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MG TD TF 1500 - Lenght push rods - adjusting screws - valve guides

My early TD (1824) engine needs valve guides and push rods.

The WSM notes two positions for the valve guide in the cylinder head - 24mm for the early and 24,5mm for the late engine. All valve guides for standard cam shaft are the same lenght. Is it advisable to seat the valve guides to 24,5mm in general?

Beginning from XPAG/TD2/17298 push rods are shorter in lenght and the adjusting srews are longer. Except short rods for grinded heads only one lenght is available. Am I right to concider that the optional short adjustment screws are only necessary in conjunction with old and longer push rods and therefore new rods need longer screws?

W_Mueller

You are corzrect. I use 24.5mm for all valve guides, without a problem.

As for the pushrods, I would be curious why they all need replacing. There are few reasons I have encountered: a bent pushrod (usually just one), or loose ends. Why do they ALL need replacing? If bent replace the one, if loose, tack weld the tip back in place.

There is a good deal in the archives on ensuring correct valve geometry.

Tom Lange
MGT Repair
t lange

At specific TD2/TD3/SC2 engine numbers for all XPAG engines, the spring recesses in the cylinder heads were deepened by 0.5mm to prevent the valve springs from being coil-bound at high revs. As heads may have been changed, it is best to measure the recess depth to know how far the valve guides should project. I hope my sketch explains this, with the .054" or .073" depths being the shallowest depth measured close to the plinths on which the rocker shaft posts are bolted.
Regarding pushrods and adjusting screws, if your cylinder head has been skimmed, you may be able to use short rods and screws. The weight saving may be beneficial to valve operation.


R WILSON

Thank you Tom and R Wilson,

I'll give the information to the machine shop.

One push rod is loose and one is bent.
W_Mueller

I did not know that the spring recesses were deepened - I always thought that the reason the guide height was increased from 24 to 24.5 was to avoid spring binding with the .012 cam. That means that on late heads the effective reduction in spring compression is a full mm?

Why not use 24.5mm on all heads, to avoid the problem? It's what I have done for many years, without a problem.

Tom Lange
MGT Repair
t lange

RW: I should have said, for clarity above, "... with the higher-lift .012 cam."

WM: As for the pushrod question, as I weote above I would have the loose one tack-welded back in tight (you will hear an annoying clicking otherwise), and replace the bent one.

I would also be sure you have addressed the reason for the bent pushrod - sticky valve? Over-revving? Sticking lifter? It's not the sort of thing that just happens...
t lange

I have added to my sketch a couple of notes which may help, and my data comes from my workshop manual. The tops of the valve guides do not change position, and the reduction in static spring compression is 0.5mm.
The 168553 cam was in use at that time, but the coil binding problem does not occur with the 120lb valve springs. Thus MG must have thought it would be or was a problem with the 150lb springs, which at that time were only fitted to the TD MkII, so the 150lb springs may either have more turns or thicker wire.
I realise that the spring force is exerted by an inner and an outer spring, so perhaps only one of each pair causes the problem.


R WILSON

By reduction in static spring compression I mean the springs compressed lengths are increased by 0.5mm - however, the valve lifts are unchanged.
R WILSON

Thank you - I now understand.

Tom Lange
MGT Repair
t lange

"R WILSON, Buckinghamshire, United Kingdom - Thus MG must have thought it would be or was a problem with the 150lb springs, which at that time were only fitted to the TD MkII, so the 150lb springs may either have more turns or thicker wire."


Ron is correct based on my measurements and calculations when looking at various TD camshafts. These are my coil-bind measurements when looking at new camshafts.

Coil Bind Measurements Of Early 120 pound And Late 150 Pound MG TD Springs:

As a refresher, the late stock AAA3096 cam has the following specifications:

0.327-inch is gross valve lift and 0.315-inch net valve lift (gross lift – valve lash)
0.012-inch is the recommended valve lash
1.847-inch is the stock closed outer spring installed height.

