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MG TD TF 1500 - Solid Copper Gaskets

First let me thank everyone for all your input and suggestions. You are a great source of information for a novice like me.

Due to time constraints and other issues I am considering putting off a complete, engine out, rebuild this year. My plan now is to replace the cam with Len Fanelli's roller lifter cam kit. Add a thicker gasket to lower my compression ratio and get my car back on the road. I already have a new set of pistons from Moss and some new hardware and accessory items from Tom Lange.

Can you all enlighten me on the gasket issue?
Do I just add a copper gasket to my existing gasket to gain back the .040" that appears to have been milled off OR do I eliminate my gasket and add a thicker new solid copper one????

Thanks again for your patience,
Mort 50 TD (Mobius)

Can't go wrong with one of Len's roll cams. These out perform stock cams, and do not have the trouble with tappet wear, a weak point of the XPAG.
The copper gasket is the gasket, no need for a conventional gasket.
I think you will be happy with the result.
D. Sander

So if I replace my old(.050") with a new solid one and add what has been milled off (.040") I should be ordering a .086" or .093" thick gasket?
Mort 50 TD (Mobius)

Ok I am guilty of being a little sloppy here. I am not trying to get back what was milled off. That is the wrong way to say it. I need to add .040" to the head thickness to get to a CR of about 8:1.

So I still need .050" + .040" = 086" or .093" thick gasket. Those are the stock thicknesses available.

Any thoughts on the thickness to use would be appreciated so I can place the order.
Mort 50 TD (Mobius)

Anybody happen to know the thickness of a compressed standard head gasket? Bud
Bud Krueger

May want to check these folks out. Used them on one of my motorcycles that needed a special gasket
Bruce Cunha

Bud, I just measured a gasket from a low time engine I took apart because of a machining mistake and it measures .050" ( I believe I got it from MOSS a few years ago).

Richard Cameron

Mort, and (say your name here),
According to "Special Tuning for MG midget Engines" the "stock" head gaskets for 1250 TDs were .045" and since all published information goes along with that figure, I've crunched a few numbers. By the way, it equates that .045" to 4.5cc volume, a very convenient number later on.
Cranking out the displacement for 66.5mm bore and 90mm stroke, the old EXPAG drove off the assembly line with 1250.364 cubic centimeters displacement or 312.59cc/cylinder swept volume. Given the compression ratio was established as 7.25:1, I had to calculate the unswept volume for that old sand cast "banana" head (AND other cavitities), and presto- 50cc, (give or take a bunch from the looks of things).
With your new bore .040" over, your engine now has another 3% displacement, 1288.25cc.
Since you say your head now measures 2.925", you had .034" milled off. Remember that convenient 4.5cc per .045" gasket? We can sort of fudge 3.4cc reduction for a .034" cut, to be very conservative on the compression increase.
If we know your cylinder swept volume is now 368.7cc and claim your unswept volume is (slightly greater than) 46.6cc, the new compression ratio comes out to be a meager 7.91:1.
If you went to the .063" head gasket, by observation, 4.5cc to .045", the .063" gasket would give 6.3cc, or 1.8cc extra, giving 7.65:1 ratio.
Going to an unmilled head puts the unswept volume to 50cc and the C.R. would be 7.44:1 with stock gasket, and only 7.22:1 with the thicker gasket.

I think your bored block, milled head and stock gasket will get you right about where you want to be, just about 8:1!!!

Make sure Len knows you want the supercharged grind for your street machine.

Jim, I'm having a problem with something. You say "Since you say your head now measures 2.925", you had .034" milled off". I believe that the stock head thickness is 3.022". So Mort's head has had .097" milled off, not .034". That combination, OB=.040" and .100" gives just about an 8.9:1 compression ratio. Hal Kramer's article in the June/2001 TSO goes deeply into this subject with some great formulas. Bud
Bud Krueger

I agree with Richard. I have a new and used Moss Gasket here. New=.050" Used=.046".

Thanks Bruce, I am already in contact with them.

Bud, Do you know how the gasket thickness is accounted for in Hal's calculations?

