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 - TD cylinder head thickness

Can I have an opinion please on the minimum thickness an Xpag head should be for a standard engine to maintain normal road use? Thanks in anticipation of a reply. Regards, Peter.
P J Wilgoss

This link should provide some insight:

http://home.brisnet.com.au/~jmiller/wolseley/tuning.html

Graeme
G Evans

The link above is great. The definitive article on heads is in the June 2001 Sacred Octagon by Hal Kramer. I don't have a digital version handy, if someone else has perhaps they could share. George
George Butz

Graeme, thank you the link is very informative. George, let's hope that article surfaces, thanks to you both, Peter.
P J Wilgoss

The problem with Hal's excellent article is that it's a TSO article, copyrighted by the NEMGTR. It's been quoted from many times on this forum by myself and others. You can find it on the TSO CD from NEMGTR. Bud
Bud Krueger

Be careful with the info in the article reached using the Wolseley tuning link, as it repeats the myth that packing pieces should be put under the rocker shaft stands if the head is skimmed - they are difinitely not needed. However, puzzled by the mention of 'three oil holes', as the outer holes would be bolt holes, and only one plate would need three holes - but put the plates in a place for a rainy day, or for posterity.
R A WILSON

Not quite sure how to interpret this. If material is removed from the head surface, does this not change the geometry of the rocker action? ... hence the requirement for the 'packing pieces'.

Gord Clark
Rockburn, Qué.
Gord Clark

No, the solution is to shorten the pushrods - an easy job.
Dave H
Dave Hill

The rockers are on a shaft, which is on four pedestals, and nothing has been removed from the pedestals - so the action of the rockers on the valves is unchanged. MG, or rather Morris, changed to shorter pushrods during the life of the XPAG engine, and later, if a camshaft was purchased from Crane, a set of pushrods was available, and they only supplied the shorter ones.
R A WILSON

The overall length of the shorter pushrods is 217mm.
R A WILSON

If you get out the paper, pencil and a ruler you will see that reducing the thickness of the head, without any other changes will change the angle of the rockers, and therefore the part of the rocker face which slides against the valve head from its design position.

If you add packing pieces, then the rocker angle is restored to the point before reducing the head thickness.

Alternatively reducing the push rods without adding packing pieces has the same effect of returning the rocker arm point of contact to the valve to its design position.
Ian Bowers

If you don't shorten the pushrods, there is a risk that the rockers will hit the pushrod cover, especially with an aftermarket cover. To shorten them, pull out the cup at the top, which will be loose anyway, and cut off 5mm from the pushrod tube. Clean the parts very thoroughly, then either bond the cup back in with cyano or weld if you are very proficient. They may loosen, but they were loose before, so it won't really matter, the can't fall out owing to the long spigot on the cup.
Dave H
Dave Hill

Reducing the head thickness ..... everything on top of the head is the same except the pushrods poke up that much extra. Unscrewing the adjuster balls (normal adjustment of valve clearances) is usually enough to nullify this. It has virtually the same effect as shortening the pushrods and keeps the angle as it should be. If there is not enough adjustment available, shortening the pushrods is the best way of keeping the angle correct. Check the adjuster ball cup does not foul the rocker at full tilt.
Bob Schapel
R L Schapel

I agree with Bob Schapel, but then it is mentioned 'keeps the angle as it should be' and 'keeping the angle correct'. There is no change of angle between the rockers and the valves, and it would be the same if the head was assembled with valves, rocker shaft, pedestals and rockers on the bench.
I did, however, find it useful to grind a small amount off the OD of the pushrod upper cups.
R A WILSON

See http://ttalk.info/WKF-Wood-XPAG-Engine-Book.pdf
Dave H
Dave Hill

Thaet link may be broken, so go to ttalkinfo, then scroll down to Woods XPAG supertuning article. This gives lots of useful information on the consequences of skimming the head and effect on compression ratio for different bore plus sizes. Page 39 covers the rocker geometry issue and gives the options discussed above.
Dave H
Dave Hill

To answer your question Pete the generally accepted max amount of meat that can safely removed is .125", however my TD had about .140" shaved off when I bought the car in 1969. I ran it for a year & a half without any issues. It had another very light skim when the hardened seats were fitted about 3 years back making the total .144". The present CR is 10.1:1. I have a roller street cam & am yet to turn the engine over but again I don't anticipate any problems. Cheers
Peter TD 5801
P Hehir

For Peter Wilgoss - would it be possible to give the bore size, or what it has been bored out by, of your engine. I can then advise a head thickness that will give a compression ratio close to 9.5:1, as that seems to be suitable for today's petrol.
R A WILSON

For information, in the following; S1 = stage 1, head thickness 74.37mm; S2 = stage 2, head thickness 73.575mm; CR = compression ratio.
Standard XPAG bore, 1250cc, S1 CR = 8.63, S2 CR = 9.34.
+ 0.020", 1270cc, S1 CR = 8.75, S2 CR = 9.48.
+ 0.040", 1290cc, S1 CR = 8.86, S2 CR = 9.6.
+ 0.060", 1310cc, S1 CR = 8.98, S2 CR = 9.73.
Hope the info helps.
(If you have bored out by +0.1" to give 1350cc, S1 CR = 9.23, and S2 CR = 10).
R A WILSON

The http://ttalk.info/WKF-Wood-XPAG-Engine-Book.pdf link is fine, but you have to copy/paste it into your browser. The 'Enthusiast' programming doesn't like to send things out of its realm. Bud
Bud Krueger

This thread was discussed between 10/09/2016 and 13/09/2016

MG TD TF 1500 index

This thread is from the archives. Join the live MG TD TF 1500 BBS now