Stu,
The Mark II head and the standard head begin live as the same identical
casting. The only difference is in how the casting is machined. The Mark
II head has more taken off the head gasket surface to make the compression
ratio higher, the valve seats are larger in diameter to accomodate the
larger valves, and the diameter of the valve pockets directly below the
seats are machined larger to match the seats. A standard head can be
remachined to Mark II specifications very easily, so your concerns about
damaging a "rare" item are unfounded.
You said four of the valves are recessed slightly. This isn't uncommon,
especially for exhaust valve seats, in these days of unleaded fuel. How
"slight" is the recession? Excessive recession causes several problems.
>From a performance point of view, recession shrouds the valves to some
extent, reducing flow past the valve. From a maintenance point of view,
excessive recession raises the valve stem tip in relation to the rocker
arm, so you eventually run out of adjstment as the recession gets worse.
Also from a maintenance point of view, a recessed seat is shrouded so
doesn't cool as well, leading to faster growth of the recession.
The insert/no insert decision needs to be based on a lot of factors, but
primary considerations are how you drive the car and how long you want the
new valve job to last. If you put a lot of miles on the car, or if you
drive it hard, or if you want it to last a long time before the next valve
job, install inserts. If none of those apply, don't install inserts.
When in doubt, install inserts.
Inserts are available in several outside diameters and several depths for
any given valve size. Your local NAPA machinists may be unable or
unwilling to deal with the narrow OD shallow depth inserts needed, but
plenty of machine shops can. Keep shopping.
On Thu, 1 Mar 2001, Stuart Keen wrote to MG T Digest:
> Question: How risky is it to put valve seats into a MkII XPAG head?
>
> I have the head off my MkII XPAG because I am putting in new valves,
> guides and seals. Today I took the head, with the valves still
> installed, to a NAPA machine shop. The machinist was courteous, honest
> (unusual today) and seemed knowledgeable. He said at a minimum four of
> my valves needed seats, thus all should be done. How did he determine
> that? Four of the valves were slightly recessed below the combustion
> chamber surface. He said this was seat wear, not valve wear.
>
> Being a MkII, the larger valves are right up against the wall of the
> combustion chamber. Virtually no room for seats. He said "shallow" seats
> would have to be installed. He did not have the equipment for this. What
> is the difference between "shallow" and "normal" seats?
>
> Furthermore, he said there was no telling how thick the casting was. To
> him it appeared relatively thin and said it would indeed be a big gamble
> to cut a seat for there was a good chance he might cut through to the
> water jacket or hit a pocket of sand introduced during the casting. He
> recommended I go to another NAPA machinist 25 miles away in the city.
>
> At the second NAPA, I heard basically the same story. The machinist
> there said he could cut into the wall of the combustion chamber in order
> to give room for a seat. However, he again raised the alarm about the
> possibility of breaking through to the water jacket or hitting a pocket
> of sand. He highly recommended that I not proceed with having seats
> installed but that I have a normal valve grinding job along with the
> installation of new valves, guides and seals. He said recessed valves
> would not affect the performance of the engine.
>
> Is this good advice or is it relatively safe to grind seats in an XPAG
> head? The previous owner STRONGLY warned me several times to treat the
> MkII head with kid gloves, because if someone makes a mess of it, I
> would never find a replacement. The MkII is just too rare.
>
> I understand John Twist and some others have installed seats in the
> regular TD XPAG head. Has anyone heard of any horror stories of grinding
> through to the water jacket, etc. Has anyone heard of problems putting
> seats into a MkII head?
>
> For the moment, I am leaning toward no seats. The head is too rare and
> the risk too high. However, I would appreciate opinions from
> knowledgeable and informed TD owners.
>
> Thanks.
>
> Stu Keen
> 51 MG TD MkII
--
Chip Old 1948 M.G. TC TC6710 XPAG7430 NEMGTR #2271
Cub Hill, Maryland 1962 Triumph TR4 CT3154LO CT3479E
fold@bcpl.net
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