MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - DECK / PISTON HEIGHT

Hi,

I have a BMC Gold Seal XPAG engine. The MOWOG pistons protrude from the block by between 12 and 18 thou.

I cant find anything in the books I have or on line suggesting what should be correct. I would have thought they should be flush.

Any helpful comments would be apreciated.

Rob

Rob King

If the block was plained I guess this could account for it?

I remember hearing people having to double up on head gaskets for reasons like this.

Interesting to hear what the more techies have to say about this.
Chris Couper

Chris mentioned possible double up on the head gasket to gain height. I would suggest a single thicker gasket. I tried the solid copper at about .100" but water leaked like a sieve. I then tried a custom composite gasket that looks just like a standard only twice as thick. Works great. No leaks and my engine is running great.

If your interested contact John at mailto:inquiries@gasketstogo.com

Mort
Mort 50 TD (Mobius)

You can also remove the appropriate amount from the piston crown.

Tom Lange
MGT Repair
t lange

Hi,

I take it from the comments so far that the pistons should be flush with the deck then?

Is there a danger of the piston edges burning? If so surely a double or thicker gasket will not help due to the shape of the gasket holes?

Rob
Rob King

Tom,

How much can be removed from a piston crown before they 'thin out' too much?

Rob
Rob King

Rob,

They should be flush or just protruding, in any case not level with the head gasket, which should measure .045".
If you look at the cylinder head, you'll see that it is flat in places and there it can touch the pistons if the clearance is too small.

My pistons were also protruding, but much more (about .040", see picture), and as I was in a hurry I made do with two head gaskets.
Later on I installed a thicker solid copper gasket from gasketworks.com and that works fine.

In 'Tuning the A-series engine' David Vizard recommends that the pistons should protrude a little bit for maximum performance.
So maybe .018 should be doable if all other bearing clearances are like new. All the clearances of a worn engine and heat expansion could cause the pistons to touch the cylinder head?

If you have the time, you could consider removing the amount off the pistons, just for peace of mind.

Willem van der Veer

Hi Rob. Can't comment on "the piston edges burning" question but one obvious danger is that if meat has been removed from both the block & the head, the valves in the open position are much closer to the pistons. This could be a major problem particularly if running a high lift cam. If so I'd definitely check to ensure adequate clearance. 0.060" minimum is recommended to allow for thermal expansion. Cheers
Peter TD 5801
P Hehir

Rob - How much you CAN remove depends on the piston and the manufacturer. I agree that the piston should be very slightly proud of the block, and my GUESS is that you will be all right at .018". Do the modelling clay test to be sure the valves open fully without hitting, just to be sure all will be OK, and be sure the pistons don't hit the head.

Tom Lange
t lange

Thanks for your contributions guys

.....I still find it difficult to believe that this issue is not mentioned in any of the handbooks, manuals or tuning guides....

I will do the valve / piston clearance test today and take it from there.

Rob
Rob King

None of you have addressed the issue of the flame arrestor ... that little point in the head casting that forms the 'heart' shape of the combustion chamber.

If you've been foolish enough to remove this point (has the effect of lowering the C/R), then expect lower performance.

In an original head, this point is flush with the head surface. If you are going to have pistons standing proud of the block, then you're going to have to cut the flame arrestor back a bit; unless of course those solid copper gaskets make up for it.

Gord Clark
Rockburn, Qué.
Gordon A Clark

Gord,

The XPAG head has no flame arrestor? See attached picture.

The BMC A- and B-series head do have them, maybe you are referring to those?

Willem van der Veer

Use clay to measure valve clearance if you have a high lift cam but to check the squish clearance - the clearance between the piston and head, use strips of solder. Solder is much easier to measure than clay. It simply holds its shape and you can just mic it.

Up to a point, less is more. .035, maybe as tight as .030, no less. The squish imparts lots of turbulence in the chamber, makes for better mixing and less pinging.

I don't remember the size of the solder I used but it was just thicker than I expected to see, maybe .050 or .060. You can tape or glue it to the piston top where you want to measure, torque the head in place and roll the piston through one stroke. Take it apart and mic it. An old gasket of the same type works ok as they really don't compress any more.

Anyway, don't be too quick to cut your pistons down or add a thick head gasket if you don't need to.
JE Carroll

This thread was discussed between 01/05/2014 and 03/05/2014

MG TD TF 1500 index

This thread is from the archives. Join this live forum now