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BSA_WM20, gavin eisler
Total Likes: 3
Original Post (Thread Starter)
#860009 10/04/2021 1:04 PM
by lemans
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

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Liked Replies
#860074 Oct 5th a 02:02 AM
by Mark Parker
Mark Parker
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.

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1 member likes this
#860075 Oct 5th a 02:26 AM
by NickL
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.
1 member likes this
#860086 Oct 5th a 09:23 AM
by kommando
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.
1 member likes this
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