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MG TD TF 1500 - new valves & seats

New valves and guides are on their way. Old valve angle seats are 30 degree. I have to ASSUME new ones are the same (anyone know different??). That makes seat insert selection mighty thin.

This head happens to be a "round hole" with smaller valves. It appears one cylinder was damaged and someone repaired it with an exhaust valve seat insert and it looks like crap; probably too loose and was then staked all around with sharp edges of seat protruding up. It ran OK, but will now be running supercharged, so sharp edges are not desirable, especially at the exhaust seat. Fortunately, the larger valves are a logical option. As it will be supercharged, I would've been happy with the small valves, but that one chamber can stand the larger valve & seat to doctor it up, so it's large valves for all.

Moss shows 34mm & 36mm for exhaust & intake. I'm considering undertaking seat installation myself and searching for seats. I realize Brown & Gammons offer $$$$ inserts, but there are no specs. As it appears the iron under the valve seats is pretty shallow, I'm concerned about the depth. One option is bore a shallow cavity and stick in a tall 45 degree insert, then machine it down flush with 30 degree angle. That may be easier said than done, depending on the new material!

There's a short list of vendors I've checked into, but 30 degree inserts are scarce.

Could just turn it over to the local automotive machine shop, but half the time when parts come back, they weren't done to my satisfaction and I regret not doing it myself.

Anyone have experience in size, insert source and methodology?
JRN JIM

25 years ago I redid my engine. (It just ran for the first time this summer). I had new bronze guides and valve seats put in. When I got the head home from the shop one or more of the seats were loose.
I needed to put the engine together at the the time and did so,

Several years ago I took the head off and sent it out to a specialist. He had custom seats made and installed them. He also took a chip off the head and verigied that tht the valves mated properly. ($400) I now hve confidence in the head rebuild.

I wound not go to a run-of-the-mill shop. Look for a shop that specializes in heads.


Jim B.
JA Benjamin

Headworks in Bloomington MN. one of the best in the country.
Tom Maine

...my local 'Hot Rod' machine shop did mine... I provided the valves/springs/guides, they did the rest... (and I get a free dyno ride as well)

gblawson(gordon- TD27667)

That's a beautiful head but I really thought the image was going to be you in the TD on a chassis dyno - so dissappointing :-)

Jud
J K Chapin

When I had my big valves fitted the machinist cut a three angle seat (please don't ask which angles) to improve flow.
For the same reason the inlet valves were 'unshrouded' by removing material in the combustion chamber. See attached photo.

Willem van der Veer

If you look closely you can see the three angles on the seat

Willem van der Veer

A good machinist makes multiple cuts, some say they make 5 to 7, but 3 specific cuts are in the immediate area of engagement, so the machined mating surface of head engages the valve seat pretty much in a well defined band in the middle of the valve seat. It is a compromise of sorts. Heat transfer is in a critical balance.
Mating closer to the center of the valve where it is thicker allows better heat transfer, but that leaves the outer exposed edge subject to higher temperature. This is also why there must be a thick edge rather than a thin, sharp edge, to keep the temp down, thicker reduces the relative surface area. Moving the mating surfaces out closer to the edge will protect outer circumferencer better, but leaves the center hotter.
The head seat accounts for approx 3/4 of the valve heat removal & stem roughly 1/4. An overheated center will transfer a disproportionate amount of heat up the stem and along to the guide, which won't be a good thing!
The narrower the band, ie. mating surface, the higher the pressure, for better seal and heat transfer, but too narrow can cut down the overall conduction, too. I see some still mention making a 1 degree interfence angle, as grinding the valve 45 and the head 46. That makes for very high pressures where they make contact and the valve will yield/conform quickly to form a sweet contact patch.
Interesting the XPAG/XPEG valves are 30 degrees versus the more common 45 cut. Some rave about 30 as high performance and gas flow. Unshrouding helps direct gas flow for higher performance, too, but to me it is more important to improve the flow as a valve cracks open, as I learned with steam; wiredrawing can occur with high velocity hot gasses blasting through. Personally, I like the steeper 45 cut because the valve wedges down tighter with more pressure for better integrity.
To each his own.
JRN JIM

This thread was discussed between 20/12/2013 and 21/12/2013

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