[SEL] piston rings and bores.

[John Culp]

I dunno, I've put new rings in totally glazed old bores that weren't 
hugely enlarged or out of round and had good results. The glazing 
itself isn't a problem with gas sealing so much as it reduces the 
oil-holding capacity of the cylinder wall and can cause problems with 
ring and piston lubrication. A much bigger problem is if your bore is 
worn till it's not round. The rings may have worn and turned as the 
bore wore so they sealed fairly well in it, but new rings won't. 
They'll bridge between high points across out-of round sections and 
you'll get tremendous blowby.

On pulling a piston out, I always look where the ring gaps are. In a 
perfectly round bore with free rings (not pinned), there is no 
preferred orientation and the rings will rotate randomly over time 
running in the engine. The gaps won't be found oriented the same way 
when you later pull the piston as when you put it in. (That's why 
orienting them in specific ways isn't critical.) Often you find the 
gaps all lined up on one side of the piston. That means the bore has 
worn to an egg shape, with the gaps lined up on the pointy side of the 
egg. That bore will have to be rebored.

John

On May 1, 2005, at 6:38 PM, peter ogborne wrote:

> I just rebuilt a small single cylinder ,four cycle engine. .....short 
> cuts ,don't take them ! Because i could not readily buy new rings i 
> made some ,no problem there but i did not take the glaze off the bore. 
> When reassembled there seemed to be lots of compression and start up 
> was easy . After an hours run absolutely no compression...first 
> thoughts it was the valves . I did the usual and shot some oil down 
> the spark plug hole....compression !That eliminated the valves as the 
> problem . Pulling the barrel off and examining the bore showed 
> considerable glazing ,there was evidence of blow by ,oil coming out of 
> the breather. So now it out with the hone ...................


John Culp
Bristol, Tennessee, USA