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not tightening

"split" return crank will not tighten on shaft

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David Bothwell 127/07/2019 16:25:53
123 forum posts

Came across this problem while re-fitting new return crank, I used a cap head screw (2.5 m.m.) and steel nut, but it will not tighten, just wondering if anyone has a suggestion? Possibly incompatible nut (7ba.) I have copied the original part( and consulted the drawings) but the original builder decided to make a split and fit a nut and bolt to grip it to the shaft. I did have the engine running with this method, but the return crank "moved" (engine would not go!) any suggestion gratefully received. Also is there a "fool-proof" way of setting the return crank in the right position in respect to other parts of the motion? Thanks in anticipation!

roy entwistle27/07/2019 16:36:54
1021 forum posts

Use the same thread on nut and screw

Speedy Builder527/07/2019 19:03:01
1801 forum posts
127 photos

Is one of them left handed.

Jim Nic27/07/2019 20:04:41
203 forum posts
107 photos

What is it that won't tighten - is it that the nut won't run down the thread?

If the engine was running and then the shaft turned in the split crank it suggests to me that the split is closing fully before the crank grips the shaft. In which case make the split wider.


David Bothwell 127/07/2019 21:00:19
123 forum posts

Thanks for the replies, I will try the solution in the morning! The nut goes down he thread but there is no tightening up I am fairly sure this is because the bolt is a metric thread and the nut is B.A.. at least I think this is the problem

Mike Poole27/07/2019 21:10:41
2048 forum posts
47 photos

Just to be pedantic BA is a metric threaddevil but your nut and bolt will need to be a match.


David Bothwell 128/07/2019 07:56:14
123 forum posts

Of course you are correct Mike, I have looked at the conversion chart and there is a difference between 7b.a. and the "metric" equivalent. thanks for the replies most useful.

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