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MG TD TF 1500 - Valve and Rocker Geometry

I often hear about the problems of T-type valve and rocker geometry, but want to know more: how to identify problems, rocker angles, the +/- use of rocker stand shims, different length pushrods, and remedies. Can anyone point me to a primer?

Many thanks.

Tom Lange
t lange

Good question Tom...I'm interested also.
Gene Gillam

Tom,

This is a neat digest of all sorts of TD/TF posts from the past: http://www.mgcars.org.uk/mgtfaq/TD-FAQ.htm

Here is an excerpt from the engine section. When you read it you have to realize that several authors are responding at once, in rapid succession.

Quote:


Rapid valve guide wear has always been a problem in the XPAG/XPEG series of engines. Lack of lubricant isn't the problem. There are no valve stem seals in the modern sense of the word, so more than enough oil works its way down into the guides. Lack of proper retention of lubrican, however, is a problem. The real problem is incorrect geometry between the rocker arm and the valve stem. This puts a lot more sideways thrust on the valve stems than there should be, which accelerates wear on the guide bores and on the valve stems.

Ideally when the valve is at half lift, the rocker arm and the valve stem should form a 90 degree angle. That is seldom the case in any production line engine, but even with all new parts the geometry on an XPAG/XPEG is almost always way out of whack. This has to be corrected by either machining the bottoms of the rocker shaft stands or by inserting shims under them, depending on which direction the geometry is off. On an old head that has slightly sunken valve seats due to many rebuilds, the latter is more common.

On a side note, the practice once advocated of shimming up the rocker shaft stands to compensate for a radically milled head is dead wrong. This does give you back a reasonable range of valve lash adjustment, but it throws the rocker arm geometry way off. The better approach is to shorten the pushrods.

Getting the geometry right does improve valve guide life, but it's still far from ideal. The friction surface between rocker tip and valve stem top is pretty small, and the rocker tips aren't very hard (especially if they have been reground to compensate for wear). Once the rocker tip wears, it imposes more sideways thrust on the valve stem even if the geometry is right. What we really need is thicker valve stems or a thrust cap to install on top of the stem. Even better, how about roller-tipped rockers!

end quote

warmly,
dave

Dave Braun

Tom
Do you have the old book "Tuning and Maintenance of MG's" by Philip H. Smith? It is a thorough examination of all things XPAG. I bought one on Amazon, but before that I had downloaded it from the internet. The entire book has been entered there.

Tom
'54 TF
Tom Norby

Tom-
Get your head out of the T series world and go to general race engine building. Pushrods and rockers is all the same so far as geometry, and any big changes require the same steps. That all is the staff of life to race mods.

That little summary from Dave is excellent.

I would add that the contour of the rocker tip is very important and often horribly abused, even on "professionally reconned" rockers. It is actually a cam, and it should be of such a contour that once the basic geometry is right, the contact point is always at the center of the valve stem. This will minimize side loading, and contact pressures on the rocker, AND give max lift.
I once built on overscale model of a valve/rocker/pushrod out of cardboard and pins and investigated this, so I could actually SEE what the shape (should) is. I did not figure out how to characterize this in a transferable way, but I did figure out how to build a machine to form the contour. Still on the "someday" list! A good modern drawing program can probably generate this, but I have more patience and ability with my ancient tools than my new fangled ones.

Refaced rockers should be rehardened if necessary, and you do not need any bigger stems or caps for proper valves (which have sufficiently hard tips).

Roller tipped rockers are of considerable cost and questionable worth, and zillions of engines have gone untold hours without them or any problems they might prevent - but a fair number have died of roller rocker diseases! Mostly Boy Racer Bling.

FRM
FR Millmore

My thoughts, there are a few vairables to consider.
1) camshaft bcr or base circle radius.
2) push rod length.
3) cylinder head height, (how much has been removed by milling).
4) height of the top of the valve stem.
5) the valve stem is not 90 degrees to the head / block surface.
6) total valve lift.
The rocker ratio is 1.38 : 1.
Trial & error is the only way to know, install 1 rocker & 1 valve with a weak valve spring, zero out the lash, measure the valve lift at the valve with a dial indicator.
Repeat this process with feeler gauges under the pedestal.
.025", then .050" or more.
If you find the lift decreasing when adding shims under the pedestal, the pedestal needs to be shortened.
I have found a 2 - 2 1/2 % increase in lift can be achieved by a .025" to .050" or so shim under the pedestal. Maybe.
Len
Len Fanelli

This thread was discussed on 08/07/2012

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.