Angular Contact Main Bearings

[Jordan]

Bevel twins with pressed together crankshafts also use angular contact main ball bearings.

With packs of shims that include very thin ones, the usual suggestion is to keep those towards the centre of the pack.

A machine called a surface grinder, fitted with a magnetic chuck, makes short work of grinding flat surfaces to fine tolerances.
They are common tools in engineering shops.

[blethermaskite]

I suppose it worked fine for Velocette, with if I remember 4thou pre loaded taper roller main bearings and a pressed up crank.
Cheers,
George

[DBDBrian]

Yes George, Velocette had crossed my mind, It's plainly obvious it worked, but it always seemed a strange arrangement from an engineering point of view, too preload a shaft with a void in the centre.
I made a pressed up crankshaft for my Goldstar , with a .006" interference fit on the crankpin. It runs with no preload, end float being controlled with a retained ball race. I have had it apart a couple of times over the years, and there are very slight witness marks on the crank pin, which would indicate there "is" movement, even with a very ridged assembly.
Brian

[blethermaskite]

When I rebuilt my Velocette Venom a few years ago it was a real faff to shim up the crank for correct preload, the shimming being placed behind the outer races of the taper roller bearings in the crankcase housing......so quite a lot of tricky measurement to get the crank centralised and loaded (and fingers crossed) otherwise it was all down again etc. etc. I assumed the 4 thou preload would reduce to near zero once the crankcases had expanded at running temperature, thus reducing the side loading on the crankshaft.......I have no experience of this with angular contact ball race mains but I assume the same principle applies?
Cheers,
George

[LaceyDucati]

Hi,

I've been involved in building race Ducati single race engines for 35 years now and have never had a crank shim break up yet, but never use shims below 0.2mm. Most of the issues with crank shims are caused by the excessive radius on the crank journal not allowing the shim to sit down flat on the face. Failure to observe this will result in shims distorting and shaking the end float loose in no time. If you use the thickest shim closest to the crank face and chamfer the inside edge of that shim until it sits flush then you shouldn't have any issues. I always shim my cranks to give less than 1 thou end float, but at least a few tenths. Marginally tighter than Ducati spec, but never had any issues.

In my opinion angular contact bearings are unnecessary as there isn't an issue with the original set up. I recon the cases expand a few thou at temperature and that is perfectly acceptable and desirable with standard ball races. Any attempt to prelaod the crank (even a thou) will load the crank and make it run out of true...try it then clock the ends of the crank. If you preload the crank the sort of figures some twin owners think (8 thou), then I cannot see how the crank will ever run true and damage to the cases will most likely result (in my opinion). The expansion difference between the crank and the case on a twin is around 3 or 4 thou, so loading it to 8thou will bend the crank well beyond a point that will return at temperature. Mind you many of the same twin owners claim the cases break if you tune them to much.... Mostly I think the twin crank, rods, pistons etc are ridiculously heavy and badly balanced. Compare a 450 rod (especially a race rod or even a large modern single) to a twin rod...it makes no sense and they don't last any longer either.

All in all I think angular contact bearings will solve nothing and are not needed. Many will note that they are not used in many "modern" single cylinder engine designs.

Just my thoughts

Nigel