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MG TD TF 1500 - XPAG cylinder head - useable thickness used head

A mate offered a used XPAG head for TC or early TD. As far as I know the thickness should be 3,018". What would be an acceptlable thickness for further use without getting in trouble with push rods or valves?
W_Mueller

http://www.mg-cars.info/mgtd-mgtf1500-bbs/td-cylinder-head-thickness-201609100919151370.htm

Tim
Timothy Burchfield

I have a memory that a stock head is 3.022", but we are close. Stock CR was 7.3:1. See the factory Special Tuning handbook for original details.

It really all depends on what you want to do. For a reliable street machine, the lower the compression ratio the better. For a bit more power, bump the CR. For a supercharged engine, the lower the better, and not above 8.3:1.

Removing 1/16" gives you 8.1:1 which is still fine, removing 3/32" gets you to 8.6:1 (Stage I), and removing 1/8" gives you 9.3:1, Stage II. I really don't like to go much above this.

And, of course, the larger the bore size, the higher CR also.

Tom Lange
MGT Repair
t lange

Depends on what camshaft you are using;
Static Compression Ratio
(SCR) Intake Valve Duration
(degrees @ .050" lift) Power Range
(RPM)
8.00:1 185º Idle-4,000
8.25:1 189º Idle-4300
8.50:1 194º 800-4,500
8.75:1 200º 900-4,600
9.00:1 204º 1,000-4,600
9.25:1 208º 1,200-5,200
9.50:1 212º 1,600-5,400
9.75:1 216º 1,800-5,600
Len Fanelli
Abingdon Performance Ltd.
Len Fanelli

Your answers are very special because you are the very specialists.

My conclusion is it should be not less than 2.96" for road use and have some reserve. Is that okay?


W_Mueller

My mentor instructed me to always leave room for the next time, and there will be a next time.The same " Next time" also goes for installing paper gaskets, glue or contact cement on one side and grease on the other side! Next time it will come apart easily & can be reinstalled using the same gasket if necessary!
Len Fanelli

I accept this wasn't the question, but the Scientific Publications TC, TD & TF workshop manual states in the Special Competition Tuning section that the absolute minimum thickness is 2.8966", and this applies to the oval and round water hole cylinder heads.
R WILSON

If your head CR is too high, there is a good way of lower it, while improving power. You can grind out the combustion chambers to simulate a Laystall cylinder head combustion chamber shape.

Grind only near the inlet valve, on the opposite side to the port and stop grinding at the plug hole. A nice radius is the aim. Don't grind further than level with the cylinder bore. Also radius "undercut" the walls at the ends of the combustion chamber. This mod is mainly beneficial with TF size valves because it helps overcome valve shrouding.

With a standard head, the bulk of gas flows across the valve (due to inertia) and hits the opposite wall, which is close to the valve. When modified, the gas has more space and hits a nice radius to direct it down the bore. Some gas can also exit the valve when it is partially open, via the undercuts. The reduction in CR is obviously dependent on the amount of metal removed. I did this to my road TC in about 1970, and to my brother's car a few years later. My road car head has 41 cc chambers and has had 1/8" off the face.

I can't find the photo I want, but have included an image from when I "dry-decked" of my brother's engine, which partially shows the re-shaped chambers.

Bob


R L Schapel

Bob, IMO I do not think the undercut is needed or warranted when high lift cams and or Roller cams especially are used, as the valve lift is more than .100" more that a stock cam.This being said un shroud the chamber out to the edge of a used head gasket, as shown with MY 37,5 & 35.5 MM custom swirl polished valves. (38 CC chamber)
Len Fanelli

Len Fanelli

What price should I pay, if the thickness is okay?
W_Mueller

It all depends on the condition of the head. What is the thickness? Is it as-is, complete with valves and springs, or a bare head? Is it a rusty mess or clean? Has it been magnafluxed? Has it been rebuilt with good valves, guides, seats, a valve job and new springs?

Over here a bare "door-stop" head in unknown condition sells for $100-150, with no guarantee. Depending on its configuration, a complete but original head can be between $250-450. I sell fully-rebuilt and ready-to-bolt-on re-surfaced heads with new valves, guides, springs, seats as needed, a good valve job with hand-lapping and seals for $1,000 more or less, depending on whether it is a banana head, round-hole head, TF head, small- or big-valve head, and the all-important thickness. I often have $650 invested in parts and labor to re-do a head.

