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[Peter Hogan 2Participant @peterhogan2]

Stuart Triple

[Peter Hogan 2Participant @peterhogan2]

Hi, now retired power station engineer, 20 years ago inherited my fathers partially completed Stuart Triple and his tools and lathe. I have been trying to free up the crankshaft by bluing and scraping the mains but to no avail. Stripped completely and put the crank between centers the center main journals are way out of whack. Too far to salvage I think.

Stuarts don't supply any more ! Ideas on a postcard please!!

Pete.

[JasonBModerator @jasonb]

peter, you have a couple of options. One is to cut from solid, I think you will be able to get the crank out of 50mm material, this method has the advantage of being able to ctr drill the four ctrs in each end.

The other option is to silver solder three blocks onto an oversize shaft setting each at the required angle. You then make up carriers to clamp onto each end with the three pin ctrs drilled and use that to turn each pair of webs and a pin. If you join MEM forum you will be able to see the pictures in this post which show how it is done

J

[Tim StevensParticipant @timstevens64731]

Triple crankshafts are sometimes made (full size) by forging the crank in one piece with all the pins on the same plane, and while it is still hot, twisting the main-bearing pins to push the crank-pins round to the required angles. So, if you are stuck for material, this might be worth a try (assuming you do have the facility to get the crank pins hot enough).

It all depends, though, of what sort of 'out of whack' you are faced with …

Cheers, Tim

[JasonBModerator @jasonb]

Tim, I think that is how the forged crankshaft that Stuarts used to supply was done.

[Mike PooleParticipant @mikepoole82104]

The BSA Rocket 3 and Triumph T150 and T160 were certainly done that way but Laverda went for a180 crank which gives a bit of an odd beat for a triple but you've got to say they were great bikes.

Mike

[John OlsenParticipant @johnolsen79199]

I made a crank for my Stuart triple by machining it from solid. As a true masochist I also machined the eccentrics on it at the same time. This required a minor change to the valve gear so that all the eccentrics could be done using the same centres.

I left the crank webs as circles and used a tube with the same internal diameter to support them all, eg you clamp on a piece over each of the parts you are not working on to help prevent any flexing. Once the crank is done you can machine away the excess to just leave the crank and a balance weight.

Actually all of my triple is machined from solid, apart from the base, which I picked up for $2 at a club auction. I have no idea what happened to the rest of the castings set!

Some of the Stuart crankshafts were welded, possibly a friction weld. I had one for a double ten were the weld was harder than any tooling I could bring to bear on it, including carbide. If I had been able to grind it I might have been able to use it, but instead I made a new one from sold.

Incidently if you have the right gear, a lot of material can be removed from a crank blank by milling between centres. This is faster and less fraught than turning on the lathe, although you need to do the latter for the final finishing.

Dean Clarke 2 (see his V12 thread here) uses a jig which is a large block mounted in the chuck with a hole in it to fit the round webs. The crank is moved out step by step to do each crankpin, so each pin is machined as if it was a simple overhung crank, with the rest of the crank either supported in the block, or whilrling around harmlessly in mid air. This works fine, but might need a big lathe for a Stuart Triple crank.

John

[Neil WyattModerator @neilwyatt]

Welcome to the forum Peter,

Could you turn the journals significantly undersize and bush the bearings?

Only you (and us) will know and presumably it won't be doing heavy work.

Neil

[Peter Hogan 2Participant @peterhogan2]

Posted by Neil Wyatt on 10/01/2018 09:24:23:

Welcome to the forum Peter,

Could you turn the journals significantly undersize and bush the bearings?

Only you (and us) will know and presumably it won't be doing heavy work.

Neil

Thanks Neil

I will give the heating and bending a go followed by machining even if it dosn't work it will be good practice as I have never machined a crank before.

Pete

[Howard LewisParticipant @howardlewis46836]

Maybe the shaft has stress relieved itself over the years, so maybe the heat/twist technique might restore order.

A forging would be better than turning from the sold, since it maintains the grain flow, and produces lower stresses.

Howard

[Peter Hogan 2Participant @peterhogan2]

Hi again,

so stress relived at 400 C for a couple of hours, put in a 3 jaw and the other end in a center. Boy was it a long way out "0.030 in places. After improving it somewhat I put it between centers and worked along from the headstock end putting in the fixed steady as i went.

So now it is "0.0015 out at worst after a day in the workshop! It's back in the oven at 400 C I'll ckeck it after that and give it another tweek

thanks to all for the advice.

Pete H

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