Would appreciate any views/experience on how much backlash is acceptable? Matching gears at +2 (as stamped on crankcase) produce about 8-10 thou measured between meshing teeth. Can reduce this by mixing gears but Woodrow implies that no backlash at all is undesirable - presumably because of no room for expansion when hot?

after reading this and previous threads as a qualified motor engineer + aircraft fitter here is my thoughts on the subject.to start with +is bigger,-is smaller.if you have a new engine at 0 (standard) the 2 new cogs fitted would be 0 (standard).in this state there would be a built in gap (backlash)which we need to know.if you have a new engine at +1 the two cogs would be +1 each as that measurement will be the diameter so each has a radius of +1/2 giving you a total of +1 (still with the designed backlash in)also i presume this backlash would have had a tollerance range).on an older engine with second hand cogs (basically with wear),on a +1 engine putting in +1 cogs may take it out of the designed backlash range. if we knew what the designed gap was we could probably use secondhand cogs of different sizes to gain correct backlash.ie on a +1 old engine with old cogs the designed gap may be acheived by using +2 cogs.thats 1 thou over but only 1/2thou wear on each cog.does anybody know what that clearance is?.to end my ramblings may be a total load of rubbish but at least it gave my brain a workout while the other halve wathed emerdale!

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Chris,

You said "If you have a new engine at 0 (standard) the 2 new cogs fitted would be 0 (standard)."

I suspect that this might not actually be the case. The markings were, I believe, put on to aid selective assembly on the production line. Thus you could have had + or - markings on a new production engine.

I stand to be corrected.

Mike

My thoughts were that it would probably be too dodgy using unmatched gears (ie gears unmatched with crankcase sizing) as overmeshing (gears too close) might produced broken teeth. Not sure what the result of the opposite scenario might be other than more noise perhaps.

Small inaccuracies in valve and ignition timing.

Regards, Stuart

mike ,that would explain why i have a +1/2 case and +2 cogs.so austin must of had a nominal tollerance(or backlash)figure to set to, not necessarly the same as what is stamped on the case then you could put in a matched pair to acheive this.does anybody know the tollerance?

Well I have been able to match the two. The crank gear was already a +1 (in agreement with crankcase) but the only marking I could find on the cam gear was an 'S' so assumed that it was standard. No idea how they became 'unmatched'.

Have sourced a good +1 cam gear so the proof will be in the pudding when I bench test. I was just curious as to whether the backlash that exists was acceptable

norman,if you had a +1 and a standard in, removing the standard and replacing with a +1 only reduces backlash by 1/2thou but i supose you have a matched pair in.and i like you want to know what was the factory acceptable tollerace for backlash(measured on the driving side of the cogs).then it would not matter wether new or secondhand gears where fitted.at the moment it all seems a bit hit and miss to me.if its any help to you my engine run very quiet crank-cam gear wise.i have measured the backlash and i have 9thou

Thanks Chris. Looks as if your backlash is very close to mine and if yours is quiet then maybe there is hope for mine. Will update when engine is bench running again