One oily cylinder

[jono]

I've stripped my engine to fix a load of oil leaks. It has done 15k miles since a full rebuild including rebore and head overhaul by Paul Exon using Colsibro guides all properly reamed to size.

The engine has been great - very strong and no oil burning just loads of leaks due to my inexperienced building techniques (front cover).

I've noticed that all pistons tops and combustion chambers are dry with light coking, other than cylinder number one where the piston top is oily along with the exhaust valve - this oily appearance extends to the valve stem and upper side of the valve head.

I've pulled the piston and all looks normal - hone marks still visible, no wear to piston - concentric rings on piston skirt all as new.

What would you advise to check - as said the engine runs very well and there was no reason to strip it other than to reslove very significant oil leaks (front cover was hanging up on the edge of the chain slipper and the cork top gasket had extruded).

Comments on possible causes/way foward please or should I simply ignore and not worry?

[rgh0]

If the head is off I would pull the valves and check the stem to guide clearances on all the cylinders to see if any differences with cylinder 1. You need to carefully and accurately measure top bottom and middle of the valve stems and the guide bores to determine the clearance versus specification. The inlets need to be at the tight end of the spec and the exhaust around the middle.

You also need to check for air leaks in the headlight vacuum system as this could be leading to poorer combustion in cylinder 1 which is not burning the oil off as well as the other cylinders

Did you use a new front cover and chain damper in you previous rebuild?. I found recently that the new covers and dampers can have this hangup problem due to the paint on the damper and the machining of the front cover when I built my last engine with a new cover and damper.

cheers
Rohan

[jono]

Thanks Rohan,

Is it implicit in your advice that it's more likely to be a guide issue? I have electric headlight lifters BTW.

I have to confess that the front cover was re faced 30 thou due to a nick next to the water port in the area around the pump which was causing water to get into the oil. I used a gasket to take up some of the lost material but, foolishly, never checked the two parts face to face before installing them. I could have easily relieved the slipper - it was just catching next to the oil plug indent in the block plate just next to the dip stick tube.

Cheers

Jon

[john.p.clegg]

When you re-fit the pistons,make sure the ring gaps are at 120 degree intervals..

John

[jono]

I did that John, however:

My machinist (old school and 70 years old) told me that this is a red herring as the rings rotate in the bores anyway

[rgh0]

Thanks Rohan,

Is it implicit in your advice that it's more likely to be a guide issue?.....

Jon

Getting the guide clearance right is critical and not easy to do even for an experienced machinist as most people doing the guide reaming are used to much looser tolerances. If pistons, rings and bores are OK and no vacuum connection on No1 then i would suspect more oil appearing on the piston on No1 is due to slightly greater valve stem clearances on the inlet. Even with the clearances right you normally see an oil trail from the exhaust stem down the port and carbon on the back of the exhaust valve but if you see it on the piston it is normally the inlet valve stem.

cheers
Rohan

[jono]

I discovered that the oil control ring in Nr 1 was in upside down!

Then, I managed to break a compression ring on another cyliner by being ham-fisted.

So I've solved one issue and created another one.

Tried the usual suspects and drawn a blank in buying a single ring. I may have to bite the bullet and buy an engine set and replace them all. Bu66er!!

[Craven]

Double check the width and depth of the rings, I had no luck in finding replacement rings.
Deep rings seem to be a thing of the past.

[jono]

The lower compression ring is 3.95 x 1.95 with a stepped, undercut, lower edge.

I can get them but only as an engine set. QED show piston sets but not in +40.

[Craven]

I think you need to look at closest fit + 40 if you can find +60 or metric size and re-gap to suit.

[jono]

I'm pleased to say that QED have fitted me up with a single piston set for ?20 (FW Thorntons quoted ?36).

I'm happy!