Motoscrubs.com

Hi
Sometime ago I purchased an old IOM 250 narrowcase racer which had been stood in a hallway of a house, after the death of its owner. On stripping the engine it was found to be in poor condition. Over a period of time I have acquired a set of narrowcase 5 speed cases and gearbox. I have now begun the rebuild. As a check on the big end we have pressed the crank apart and discovered it is as new condition (the engineer who stripped it said it has never been run). Unusual considering the state of the rest of the motor. On further investigation the crank appears to be from a 175/200 A type rather than a 200/250 B type. Although the stroke appears to be the same for all small 4 strokes engines (57.8). My questions are (a) are my suspicions correct? (b) Can this crank be run in 250 5 speed cases? (c) Is it as durable?



Many thanks in advance
PaulM

I suppose it depends on the type of use you intend. If its just to parade and be ridden at roadster speeds and revs I can't see a problem. A sensible compression ratio ( 10 to1) and restrict revs to 9k ,don't run below 3k , allow to warm up well , I should give years of fun. If you intend to race in anger it will likely survive a season if well prepared but should then be stripped and checked. What class would you contest?.

Under no circumstances should you use this crank in a 250, first it is 4mm narrower than a 250, this means that each side is 2mm shorter i.e the bevels will not line-up, second, it is balanced for a 175 piston, and you will run it with a much heaver 250 piston, the balance will be miles out, it would vibrate like a road-drill, and third, it has no centrifugal sludge-trap, the big-end bearing would not last very long and more than likley take out the piston and cylinder with the swarf from the big-end bearing, or worse seize-up, which would destroy the rest of the engine. I hope that I have not scared you too much but mixing-up engine parts is not recomended without experience.
where are you, as I may be able to help if I can.

Thanks for the advice. I suspected there would be a catch. I will look for a 250 crank or bottom end and put that with my 175 bits for now.