Hitchcocks Forum

Paul, I’ve just watched your video and have a question I’ve wondered about for a while.

When you true the crank, what stops it wandering when fitted?
Obviously I appreciate it’s held in situ by the bearings. But what stops the force of the fire stroke putting the two flywheels out slightly again? I guess if they are trued, then they shouldn’t really wander?

Then again if they wandered that easily, all engines would be buggered pretty quick!



John

Hi John, when pushed to their limits, cranks can whip and flex. When we started to get the other side of 40 BHP @ rear wheel and 7,000 RPM from the 500 racer, I decided we ought to use something a bit stronger than a lightened 60 year old set of flywheels and we had a crank specially made to our specs. I often split [and rebuild] Bullet and other cranks where some degree of fretting in the crankpin holes is evident, due to flexing and whipping taking their toll - hence there being crankpins with oversized shoulders available, should they be required, to ensure a tight fit in the flywheels if this has taken place. Also, when I was a marine engineer, for engine survey work, we used to measure 'crankshaft deflections', where a very limited amount of sag, or runout between pairs of webs was permitted, but measurements exceeding permitted tolerances may have indicated worn main bearing shells in that vicinity and remedial work would be required.
Here is 'Some Welsh bloke' working on a Bullet crank ...
https://youtu.be/59OkzoyMmRQ

Here's Michael Waller of Britannia Motorcycles rebuilding and truing a BSA C15 crank, exactly the same principles apply!

A.

https://www.youtube.com/watch?v=ocOMEXGes-8

Thanks both. Paul, I watched your video before posting. I’ve also just watched the other video. I can see that if they are not trued, you’ll have great fun fitting them in the crankcases...

So it’s all about tolerances.

Hi John, also, the truer a crank is, the less strain on the main bearings and the engine will be smoother running as a result. I have stripped some previously running Bullet engines to find terrible runout on the mainshafts and usually, a vibrating Bullet engine will be due to this, rather than balance issues, certainly on the ones I have worked on.

Thanks Paul

It must be difficult truing these crankshafts up:- http://www.marinediesels.info/2_stroke_ ... kshaft.htm

papasmurf wrote: ↑

Tue Dec 15, 2020 12:53 pm

It must be difficult truing these crankshafts up:- http://www.marinediesels.info/2_stroke_ ... kshaft.htm