I have learned not to trust someone-else's "fully re-built" head, because I always dismantle and check everything, and am almost always disappointed in some aspect of the workmanship. I have found some valves with little meat left, or used springs that were not even checked for tension, or shimmed. I found heads where the o-ring seals had been nicked in installation (or incorrectly installed); I found that 2 valves in a "fully-rebuilt, ready to bolt on" Laystall-Lucas head had nicked stems where the valve-grinding machine kissed them, creating a weak point which certainly would have broken, resulting in major engine damage to cylinder, bore, and head. So caveat emptor...

Tom Lange
MGT Repair
t lange

Thank you Tom,

I've forgotten to say that it is only a bare banana head for TC or early TD as a spare, which is not clean but with little surface rust. In this case the thickness is not sufficient. So it is not a good deal.
W_Mueller

Hi Len,
So you suggest un shrouding equally near the exhaust valve too? I have never done that, although I see no disadvantage providing CR is not reduced too much.

If the head in the image is 38 cc (presumably with the "maximum" off the face?) it shows how much I have removed from the chamber if I only un-shroud near the inlet (and undercut) and I have 41 cc chambers!

My method is really a copy of the Laystall. Wouldn't it be great to be able to test lots of different ideas! I guess the factory competition guys would have got a real buzz out of that sort of thing.

On a different topic, what is your opinion of the AEG122 cam? I have just written an article for the local club magazine in which I point out its similarity to a standard TC cam? (Approx only 2 degrees different on each number although slightly higher lift.) The factory suggested a modified exhaust with AEG122 so I figure that also applies to a TC cam? I know you specialise in roller cams etc, but I am not trying to compare with those.

Cheers from South Australia,
Bob
R L Schapel

I just skimmed the thread, so pardon if it's been mentioned already, but stock thickness is 3.022" on all XPAG heads. To the best of my knowledge, 1/8" is considered the most you want to shave on a road engine. To do more would likely require a lot of custom work.
Steve Simmons

Steve - thanks for confirming what I said above. My only comment is that Mark II heads came with higher compression ratios, and had thus been factory-shaved to less than 3.022".

Tom Lange
MGT Repair
t lange

Another limiting factor is the angled spark plug hole. If you skim too much off the head you will cut into the threaded area and the end of the spark plug will stick out into the combustion chamber. When that gets hot it will glow and cause pre-ignition.
Barrie Jones

For my Information. If a head has been skimmed close to the limit, can you correct this with a thicker head gasket.

I have seen companies that sell solid copper head gaskets in various thicknesses.
Bruce Cunha

Yes, you can use a solid gasket, available as you note, in different thicknesses. But it is not an unalloyed solution, since a solid copper gasket compresses very little when compared to a stock composition/asbestos head gasket, and cannot be expected to help seal any slight imperfections. Both head and block must be perfectly smooth, and torque must be applied with great precision. Most old head studs have stretched, so replacement is advised.

Another procedure is to use two head gaskets instead of one. Again, that is not without risks, and the above caveats must also be noted. That being said, I did that to an engine in 1972, and according to the owner, it still has fine compression!

Tom Lange
MGT Repair.
t lange

Tom's point is important. Solid gaskets can only be used if the head and block are VERY accurately matched. I use solid gaskets on my race TC but I lap the head to the block with coarse valve-grinding paste. It takes a long time to do with an alloy head and even longer with a cast iron head! A bit of a procedure with bottom end intact... I use masking tape in the bores/pistons and masking tape/plastic sheet on the cam-follower side. I use bearing blue to check the surface fit. Water galleries are not a problem, particularly when the engine is dry-decked. Accurate, lapped surfaces are still only as good as the accuracy of the solid gasket.

Anyone who has lapped a head to a block will tell you that even the most accurate machining will not initially give a good blue marking. The head will also easily lift off, even after a bit of lapping. It is only after a couple of hour's work, or more, that the head becomes difficult to lift (due to suction) and the blue marking starts to approach a good match.

Bob
R L Schapel

And additionally back in the day the T types raced in the SCCA had a wet deck and no head gasket, to raise the C/R in sprint races. I believe it was due ( in the stock class) of a rule of not milling the head! Oh and when doing a valve job never removing the carbon from the piston crowns or chambers.
Len
Len Fanelli

Fascinating stuff Len. I didn't know that, although here in Australia rules were probably different. I planned to run my current race engine gasketless but decided to use a "shim" to decompress as the CR turned out to be a bit high for the boost.
Bob
R L Schapel

This thread was discussed between 06/02/2020 and 16/02/2020

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