Triumph twin crank idea

Now then chaps, in my experience, Triumph twin cranks are quite strong, but when pushed too far, will break, and when they do, it's usually through the sludge trap, which when you look at it, isn't a surprise as the crank is quite weakened there.

Billet cranks are available but are very expensive and heavy. Personally, I like the 'up to 68' lightweight crank.

I've been toying around with this idea for a bit and thought I'd throw it out there for scrutiny. Here's it is:

The sludge trap is not needed these days with modern high detergent oils and especially if you also use a good filter.

So, remove the sludge trap.

Bore straight through the crank, so you can see all the way through and it's a nice clean bore.

Press in a specially turned up plug that passes all the way through as an interference fit (maybe heat the crank up?)

Drill an oil way down it and cross drill the big end feeds into the oil way.

Now weld up the ends of the pressed in plug, weld over the hole and fill in with weld on both sides so it's part of the crank cheek.

Machine off the excess weld nice and smooth.

Re balance.

Job done.

Am I overlooking something blindingly obvious here? It seems to me you'd have a bloody strong crank for very little cost / effort.

What say y'all ??

Obviously cutting a new crank from billet is a better solution.

Boring, drilling, pressing, and welding on an old crank isn't going to make it as as strong as a new crank. All that could go wrong in that process wouldn't be worth the difference in cost.

I think I've heard of it being done.

As he says, doesn't sound like the wisest course of action.

Well of course a billet crank would be stronger!

But in the UK the best pice I can find is still over £1700. And a lead time of "who knows"!

Unless you're going for a big Nourish conversion, these billet cranks are overkill in a Triumph.

The bodge I'm talking about wouldn't be as strong as a billet, but I'm thinking it would be plenty strong enough for your average thrashed 650/750 and more to the point, wouldn't cost more that £200-£300 from a pragmatic engineering shop.

Well... maybe...!

What's doubtful is whether it has any effect.

What I can visualise is it might hold the parts of a broken crank together enough to save the crankcases.

Triton Thrasher said:

What's doubtful is whether it has any effect.

What I can visualise is it might hold the parts of a broken crank together enough to save the crankcases.

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Indeed, that's what I can't work out either!

I think just welding up the sludge trap tube plug would actually irradicate the inherent weakness.

As to the value of the 'plug' I'm with you in being unsure, but some cranks are held together entirely by the press fit of shafts. So, if the tolerances were correct, it should add strength... I think!

Not sure I'm convinced enough to try it though to be honest!

Make sure to post photos when it's done.

...and also when it's done with!

Fast Eddie said:

and more to the point, wouldn't cost more that £200-£300 from a pragmatic engineering shop.

Well... maybe...!

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Let's see some actual quotes instead of numbers being pulled out of the caboose.

Putting a rod through the sludge trap might strengthen the shaft if it was a neat fit, otherwise it will probably send you crazy getting it right. How many one-piece Triumph 650 shafts have you broken ? Even the 3 piece big journal shafts stand up well to a bit of abuse. Triumphs are like Nortons, if you rev them too high they will cop it but they won't like it. The only reason I ever persevered with my 500cc short stroked 650 was reliability. I feel I did not ever really need to go that way. A normal one-piece 650 shaft is OK. My friend still uses them, however he never revs his Triumph above 6,500 RPM. It is set up to pull hard and he runs very high gearing - same deal as with a Commando engine. I rode it years ago and absolutely blitzed a good guy on an H2 Kawasaki two stroke.

It has been done with the Norton crankshaft, and also with a Honda v-twin. I think Jim Comstock has tried it with a Commando crankshaft, but I could be mistaken. I know someone has done so and told me about it, but I could have the guilty party misidentified. If my memory is correct (and it isn't always) the Honda crankshaft was in Ely Schless' Duconda road racer, a marriage of a Honda 650 Hawk bottom end and Ducati top ends.

It would clearly require a tight fit to work properly. I'm not sure how much it helps with a Norton. They usually break at the base of the drive side shaft, and one might suspect that it is more due to the output shaft flexing than from overall crankshaft flex. If so, the bar isn't going to do much to help in that specific area. In fact, by stiffening the central part of the crankshaft, you might actually make the flexing at the output shaft worse. On the other hand, if you've ground the journals offset to a smaller bearing size to get a little more stroke, a press fit bar might help a lot.

Could be different for the Triumph crank. Where do they break? If it tends to break at the rod journals, the bar might help.

Ken

Ken, in my observations most break through the sludge trap plug area, hence my thoughts that simply leaving the sludge trap out and welding up that area would be an improvement.

As Alan says, they only break if you explore the upper rev ranges. In reality, the cases aren't much stronger than the crank (so you could argue Triumph got the design package well balanced), the cases can break either through the cam bushes, or through the drive side main bearing. So once you've strengthened the crank, you just move on to the next weak link!

Alan, I believe the bolt up crank is not the chocolate fire guard that many claim. Perhaps it might flex more, but I don't see why the bolt up crank is weaker per se. The Norton crank is bolt up...

In more support of the bolt-up cranks, the Maney crankshaft is the same bolt-up design as the stock Commando crank, but much more durable. I think that's mostly due to the use of higher spec steel, more precise machining (particularly the sludge trap part of the journals), good surface finish, and the Tuftride process. It is possible to break a Maney crank, but you really have to work at it. It's also possible to break a Nourish one-piece crankshaft if you try hard enough. There's really nothing inherently wrong with the bolt-up crankshaft, as long as it is designed and manufactured properly.

Back when Steve was just making steel flywheels and fitting them to stock crankshafts, part of his process was to have the stock halves Tuftrided. I used to have my stock crankshafts shot peened for a similar reason, and it did make them last longer. Do the Triumph guys do something similar to improve longevity?

