Classic cars forum & vehicle restoration.

[Ray White]

The pistons in my XPAG engine are standard. Nothing unusual about that but the pistons are stamped OME (which I am informed stands for OMEGA pistons) and have what I believe is an unusual ring set up.

As you can see the middle groove has three very thin rings instead of one compression ring.

I have been told these pistons are for "racing" and that would tally with the car's competition history (back in the 1950s).

The four vertical "V" grooves are also unusual.

There is a fair amount of wear present with at least 0.006" clearance in the top ring groove so I shall be renewing the pistons. The new ones come with four rings which seems excessive so I shall probably omit the third compression ring.

I thought others might be interested in my discovery?

fallout pipboy icons

[peter scott]

I have used that type of ring in the past. They are very good because they are not fragile like cast rings. The only down side is that need to align the gaps such that they set apart.

The gapped rings like your third one are the most fragile.

Peter
_________________
http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[Bitumen Boy]

Interesting, I'm sure one of the Triumph engines I've had apart had rings like that.

[Ray White]

I have my faults but dishonesty is not one of them. As a consequence I tend to take other people at their word and I tend to suffer the more for it. The previous owner told me to my face that the engine had been rebored. In fact the pistons are standard size XPAG and the bores are worn to +0.027".

A straightforward rebore is not possible as one of the cylinders has been sleeved. There is insufficient material to take it out to +0.030" so a new sleeve will be required.

This engine must have suffered a catastrophic event at some time because besides the damage to No.1 cylinder, there is a lump missing from the bottom of No.2 as well.

On the plus side; it could be a lot worse!

[ukdave2002]

Hi Ray

The bore (I think its 66mm) would require around 0.008" piston/bore clearance, could you get away with a rebore to suit+ 0.020" pistons and avoid having to put a new liner in?

Its needs a expert to confirm, I'm suggesting it because these Morris / MG blocks were not the best cast iron in the world and insertion of liners can introduce other problems namely cracks.

The second compression ring looks very similar to the Hepolite "Oilmaster" ring set, is the top ring stepped?

Dave

[Ray White]

Dave. The standard bore is 66.5 mm and it now measures 67.2 which is +0.7mm = +0.0275".

The existing liner has been worn quite thin already so I doubt it could take a rebore.

In any event I think a +.020" rebore would be right at the limit and I suspect the machinist would prefer to go to +30 to allow for a bit of wriggle room especially if a new liner is required. We don't know what horrors have been hidden so I plan to leave the engine with XPAG specialist Peter Edney.

[Minxy]

Three separate rings in one groove isn’t unusual mainly from companies who sold replacement rings to fit standard pistons in worn bores to compensate for wear - a company called ‘cords’ springs to mind. Often they were ‘cupped’ so if you took the ring in your hand and squeezed the gap closed the ring would arch. When fitted they had to be fitted alternatively.
_________________
Hillman minx convertible. Lanchester LD 10

[Ray White]

Having spoken to Peter Edney (the XPAG engine expert) this afternoon I have agreed for it to be collected on Friday. The plan is to chemically clean the block/crankcase and remove the liner. It will be replaced with a new "top hat" liner and all bores machined to + 0.030". I will also be sending the new pistons which should be with me tomorrow.