do what experience dictates works best. 

Is the consensus of them as knows that .002" of preload is about right for angular contact bearings mounted thrust face to thrust face at the front of the crankshaft?

[Chris KC]

Colin I reckon if you set the pre-load at 0.002 - 0.004" you'll be in good company.

["Slack Alice" Simon]

I took my engine apart, renewed the rear main and put a little bit of preload, about 4 thou on the fronts.

One of the people I regard as an Austin 7 guru is used to my turning up with something running "like a bag of nails."

When I went to his yard with this car, he checked under the bonnet to see if it was still an Austin engine!

So that is another vote for a little bit of preload on used front mains.

Simon