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Crankshaft repairs

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ronan walsh07/08/2018 01:36:12
541 forum posts
32 photos
I have a crankshaft somewhere from a jawa speedway engine. I would imagine it had to cope with 60hp passing through it. There is no splines, serrations, tapers,rivitted flanges, cotter pins or anything else holding it together. Both the mainshafts are press fits into the crank cheeks, and the big end the same.
IanH07/08/2018 07:32:26
70 forum posts
56 photos
I have redone the bottom end of a JAP 60 degree twin and used what is now I believe a very well established approach. New shafts are made out of EN36B and then carburised, you leave additional material on where you don't want the shafts hardened e.g where it is threaded. Then turn these details and send the shafts for hardening, finally the hard surfaces are ground to size. Tapers are abandoned being replaced by press fits.
Clive Foster07/08/2018 09:44:09
2126 forum posts
73 photos

Effective though press fits are you need very careful control of sizes and surface finish if full grip is to be achieved. Its very easy to scrape things a touch or even, god forbid, start things slightly out of line initially stretching the hole as the shaft goes in.

Slow tapers give almost identical grip for the same pressing loads but the actual pressing distance is far shorter and they are mostly self aligning. Its much easier to to do a good job.

Testimony to the potential issues with high strength press fits are the various patent expansion techniques, such as the SKF oil injection process, intended to sidestep potential issues with surface damage whilst obtaining the maximum retention forces. The BSA riveted flange construction was devised to allow a light press fit sufficient to carry basic crank loads to be used with the riveted flange handling dynamic peaks and ensuring that the shaft couldn't pull through. As with most complicated efforts failure results in a really sorry mess!

If going for parallel press I'd be inclined to cool the shaft to make fitting easier. But in a normal workshop you have condensation issue to contend with.


not done it yet07/08/2018 09:54:57
4470 forum posts
16 photos

Perhaps a shrink fit might be more appropriate in this century? Liquid nitrogen is relatively freely available these days.

thaiguzzi07/08/2018 10:15:57
686 forum posts
131 photos

Good topic. Excellent posts. Enjoyable reading. Thanx.

duncan webster07/08/2018 10:26:53
2525 forum posts
49 photos

Welding of alloy steels is best avoided. The rapid heating and cooling gives rise to a hard and brittle zone. Yes it is possible to avoid it but not easy

John Rudd07/08/2018 10:38:22
1367 forum posts
58 photos
Posted by not done it yet on 07/08/2018 09:54:57:

Perhaps a shrink fit might be more appropriate in this century? Liquid nitrogen is relatively freely available these days.

I've seen shrink fits achieved by heating with oxy-acetylene and cooling with a CO2 extinguisher....

John MC07/08/2018 16:52:47
272 forum posts
37 photos

Alan W2, I would use a parallel interference fit for the shafts with a "top hat" on the end of the shaft, that will stop them moving in the wheels on assembly and in use. The "top hat brim" only needs to stand about 0.1" above the diameter of the shaft and about the same length, counter boring for that will not weaken the flywheels. Make the diameter for the O/D of the brim a clearance so you do not have two fits fighting each other. EN24T would be good for the timing side, the drive side has the drive line shock absorber on it, ideally case hardened EN36 would be best, although I'm sure 24 will be OK for occasional use.

I believe the reason new bearings are so expensive is because the bore of the bearing is ground to a small (0.001"/1.000". My approach would be to look for a modern bearing that the new main shafts could be made to fit and if necessary bore the crankcases to suit the O/D, a spacer would make up any difference in length.

To bore the flywheels I would bolt them together, located from the crank pin hole. A dummy pin may be need to do this. Its vital that this is done accurately, if not the wheels will not true up properly.

Hope these ramblings are useful.


Howard Lewis07/08/2018 20:59:41
3123 forum posts
2 photos

How about dry ice (Solid CO 2 ) to cool the pins before pressing into place?

Don't know where you would IT get these days, (Used MANY years ago to cool the interior of a coach whilst working Son a thermostatically controlled heat/ventilation system)

Google might help finding a supplier local to you.

Needless to say do not touch with bare hands, and keep the block well wrapped, and in a cool p-lace, when not needed


Brian Sweeting07/08/2018 21:35:23
413 forum posts
1 photos

As a novice to this type of work might I suggest getting some basic material to practice the machining work and technique before starting on the final piece.

Will Noble23/12/2018 20:06:20
39 forum posts
7 photos
Posted by Alan Waddington 2 on 05/08/2018 22:07:53:

Started restoring an old Velo engine today......

Hi Alan. I'm a Velo MAC owner. I'm currently glowering and cursing, awaiting the chance to finish a rebuild of a snapped (thrashed) crankpin and that's subject to finishing the garage/workshop rebuild on my new (to me) house. I'm a little rusty on the detail (and overseas at the moment) but I'll try and help if I can. I can only commend the Velo Club for help and an absolutely unbeatable source of technical help and information. It's a goldmine for a Velo owner and there are guys on there who have SERIOUSLY modded Velo engine to run on Nitro and Methanol for sprinting. They'll know what to use - but I never saw a standard Velo mainshaft fail from normal use, only pull through the flywheel (mine included) from the cush drive hammering at it - 'cos, as stated before, I thrash it!embarrassed

Someone has mentioned 'top hatting' the inner end. I would! The alternative was a rollpin - but as it needs a new one..............

Someone also mentioned RADCO's book. The best source of info to be had between two sheets of cardboard, even after all these years. I have seen an electronic version somewhere.

The shafts were, I'm sure, case hardened/nitrided, definitely not through hardened. They need to be tough with a hard skin for the likes of the cush drive sliding parts. All the rest of the attached parts are fixed and don't rotate against them.

Have I missed bits, or which engine is it? Looking again at the pictures, there's a taper on the drive side shaft? - or is it so badly corroded? If tapered, is it a K series motor?

It's a very Velocette trick but if it's a MAC, or some KSSs (not the only culprits) the inner (1"ish) end of the mainshaft outboard of the bit held within the flywheel was ground on a fine, 0.001" per Inch taper, as was the bore of the 22mm bore main bearing. An original stock main bearing had two notches cut in the inner race to show which way round to fit it. They were, a while ago, made of unobtainium but I think (?) the club have had batches made again. They aren't cheap!

If you are making new shafts and originality is not a concern, then I'd be putting the biggest bore, single shouldered, roller bearings in there that wouldn't weaken the crankcase by having to take too much out the web bore and forgetting the expensive originality option.

The flywheels were malleable iron. It's very forgiving but has been known to lead to other problems with elongation of holes around crankpins - which is probably why my (Alpha version) bigend snapped. Veloce specified a 2 piece pin. The core was toughened steel, the bearing sleeve was through-hardened (EN36 ballrace steel?) so the pin flexed under load, the sleeve took the wear. The only version available to me was one-piece case hardened and it snapped at the root of the bearing surface. Oval crankpin hole didn't help, 2 piece pin would have survived but cheap construction method sealed its fate.

Either way, the Club is your best source of info.

I hope some of the above helps. I'll offer anything I can. If any of it's wrong - a big boy did it and ran away-----------------

Please keep us (just me if no one else is interestedsmiley) up to date with your progress.


Edited By Will Noble on 23/12/2018 20:14:43

Alan Waddington 223/12/2018 20:24:06
492 forum posts
87 photos

Thanks for the contribution Will, very informative. It is indeed a K series engine, ‘34 KSS. Progress has stalled slightly due to work commitments, and dragging home a ratty J&S 540 surface grinder which has meant some fettling and re arranging the workshop to fit it in.

I will update the thread as and when any progress is made.

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