14-02-2018, 07:47 AM
(This post was last modified: 14-02-2018, 07:58 AM by steve davidson.)
(12-02-2018, 07:08 AM)Bob Culver Wrote: It is not primarily reduction of gas pressure but the ring planing on the oil wedge on the upstroke which reduces pumping.
If large enough to admit significant gas, reduction of ring pressure would occur in both directions.
It means further loss of already very short skirt but a very small relief below ring could be added. Your club likely has old pistons to copy. With modern oils small drillings do not block.
thanks Bob, good idea, I'll have a hunt around for some old ones and see what they had
(12-02-2018, 10:53 AM)AllAlloyCup Wrote: For what it’s worth I once replaced pistons on my Morris Eight
1934, plus reground the valves, but did not change those or the valve guides.
Result : a smoky engine and I was told that the improved pistons
we’re sucking oil up the very worn valve guides( (( I was 18 and it was my first car!)
I had no garage and can still remember how cold my fingers got
changing the valve guides in situ, with a brass drift,in the street, one
very cold March day. After all this the rusty M8 Tourer failed it’s
10 year test as the examiner did not like my Dexion repair to the chassis!
Just as well there’s no emission tests for A7’s or M8’s....
Bill G
Hi Bill,
Great to hear that these problems have haunted all of us! I've heard similar theories before but it's always struck me that for this to be the case the piston must be on the down stroke (creating a negative pressure) so either there has been detonation and all the valves are closed or the inlet valve is open and any oil being sucked in from the valve stem is "competing" with air/fuel from the inlet port. To me that would seem more likely if it was an OHC engine. But I'm a simple civil engineer, the mech guys know more than me! Thanks for the input.