British-Cars.org

Welcome to the DMR Site for British Car Information.

Recommendations

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 - Piston/bore sizes

Hi all, I'm about to order new pistons and rings for my bored out XPAG 1250, the old pistons measure 2.870 but are dished, before I order the wrong sizes does this sound like it is 1500 40 thou over? The pistons are very tight in the bore, probably why it stopped 40 years ago. I can hone the bore out a little to clean it up.
Any thoughts, comments, measurements?
Regards, Don in Hot (39c+) South Oz
D J Walker

Hey Don,

I don't know if you deserve a response or not. Its -19.7° as I write, and I feel you're pulling my chain on this.

Weather aside however, for the XPEG, Blower gives the standard bore as 2.835; first over-bore at 2.854; and a second over-bore at 2.874.

Your pistons atre 2.870 so, allowing for an interference fit of .004, I can only guess that your pistons are indeed +.040".

An interference of .004 seems to be OK. Try the new piston w/o the rings installed and see how it fits. Fit the rings. Use copious quantities of automatic transmission oil in the bores, and press the piston home. If its still tight, then as you've suggested, run a light stone or emery paper down the bores and try again.

Dished pistons are no longer used. Newer pistons are flat and will raise your C/R and should work just fine with today's fuels.

However, I would be suspect about the dished pistons. Check the head thickness. For the XPEG, it should be 2.99 inches. If the head is anything less, and you plan to use long reach plugs, check for interference.

On my last rebuild, I was able to find a set of UK pistons with the oil control ring below the wrist pin. I'm not up-to-date on what's available these days, but for years, that was the configuration of choice. US-made pistons always seemed to have 3 or 4 rings above the wrist pin.

Gord Clark
Rockburn, Qué.
Gordon A Clark

Thanks Gord, sorry for mentioning the weather but it's only going to be 30c tomorrow!
Thanks for reinforcing my thoughts re. the bore size, I thought the head was XPAG, the thickness is 2.965.
As the 1250 has flat top pistons I was hoping the larger bore would raise the compression to about 8.5:1.
Now the next Q, should I order the 1500 gasket set? I can't see any difference apart from the bore size.This car was re-bored and sleeved in 1972 then bang! I think it blew a head gasket as I can see tracking on the old gasket.
I'll be getting the pistons and rings from LBC.
Regards, Don
D J Walker

The only way to really do this correctly is to clean up the bore (hone), then measure at least 90 degrees apart in a few spots. Then measure the pistons and do the math. New ones could be somewhat different in diameter, etc. George
George Butz

This thread was discussed on 17/01/2012

MG TD TF 1500 index

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