Originally Posted by Andrew R Stewart

Brian- measurements are the solution. The shell is very likely a common JIS/English spec (with a 68mm shell width) (every Lotus I sold was). So that's likely the BB's threading and shell width need. Then you can measure the axle length best by removing the arms from the axle and directly measure end to end of the tapers. Note if there's any off set or difference in the axle's extension between the RH and LH sides, Bbs of that era often have the RH side of the axle extending further out from the shell then the LH side does. If there is an extension difference figure out that amount and add that to the overall axle length to get what a symmetrically spaced/extended axle will be to place the RH arm in the same location that the OEM axle did.

If you can't remove the arms to do a direct measurement then you aren't ready to do the remove and replace job any way. So go get the crank arm bolt and extractor tools first. But there's an indirect way to measure the axle without removing the arms first. To do this you need to measure across the outside faces of the arms right at the extractor threads (where any dust cap screws in). This is the axle length AND the dimension from the arm's outer faces to where in the tapered hole that axle end sits. So remove any dust caps, remove the nuts/bolts retaining the arms and using a depth gage device (easy to make with a spoke/rod/even a wood match) and measure from each arm's outer surface (where you did the earlier outer measurement) to the end of the axle deep inside.

I did this just yesterday and came up with this set of numbers. The outside to outside was about 142m. One arm's outer face to axle end's depth was about 13mm, the other was about 10mm. Doing the math I calculated the axle is a 118mm one. As this was with a modern symmetrical BB axle there was no offset to consider. What was an issue was the depth difference as the shallow side (the RH as it turned out) was because that side arm's tapered hole had been enlargened from repeated overtightening after the arm likely had come loose from poor maintenance over the years. The RH side's axle was flush with the arm bolt's seat. I suggested the customer to replace the crank set before replacing the BB (as the new crankset likely will take a different Bb axle length for correct chain line).

Last details to consider and note before taking anything apart and spending $ is the chain line as it currently is as well as the RH arm and ring's clearances with the chain stay. The chain line can be fudged a couple of mms with out problems unless you end up with the cranks/rings contacting the frame. Andy