Motorbike engines!!

[apprentice87]

One for Steps here!!

I stripped my villiers bike engine, and had a look at the crank and there was about 3 thou radial play in the big end. On the one hand i thought it was not a formula one car, but on the other you could very clearly hear the "knock" echo through the cases as you worked the "slack" up and down, so with the motor running, i can only assume it would have been fairly horrendous. It would not get any better either and i could do without the motor throwing a rod.

Am i thinking along the right lines or being too fussy??

john..

 

[Sidewinder]

Hi John,

What diameter is the crankpin?

Typically for hydrodynamic bearings .001"/" clearance is typical, so on a 3" dia. pin .003" would be fine for a "normal" bearing.

The oil has to go somewhere, and the bearing has to be able to surf remember.

 

[apprentice87]

Hi Paul,

Problem is, the crankpin is only about 3/4" diameter, so the clearance i got must be miles too much. Now i got the crank out i suppose it must be 4 or 5 thou..

When i was about 10, someone gave me a Honda 50. On the way home [i was with my mums boyfriend] we passed a motorbike shop the boyfriend used to do the books for. [he was an accountant] The owner came out and listened to the bike and said "the crank is knackered" My mums boyfriend went; "Shhh, do not tell him that, he will end up spending all his money on it" Seems like some things never change!!

If i have the crank done i will, i think, have to have a rebore as well. Seems to me that if the new conrod was by chance any longer then the old one, that the rings would then smash into the wear ridge at the top of the bore, and that would be that...

john..

 

[Sidewinder]

The conn rod shouldn't need changing if you are just getting the crank ground?

It is using white metal bearings yes?

The crank pin centreline should be re-ground to the same location as the original.

The crank grinding machine company I used to work for have now gone away from all mechanical means of generating the throw on cranks pins, the shaft is rotated around it's true centreline i.e. about the main bearings centre line, and then the pin is ground orbitally by the wheel moving in and out to follow the pin around its orbit.

The old ways were to mount the crank in fixtures so that the big end journals rotated about grind centreline.

A regrinding machine should be able to track the original pin, or if the original geometry figures are available it could probably be set to that.

 

[apprentice87]

Hi Paul,

It is a roller bearing crank. To be honest i am not sure what they do, grind out the conrod and then have a bigger pin,, or a new conrod instead, i am not sure. I know you can buy a conrod kit complete with the bearings and crank pin, but no idea how the crank comes apart. I know it is pressed up, but there is nowhere really to get a grip to press it apart in the first place. I only got a ten ton press aand i have no idea how tight these things are assembled either. Besides, my press is not all that rigid, should fabricate one myself!!..

I am going to take it to a place that speciallises in doing these exact same cranks for in 210cc go-karts. Judging by the cranks they make from scratch they must be pretty good!!

Glad i took the crank out though, as there is a circlip missing that is supposed to locate the mains..

Like the sound of the new fangled crank grinders!! So far as i am aware years ago you had to mess about with "throw blocks" because the crank had to be set to revolve around the axis of the crankpin as you say..

john..

 

[Sidewinder]

Yes John, throw blocks caused me loads of work!

With automated loading, any misalignment between the loader & the machine would result in damaged mains.

The mains have to be finish ground before the pins, so you can get the location correct for reference.