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#860009 10/04/21 1:04 pm
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lemans Offline OP
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I'm one of the suckers who bought a 90-drg crank from Chris B. Visited his premises twice, looked like a decent chap but didn't understand the basics of running a mechanical workshop. alas.

so I'm stuck with a crank that doesn't fit. (at the moment it is used as an expensive presse-papier)
1. the pistons don't clear the central web
2. the bigends are approx 2,5 mm to close, they are spaced to close to each other. so the conrods run off-center to the bore.

we (someone I met recently and I) are now trying to remodel it to an usable construction/form/crankshaft.
the idea is:
1. to split the central web and have a new distance-piece made to fit. and use bolts (7~9)
2. turn down, reduce the central web, easy.
3. rebalance the lot

keep you posted.
regards A

[Linked Image from swf-ignitionsystems.nl]

[Linked Image from swf-ignitionsystems.nl]

Last edited by lemans; 10/04/21 1:25 pm.
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Cutting it in half and fitting a spacer seems very drastic to me and would involve a huge amount of work and could result in something weaker.

How much interference is there between the piston and the flywheel? Could you not use a combination of reducing the flywheel's diameter and/or shortening the pistons skirt to get the clearance needed.

You say that the big ends are 2.5mm too close, in effect you need to find 1.25mm per side, it might also be possible to use a combination of removing metal from one side of the small end and/or from the inner gap of the small end on the piston, just 0.65mm from each of those components would get you over the line.

If my memory serves me right, I seem to recall that Triumph moved the big ends inwards on the TSS crankshaft to put more strength into the outer crank webs and the piston spacing remained the same as the T140 and that the small ends were offset to make it all work.

Just my thoughts.
John

Last edited by John Harvey; 10/04/21 5:41 pm.

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Originally Posted by lemans
......2. the bigends are approx 2,5 mm to close, they are spaced to close to each other. so the conrods run off-center to the bore.......
So you are talking 1.25mm (.050" appx) off center per side. I don't like it when I see it but many engines I have seen over the years have rods that do not line up exactly centered between the pin bosses.

Also, there are engines out there that are manufactured with offset rods to correct for this sort of misalignment, say for example when the bore centers are moved to allow for a larger bore in an existing block and lower end. I don't like that very much either.

So hopefully it doesn't sound like I'm suggesting things I don't really approve of, but more to let you know what is out there and hopefully provide some context as you go about finding a solution.

Does the crank fit the cases ok or is it 2.5mm too narrow as well?

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lemans Offline OP
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Crank is also to narrow. First suspicion was the spacer to set axial float. Had to be a lot thicker than anticipated. But it didn’t set of any alarms.

All options will be considered. First idea was to offset the small-ends. Offsetting the bigends will give a better losd profile but is more difficult to achieve.

And bolted cranks have been used for ages.
Keep you posted

First to do is to make a 2D-autocad drawing.
That will keep me buzy for the next couple of weeks

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2.5mm isn't much. It would be 1.75mm each side. What stroke is it? If it's long stroke chose pistons carefully. Ed V's B44 pistons are great because the skirt is high. Other B44 pistons can have a lower skirt and come out of the bores at the bottom. His are short and I don't know any that are better quality.

Otherwise cut or shape the bottom of the piston. I did that putting T120 pistons in a T150.

The piston will slide across on the pin. Has it room to do that? 1.75mm isn't much. I've seen A65's with the crank so badly aligned under the bores the rods so hard against the piston bosses it spun the L/end bushes out.

I cut the tops off old pistons to check that on mine, the alloy rods are wide. I doubt it matters much as long as it has some clearance.

I would not ever cut that crank. You can with a stock one because the halves spigot into the new flywheel and keep it true. Early and late cranks do vary in width? Which doesn't mean that matches any really, but worth checking. Pins would be spaced the same though.

This shows this type of B44 piston. On a std stroke crank there is lots of room. This one isn't good on the other side when cheap Nicasil came off. It can be cheaper just getting the best job even if it costs a little more. If you use those pistons you have draw on them with a felt pen and cut them off. It doesn't look like it's hitting by much.

[Linked Image from i.postimg.cc]


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Use 6in centre rods, Carrillo will make them for you.
Expensive but allows choice of pistons etc.
Also can have centre drilling for small end oiling.
It's what we did with 89-91mm stroke.
Used both t140 and B44 pistons, also A70 ones.
I tend to agree cutting the crank will be a lot of work, although
it will be centred properly the difference between just milling a
small amount off the piston pin boss will make little difference
to how the motor works for general street use.
You could also try a set of slipper type pistons, they rattle a bit
but are very strong.

Last edited by NickL; 10/05/21 2:29 am.
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My memory failed me as the TSS crank had larger big ends but centres were the same as a T140. It was the cylinder centres that were 1/2" further apart so the small ends were offset by 1/4" each side. Your looking at approximately 75% less offset than Meridens most powerful production twin.


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Fit a shell bearing to one of your rod big ends, then look at the clearance between the edge of the full thickness of the bearing lining and the edge of the rod. There is a sizeable chamfer on the edge of the bearing that can be safely milled off plus there will be a gap between the side of the con rod and the edge of the shell. The tang on the shell is for ordination only, it does not stop the bearing from spinning in the housing, the over stand (crush height) or extra length of the shell bearing over the housing crushes the shell into the con rod to stop it from spinning. So you can grind the tang off, mill off the chamfer and move the con rods outwards within the limit set by the bearing journal crack resisting radius on the crank, any remaining movement can be achieved at the small ends.

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The crank takes 6" A65 rods doesn't it? MAP steel rods would fit if that's the case. Long stroke can also mean notches out of the bore for rods, more if bulky alloy or stock bore. As long as they have piston to rod clearance at the pin the engine will not know. It's a tiny amount of offset. The spark plug isn't in the middle.


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lemans #860123 10/05/21 10:22 pm
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My comment relating to rods is based on most blokes using Norton rods on long stroke cranks.

Kommando's solution is great if you have the ability and mill. For the street just milling the piston
boss a couple of mm is fine.

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thanks for all the pictures and advice, some send directly by mail.
first is to take all measures and make a 2D-drawing, and check all sizes with you.
and then decide what to do, (money is one of the parameters)

first parameter for the conversion is: I want to ride this bike without any nagging thought in the back of my head, please be carefull it might grenade.

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What bore and stroke are you using?


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I would think the chances of grenading will be several orders of magnitude greater if the crank is cut and welded/bolted than running rods 1.25mm off centre in the bore


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I would think the chances of grenading will be several orders of magnitude greater if the crank is cut and welded/bolted than running rods 1.25mm off centre in the bore
The pistons can be shortened to clear the flywheel, just hand tools required. How short can a piston be? This is the piston as fitted to my race bike
[Linked Image from live.staticflickr.com]00501-GM-piston by Sigma Projects, on Flickr


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I can imagine it being cut, but I have no idea how it would be aligned and bolted. Norton use a big pin. Triumph and BSA use the flywheel spigot. It would be easier cutting and bolting an A65 crank on a new flywheel. From memory the T120 pistons in a Trident needed hacksawing at an angle then trimming up.


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