Hi Guys, i'm hoping someone might be able to help with a problem i'm having.  I'm assembling a cub that i brought in (many) parts.  I've had the crank rebuilt, cylinder re bored and  I'm fitting a new piston.  I've just reassembled the top end and found that when i turn the engine over it's getting stuck fast at tdc (or thereabouts).  I've taken the head off and removed the pushrods.  Then  put the head back on (minus pushrods) - still fouling!  removed the head and (at the point i met resistance) and piston is approximately 1mm above the top of the barrel.  If i continue to turn the engine over (from when i met resistance) and with the head removed it travels another 2mm approx?  I's a flat top piston and oval head.

Any ideas why this might be happening?

QM

The piston does stick out at tdc as long as the ring does not The piston should miss the copper head gasket and fit the recess  on the head What piston are you using and was the head skimmed?

Hi naichuff and thanks for your quick response.  The rings are below the top of the barrel and the pistons clears the gasket without lifting it.  What i didn't do is place the piston in the recess of the see if it's ok.  What i've since done is place the head over the studs and turned the engine over without putting the nuts on.  When it gets to tdc it very slightly lifts the head so i suspect it's fouling.  No i don't think the head has been skimmed but can't be sure.  I suspect the top of the barrel may have been very slightly (in comparison with another barrel i have),  The piston i have is a low comp.  The barrel and head that i'm using had a high comp fitted but i thought they were all interchangeable?

The  pistons are interchangeable One has a flat top the other has a dome but round the edge is flat The head is flat on the mating surface then drop into a recess with the dome where the valves are If the head or barrel have been skimmed the piston will hit the head as there is not much clearance       

Edited May 7, 20179 yr by naichuff