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MG TD TF 1500 - Correct length Pushrods for a Late TD

OK - SO I'm confuded.
I emailed to Smith Brothers and they say they do not have any TD pushrod in stock and ask I send one to them so they can duplicate it.
The trouble is I am a bit concerned as to the correct length. I may have rods that are too short.
One - The followers were not rotating. (If that has any to do with it)
Two - 4 of the 8 rods look to be slightly bent
Three - At 8 3/8th" overall length these are shorter than the rods sold by Moss for oversized lobs or head with reduced thickness.

Moss Quote:-
NOTE: Early TC-TD pushrods had an effective length (ball seat to ball end) of 8.681”, the later TD-TF
Pushrods had an effective length of 8.62”. When a cam with oversized lobes or a cylinder head with
reduced thickness are used, our short pushrod set with an overall length of 8.531” will suit.

Neither of which describe my engine I believe. Lift on the lobe is only 0.237" at best So it is not high lift.

There were also 4 short tappet adjusting screw which were on the end of the adjustment and are being replaced with longer ones now.
The engine was worked on in England in 1990 and I believe the cam was then replaced with a touring cam???
When I built the engine in 1974 all the adjustment screws were the same length? So I am at a loss to understand it now.
With the pushrods I have installed now - the tappet adjustment is about centered on the longer adjustment screw when set to 12 thou.
Is it safe to say that the 8 3/8th" overall length pushrods will be OK to replace with like and install?

To be fair - I Have to say I did not notice the engine ever running poorly.
Confused.
Rod

Rod Jones

Rod one piece of info you should have in order to get a definitive answer to your question is to determine exactly what the existing thickness of your head is in order to determine just how much meat has been removed. Do you have spacers under the rocker posts? If you have & you feel that the pushrods are too short then removing the spacers may solve your problem. However as you state that 4 of the 8 pushrods appear to be slightly bent if anything this may suggest that they are too long. Spacers were often fitted in an attempt to deal with shaved heads so that original pushrods could still be used. However if the tappet adjustment is centered on the adjustment screw as it is on your longer screws then this would seem to be correct for your head & gasket. It seems strange to have adjusters of different lengths. It may be that your bent rods are the result of the earlier work. In any event knowing the exact thickness of your head would be a good place to start & I believe it's essential in determining the desired length of the pushrods. Cheers
Peter TD 5801
P Hehir

Extract from Neil Cairns "Engines for MGs" might shed some light.

"At SC2/17432 and TD2/17298 the pushrods were shortened to accommodate longer rocker adjusting set screws. The threads were longer, and both rods and adjusters were only supposed to go with each other, not be mixed with earlier parts."

This link will also enhance your research and commence a complicated fact finding mission, dont ask me, I created my own mystery when I fitted an XPAW head on an XPAG block.

http://www.team.net/mharc/archives/html/mgs/1998-09/msg00008.html

Graeme
G Evans

Peter,
The head I do not believe has been skimmed at all. When I measure it - it looks to be 2.865" or 72.77mm but it is really difficult to do it with the head on the engine. It does have the larger TF valves fitted as it was stage tunes when first put together. There are no spacers under the rocker posts.

Thanks for the link Graeme - I think :)
Looking at the ware points on the rockers everything looks OK - position wise. I just wondered about the pushrods being sold by Moss, which are all longer than mine?
Would it have any bearing on the fact the tappets are not rotating?
How do you check for the correct valve train geometry? I did read some ware you want the rocker to contact the Center 1/3 of the valve head - from full open to full close. Certainly the wear mark on the rockers is dead center on the pad. Apart from the one that showed a deep depression.

Rod


Rod Jones

Wow! If your measurements are correct, using the table I've uploaded below (originally from Bud I think) 2.865" is a long way from standard (3.022") for an XPAG block. Cheers
Peter TD 5801



P Hehir

Block should read head.
P Hehir

The table is from Hal Kramer's TSO article. Rod, if that's your head measurement, you're in some serious territory. That's about .150" off of the head, or about .090 off of a MKII or TF head. Might explain your short pushrods. Bud
Bud Krueger

Holy cow? Thanks Bud, Steve, for the info. I thought it was STD??.... When put together in 1974. I purchased the car in bits from a good friend. He did say it was setup as tuned and when I put it together I did no measure the head - just assembled it. It has the larger TF valves & hardened seats (now). It had 1 1/2 carbs and twin fuel pumps, 4 branch extraction exhaust and really went quite quick. Over the years I have put it back to STD and removed a lot of the none STD stuff.
I guess the only real way to confirm the thickness is to remove the head and do a proper measure. Makes me wonder about that compression test I did a few weeks back which seemed rather low.

I measured the head thickness on the engine just above the water outlet in the front. There seemed to be a little overhang there?
Maybe I got it all wrong.
There are no spacers under the rocker towers and the only thing I found when removing the rockers was split washers instead of the bent tab washer at each tower bolt.
I guess I will have Smith Brothers make me a new set of 8 3/8th
Rod Jones

Rod - if I remember the geometry, the rocker is supposed to be perfectly horizontal at mid-point on the cam. I use a dial indicator and measure the total rocker movement, find the half-way point, and see if the rocker is horizontal. That will tell you if you need short, long or standard push rods, or custom ones.

Also, as Mort will attest, push rods are best measured by inserting a ball bearing in the cup, measuring the total length, and subtracting half the diameter of the ball. The cup end is imprecisely machined and notoriously inexact if you use it for measuring overall length.

Tom Lange
MGT Repair
t lange

Thanks Tom,
And sorry I called you Steve Peter :) Don’t know where I was....
Tom, I note the valve stems are not vertical, so do you mean horizontal to them? Not sure if I can measure that correctly.
On the Smith Bros site they talk about roller rockers and stem mounted rockers However, is it correct to say the rocker type we use should be in the same position??? Actuate the valve by applying pressure at the point as near to center of the valve stem as possible. I presume to prevent applying force to the valve guides? Looking at the tips of my rockers the scuff mark is in the center of the tips.
This would seem to be correct.
I take the point about the ball bearing to measure the “effective length”. The Smith Bro's site says to measure with the correct size ball bearing and then deduct the Dia. of the ball from the total length. I think I will just send them one of my strait ones.
I am still trying to get my head round what was done back in the day to make them this short. At least this way the geometry should still be in the right place with no pillar spacers fitted. It never occurred to me the head had been skimmed without them.

Rod







Rod Jones

That's OK Rod. I've been called a helluva lot worse!
P Hehir

This thread was discussed between 16/08/2014 and 17/08/2014

MG TD TF 1500 index

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