Stiff piston downstroke

[bmwnunkie]

Posting for my Father.

He is pulling apart and putting back together a BSA (C15 ), single.

Having taken the engine to pieces and put it back together again... it is apparent that , when cranked ( manually ), that the downstroke of the piston is quite stiff in comparison to the upstroke.

Does anyone have any theories as to why this may be? The crank seems straight, as does the valve pushrods.

[ducati2242]

Are you sure you are cranking it in the right direction as it would if it were running .

[bmwnunkie]

Yes, we are cranking it the same way as if the piston was being turned by the kick start.

[rhyatt]

I can imagine some resistance when coming up on the compression stroke.
If you take the plug out , is it easy then?
But you say it is going down that it feels stiff, could the pushrod be back to front?

[bmwnunkie]

I'll get him to check the pushrod... he can't see very well... so....

That said, this is without the cylinder head on... so the spark plug doesn't come into it... so no resistance should be felt as far as we are concerned.

I'll mention the pushrod... get him to check... thanks

[ducati2242]

no head on so nothing to push or pull . Is it a rebore and a new piston . personally as there is no head on i would take the barrel back off and see how it moves with nothing to restrict it . if it is then turning freely you will know if it is the piston in the bore . if it has new rings and a rebore you could get more drag going one way that the other until things bed in .

[rhyatt]

I'm unsure if these pistons are offset, is there a FRONT arrow or anything on it

[neil1964]

I this a bare crank in bare cases with nothing else hooked up except rod and piston?

Presume piston is lubed in the bore, rings are the correct way round?

Rebore done by experienced person with the actual piston to be used in their hand (ie correct clearance for said piston)

Odd.  Need more info please.

[bmwnunkie]

Many thanks for your reply.

Unfortunately, my Father is doing this ( he is 89 ) and a lot of the information I am getting is a little... errrrmmm confusing to say the least. That said, yes, this is a bare crank case with nothing else hooked up... it hasn't been rebored but my Father has changed the rings for new ones and all appear to be the correct clearance for the piston.

So, in effect it is the piston , for the bike with no re-bore. All that has been changed is old rings for new ones.

The stiff downstroke is just noticeable as the upstroke is without any resistance.. not sure what else I can tell you, if I'm honest. Personally, I think he needs to break it all down again to check everything but what do I know?

[neil1964]

Given what you have said, I agree.
Check the ring gaps, check the rod is centred, rings are correct way round (presume compression rings have taper correct way?
But yes. 
My father is 85 now, an engineer, and I'm grateful that he taught me a lot of this stuff as a teenager, I can't imagine him doing engine rebuilds now!!

[Derek996]

Just a thought. On the upstroke the flywheel counter weight will assist in pushing the piston upwards but will resist the piston being pushed down.