Ken

The bolts that hold the Triumph crank together are 1/4 inch, the ones in a Norton crank are 3/8 inch or 5/16 - whatever, they are larger. I suggest that if you spin any Norton or Triumph crank fitted with appropriate weights on the journals, to 8000 RPM in a lathe - by shining a strobe light onto it you could watch the flywheel grow, It probably stretches the bolts more than deforms the halves of the shaft. If you rev the tits off the motor, do billet cranks also fail at the mainshafts due to flex when you use Superblend Bearings?

acotrel said:

The bolts that hold the Triumph crank together are 1/4 inch, the ones in a Norton crank are 3/8 inch or 5/16 - whatever, they are larger. I suggest that if you spin any Norton or Triumph crank fitted with appropriate weights on the journals, to 8000 RPM in a lathe - by shining a strobe light onto it you could watch the flywheel grow, It probably stretches the bolts more than deforms the halves of the shaft. If you rev the tits off the motor, do billet cranks also fail at the mainshafts due to flex when you use Superblend Bearings?

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Actually, the two one piece Norton cranks that I know off failed at the drive side rod journal.

The 1/4" fasteners on the Triumph seem pretty small to me. Back before the MK3 crank came out with the 3/8" fasteners, a lot of us used to ream the 5/16" holes in the stock cranks out for 3/8" fasteners, and use high quality NAS fasteners. Does anyone do something like that with the Triumph cranks?

Ken

Eddie the lightweight Triumph crank which was used in the Saint 650, did not make the bike perform better. In fact those cranks can be bought very cheaply. A crank with the heavy flywheel is much more desirable. You have a choice, you can go high revs, high horsepower, light crank and multi-speed gear box. Or you can go cheap and nasty and build the motor to pull harder. Whenever you raise you upper rev limit to get more power, you increase the cost. My friend has the quickest 650 Triumph I have ever ridden and in the old days, I had a lot of them - all hotted-up.. The motor is built to pull harder and never goes over 6,500 RPM. He is currently converting it to 750cc and is having all sorts of stupid trouble.
My short stroke 500 could always stay with his 650 during races, however I could never convincingly beat him. My gear box was the main problem. It was 4 speed close ratio. I could choose where I was going to lose races - at the starts of the straights, or at the ends of the straights. Without 6 speeds, there is no happy medium. I rode the 500 in later years after it had been fitted with a 5 speed Triumph cluster. It was better but not perfect.

The bolts in the Triumph crank are positioned so that their heads and nuts are too close to the metal. Small nuts are already used, but you would never get a spanner on bigger ones. I once used Allen headed high tensile bolts, but I understand they are not so good in shear situations. You might be able get a spanner onto a bigger nut if it had 12 points instead of the hex, or perhaps you could relieve some metal. One way which might be possible, would be to buy some high-tensile tubing then thread the inside so it can be used as a nut, torque it up and then cut the excess off. - All to bloody hard ?
There is only one way that any early Triumph is better than a Commando - Triumphs have separate cams for inlet and exhaust. So they are much easier to manipulate.

I suggest there is an easy way to make a Triumph 650 beat any 750 Norton. Simply do a Fullauto with the cylinder head and make it to a Commando configuration with downdraft inlet tracts and squish bands.

acotrel said:

I suggest there is an easy way to make a Triumph 650 beat any 750 Norton. Simply do a Fullauto with the cylinder head and make it to a Commando configuration with downdraft inlet tracts and squish bands.

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Agreed, I did that on an 89mm stroked motor and it was good!

Heck of a lot of work though.

Much easier and better results by going the Nourish route !

The Nourish direction is the Weslake head ? - 4 valves per cylinder in a large single or twin usually gives a 10% increase in power across the usable rev range. However I know of one Weslake (Rickman ? ) head which did not seem to make any difference to a 650 Triumph compared with the two valve version. It was in the hands of a very competent A-grader. My feeling is that downdraft on the inlet tract is not steep enough on the Weslake head. Theoretically it is a much better way to go, however in practice . . . ? The other thing with the Weslake head might be the inlet port diameter - too large and you lose a lot of pulling power - makes the motor all top end. A Triumph crank in a Commando might be interesting. Sometimes a smaller capacity motor can be much faster than a large one. It might cop more revs without destroying itself, spin up quicker and be overall more efficient. Whichever way you go, the gearbox is the heart of the bike. If you don't have that you will always be struggling.
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Sure, if you go and buy some knackered old 680cc Rickman kit and expect it to be a Fireblade beater then you're gonna be disappointed.

But the Nourish route opens up a whole new avenue of oomph. If you do it as a proper 'package' there is simply no comparison with a Triumph motor...

Firstly, you get 4 valves per cylinder, you get a proper modern combustion chamber design that gives you a high CR with flat pistons, you can take the ports out far bigger than a Triumph and fit (and utilise) 38mm carbs (or bigger). The Nourish crank is infinitely stronger than a Triumph and you can stroke it up to 93mm in Triumph cases and have any configuration you desire (I only ever went as much as 89mm) and you can take the bore out to in excess of 81mm. The valve to piston clearence is easy to make as big as you want for whatever cams you want.

Gearbox? We're talking Triumph so you can fit a perfectly good 5 speed box using stock parts. Brand new clusters are still available for around £700... compare that to a TTI!?

I built a 906cc engine based on the above that had monstrous torque and over 85rwhp (using the same dyno that measured my current Commando at 64rwhp).

If you can match that with a Triumph based engine then you need to start talking to Stephen Hawkins as you will have disproved the laws of physics ...