Joined: Nov 2017 Posts: 6 Threads: 2
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Any suggestions, my 1936 special keeps oiling up No.3 plug, this is now so bad it is covered in just a couple of miles.
The engine was rebuilt no more than 1000 miles ago, rebored +20, new pistons, rings etc. Have checked compression and all four are between 7.0 and7.6 bar. No oil baffles are fitted.
I have pulled the offending piston suspecting broken rings but they are fine. I have checked ring gaps which were set at 0.006 to 0.008 when installed, and find the oil rings are now 0.010 and 0.015 which seems a bit odd.
If this was OHC I might suspect valve guides but vintage car friends (non A7) suggest with side valve oil through the guides is unlikely.
Any suggestions?
Joined: Aug 2017 Posts: 919 Threads: 6
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Location: Scottish Borders
Perhaps a piston ring is the wrong way up so pumping oil up instead of pushing it down.
If it is a high compression (Late Ruby) head, they are more prone to oiling plugs.
Jim
Joined: Nov 2017 Posts: 6 Threads: 2
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Jim,
thanks for the reply, having taken a careful look at the rings and some new spares I have, I cannot find any indication of an up or down on them. Any suggestions?
Joined: Feb 2018 Posts: 80 Threads: 15
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Hi Jim,
I understand your frustration. Austinwood is correct in that I had a thread about this a few months ago. In my case there were no "dots" or “pips” on the rings to indicate which face should point toward the top of the piston. There were .030 marks and I put these facing up, big mistake!
What I learned was;
1) If there is a “pip mark” it must face toward the top of the piston.
2) If there is a step or chamfer on the inner radius the chamfer faces toward the top of the piston. This allows any “compression” that sneaks down from the combustion chamber to push the ring out toward the cylinder wall.
3) If there is a step or chamfer on the outer radius the chamfer must face toward the crank. If it were facing the top it would provide an edge for the compressed gas to push the ring away from the cylinder wall and thus allow oil to move in to the combustion chamber and foul the plug. This was the case in my engine.
The chamfer on mine was very small, maybe .005 so not immediately obvious if you’re not looking for it.
Once I switched mine around it was all good. As a dirty horrible bodge you could run a hotter plug in number 3 until you have the time to inspect the rings (again).
Will be keen to hear how you get on.
Joined: Aug 2017 Posts: 2,748 Threads: 31
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Location: Auckland, NZ
21-05-2018, 07:46 AM
(This post was last modified: 21-05-2018, 07:50 AM by Bob Culver.)
There seems to be some confusion. Externally stepped rings are fitted step down (incidentally such rings have a much increased gas leakage path) Slightly taper face rings are fitted small external dia up as per many sources. At moderate miles such rings show wear at the bottom only. The ring rides the oil film on the up stroke. (A common somewhat desperate practice was to file or oilstone a very slight bevel on the top edge of rectangular rings.
Is the plug oiling and causing misfire or vice versa?
I dunno if any anomoly with the oil jet would affect just one cylinder