Mort 50 TD (Mobius)

If you want, I can give you Hal's contact information. He is our expert with XPAG heads and compression ratios.
D. Sander

Let me try this, Mort - In Hal's paper, the basic definition of Compression Ratio is:
[(Displacement + Combustion Camber Volume)/Combustion Chamber Volume]:1. Hal then rewrites this as:

CR={[(1/2)B/10)]^2 * pi(S/10)]/Vcc}+ 1:1

CR=Compression Ratio, B=Bore in mm, S=Stroke in mm, Vcc= Combustion Chamber Volume in cc (which consists of the combustion volume in the head and the head gasket thickness), pi=3.1416.

It's the term '+1:1' that clears out the gasket issue, in that this '1' is really (Combustion Chamber Volume)/(Combustion Chamber Volume).
He then establishes the 'simple' first order linear equation, Vcc=100(HT-2.522) to relate the Combustion Chamber Volume to HT (Head Thickness).

One can then continue to compute the CR by inserting variables HT and OB (OverBore in inches).

Hal is one very clever person. We're lucky to have him share his abilities with us.

Hope this helps. Bud
Bud Krueger

Oh Crikey Bud...

You made my Braine hurt :)
R D Jones

Jim, Bud,
In my original post above I mentioned .040 as being milled off. I was thinking that the original HT was 2.966" and since mine is 2.925 I got the .040" figure(rounded).
Bud has pointed out that the original HT is 3.022" so Jim's computation is based on my wrong numbers.

Bud, To answer my own question above about the gasket thickness in the formula; on page 18 Hal states "(Vcc) is the combustion chamber volume in cc (which consists of the combustion volume in the head and the head gasket thickness)".

So, CR=(Displacement + Combustion Chamber Volume)/Combustion Chamber Volume
CR = (D+Vcc)/Vcc

Standard TD
Displacement = Bore Area x Stroke = Pi x radius squared x stroke = 3.1416 x 3.325cm x 3.325cm x 9.0cm = 312.59cc
Vcc = 50cc per Hal's table and includes the gasket.
CR = (312.59+50)/50=362.59/50=7.25
This matches the WSM Specs.

My current set-up at a bore of .040" Overbore or about 1mm.
Displacement = Bore Area x Stroke = 3.1416 x 3.375cm x 3.375cm x 9.0cm = 322.06cc
Vcc = 40cc per Hal's table and includes the gasket.
CR = (322.06+40)/40=362.06/40=8.05

So if the goal is a CR of 8:1 I am virtually there and should be using a standard head gasket.

Someone give me a reality check here.


Std Bore = 66.5 mm = 6.65 cm
The radius is 3.325cm

My bore is 66.5+1 = 67.5mm = 6.75cm
My radius is 3.375cm

Stroke is 90mm = 9.0cm
Mort 50 TD (Mobius)

Since the Workshop Manual doesn't specify the head thickness, I dug up a number for a 1250 XPAG off the internet, 75.16mm or 2.959". I just got off the phone with Blaine at Moss Motors/Jackson Racing and he said he didn't have that info on hand. After further searches online, the figure 3.017" comes up in a number of references for the TD. The dimension 3.022" is given as the TF 1500 head. The 2.959" figure is likely for the TD TF 1250 "round hole" head with higher compression 8.1:1 and larger valves. I just miked our round hole head with smaller valves, that has been ground, and it is 3.005".
OK. Worst case scenario- if .097" was shaved off and we "overestimate"* that it reduced the chamber by 9.7cc, a new calculation would be:
(322.06cc swept vol. + 40.3cc unswept vol)/40.3= 8.99:1

I say "overestimate" as the "footprint" of the combustion chamber is much smaller than the oval hole in the head gasket, so the calculations are very much on the conservative side.

The reason I called Blaine is to get Moss/Jackson Racing's opinion on the compression ratio, and he said a ratio of 9:1 or under is safe. 10:1 is getting dangerous.


My calculator must be broken. Above I stated that:
CR = (322.06+40)/40=362.06/40=8.05
I forgot to add the 40 in the numerator. The correct calculation is:
CR = (322.06+40)/40=362.06/40=9.05

So I am running about 9:1 and there seems to be a difference of opinion as to whether it should be 8:1 or 9:1

I'm all ears.

Mort 50 TD (Mobius)

Last shot:
WKF Wood, The XPAG Engine, XPAG Service Data:
cylinder head depth - 76.75 mm (3.022 in.)

W.E. Blower The MG Workshop Manual, page 484:
Service Data Model "T.D." Midget Depth of head 76.75 mm

Fini. Bud
Bud Krueger

This thread was discussed between 21/01/2014 and 23/01/2014

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