BSA Owners' Club Forum

The BSA Workshop => Singles => Topic started by: MICKREEVES on 13 October, 2017, 10:24:38

Title: B44, Bigend, Piston. Bad vibration.
Post by: MICKREEVES on 13 October, 2017, 10:24:38
My B44 vibrates badly on a light throttle opening, opening to accelerate stops the vibration,
closing throttle to overrun stops it as well. This is most noticeable at high RPM.
So bad, bits are dropping off or breaking.

I thought it must be bigend play, or possibly smallend or mains.
I have stripped the top end this morning.
No play to be found at all. I  cant mend something if it aint broke!

I wonder about balance factor?  The piston is 71/798, 1/2 mm oversize. It looks quite normal.
I see bsa part numbers. 41/794 standard, 41/795 + 10 thou, 41/797  + 20 thou (1/2mm)

The engine is not loose in the frame. I cant think what it can be.
I will order a new rocker gasket, and rebuild when it comes.
Any suggestions? Do they ALL have this fault??
Mick
Title: Re: B44, Bigend, Piston. Bad vibration.
Post by: MICKREEVES on 13 October, 2017, 10:41:02
answering my own question!
I checked again.
Gripping the piston with both hands, push up and down, moving around and sideways.
Finally found a point where a very small amount of play was felt. Enough to hear a small click.
Im guessing that that is the answer?? ANY play is not acceptable??

Suggestions for a good supplier for replacement bearing please.
Mick
Title: Re: B44, Bigend, Piston. Bad vibration.
Post by: JulianS on 13 October, 2017, 11:53:15
Just read a contemporary road test of the 1970 441 Shooting Star. The tester reported lots of vibration.

On piston part numbers - the numbers in the BSA parts lists are for a complete piston - including pin and rings - the number you see on the piston is the number for the bare piston.

You could try Alpha Bearings for a bigend .
Title: Re: B44, Bigend, Piston. Bad vibration.
Post by: ANDY HIGHAM on 13 October, 2017, 16:27:11
There is no reason why a B44 should vibrate badly.
Don't assume BSA correctly balanced the crank, they probably balanced the prototype then made all the production ones "similar"
My B31 (400cc) crank/rod/piston assembly was balanced by Phil Pearson and the bike is smooth as silk all the way to 80 + MPH
Title: Re: B44, Bigend, Piston. Bad vibration.
Post by: MICKREEVES on 17 October, 2017, 10:27:23
answering my own question!
I checked again.
Gripping the piston with both hands, push up and down, moving around and sideways.
Finally found a point where a very small amount of play was felt. Enough to hear a small click.
Im guessing that that is the answer?? ANY play is not acceptable??

Suggestions for a good supplier for replacement bearing please.
Mick
Title: Re: B44, Bigend, Piston. Bad vibration.
Post by: MICKREEVES on 17 October, 2017, 10:34:19
I have stripped the engine.
I dont think a new bigend was really needed. There is JUST perceptable movement.
Now I have it stripped, I will fit a new alpha bearing.
I suppose the whole job was a waste of time and money, but I still want to get rid of the vibration.
I might try making a weight, 58% of the rod and piston (is this correct? ) Then drilling the flywheels as needed.
Mick
Title: Re: B44, Bigend, Piston. Bad vibration.
Post by: ANDY HIGHAM on 17 October, 2017, 18:16:12
Not quite. The piston assembly and the top part of the rod is reciprocating mass, the lower part of the rod is rotating mass. Also, is 58% correct or just what was thought to be correct 50 years ago?
Title: Re: B44, Bigend, Piston. Bad vibration.
Post by: JulianS on 17 October, 2017, 18:24:47
You need to make sure, when fitting new big end assembly, that the crankshaft/flywheel assembly is as near to perfectly aligned as possible.

If not so then expect vibration even if balance is correct- aim to get it within 0.001 inch. Bad alignment can also mean loss of power.

I would suggest you consult a specialist and get it dynamically balanced rather than static.

Basset Down Balancing did a B25 for me some years back and the improvement was very noticable.
Title: Re: B44, Bigend, Piston. Bad vibration.
Post by: MICKREEVES on 18 October, 2017, 10:16:35
I have shipped my crank to Alpha for replacement and balance.
Mick
Title: Re: B44, Bigend, Piston. Bad vibration.
Post by: DEREKANDERSON8 on 18 October, 2017, 19:24:50
I have shipped my crank to Alpha for replacement and balance.
Mick

I take my hat off to you mick! you certainly don't mess around when you have a problem to deal with, from having a bad vibration to the crank being sent to SRM for repair 4 days have passed!

Excellent I hope you keep us posted  :D

regards

derek
Title: Re: B44, Bigend, Piston. Bad vibration.
Post by: MICKREEVES on 27 October, 2017, 18:36:16
Still waiting for the crank back from Alpha.
Mick