The early 120 pound spring pack (installed inner and outer springs) coil binds at 1.340-inch spring height and the late 150 pound spring pack coil binds at 1.400-inch spring height (late springs are heavier gauge wire). The inner spring uses a stamped sheet metal centralizer and it coil binds before the outer spring; so the outer spring might appear to have slight coil clearance at coil bind.

Stock rocker arms have a 1.491 inch ratio by my measurement when lash is included.
WHT

Very interesting numbers. If I take the 1.847" spring installed height and compress by the 0.327" valve lift, the spring compressed length becomes 1.52". Thus it does not appear that either the 120lb or the 150lb springs become coil-bound.
If the 1.847" installed height is for a later cylinder head with the extra 0.5mm recess, then for an early head the installed height becomes 1.827". The spring compressed length then becomes 1.5", so again no springs will become coil-bound.
Perhaps MG were worried about something that did not happen, or there was a different reason, or have I misunderstood the information.
R WILSON

The old springs have some light spots and the machine shop recommends buying new parts. When buying new springs, I did not see any option for early and late items. Only FTFU makes a difference (423-410 for the TB to TF, and 423-420 for MKII and TF). The springs MOSS 423-420 are unknown at MOSS. MOSS does not mention any hints while selecting 423-410.

If I sum up your information, would it be necessary to deepen the valve seat, as depicted in the picture by R. Wilson, if the springs come with a high of 1.400" ? I guess the worst case is when springs block.

Or shall I just use the old springs in future?




W_Mueller

I do not know why Moss and some other suppliers do not offer the 120lb and 150lb springs - perhaps they only supply the stronger springs. I don't know who FTFU are, but if 423-410 are the 120lb springs, they were not fitted to the TF. Perhaps Moss do not offer any hints (warnings ?) because as we have found, neither springs actually become coil-bound on the early heads.
When I bought my TD it had an early oval water hole cylinder head, but with the larger valves and 150lb springs. Coil bound springs would have resulted in broken rocker arms, but I don't remember ever breaking any.
You could assemble one valve with springs on your cylinder head, and measure the installed height. You can then use your valve spring compressor to screw down the springs until they become coil-bound, then remeasure the spring height. Then calculate the difference, and if this is greater that the valve lift by 0.1" or more, then as suggested by the information above, you have nothing to worry about. For valve lift you can use 0.327" as given previously, or you can measure the lift on any lobe of your camshaft, then multiply by 1.5. Ignore the valve lash or clearance.
Regarding the use of old springs, how old are your springs, and how many miles have they done? I now use 120lb springs, and the inner sleeves have been shortened as per MG tuning data. The springs have been in for at least 100000 miles over 25 years, and been dismantled and re-assembled several times. I don't rev the engine to a high RPM, and there does not seem to be any valve bounce. I do accept that others may consider my use of old springs to be sacrilege.
Hope the above helps.
R WILSON

Should have said - do you know the original free lengths of the springs - can you ask the supplier?
R WILSON

Moss USA lists two different spring sets (they all come from XRN in the UK):

423-410 for TC-TD (120#)
423-420 for Mark II - TF (150#).

I use the lighter springs on all my rebuilds. I only use a 24.5mm guide height, and have never had a problem of any kind.

Finally, I also frequently re-use old springs, after measuring their free and compressed lengths. If necessary I use shims underneath to return them to their full pressure. If the springs are wildly different I replace them, but have had no problems re-using old ones.

Tom
t lange

Thank you

It seems the valve springs are very reliable and can be left in the head.

Whats about rocker arms? The rocker shaft has some worn and will be replaced - the machine shop wants to mill the faces of some rocker arms. Will that work with that material or will it become soft?

W_Mueller

The correct way to fix the rocker tips is to weld them up with wear-resistent Stellite rod, and then re-grind the tips back to the proper aspect. If you just grind the existing tips you will grind thru the hardening, and they will be ruined in short order.

If you grind the existing tips you will also upset the rocker geometry.

Tom Lange
MGT Repair
t lange

There has to be a danger of doing that, sure enough. What you usually have on the end of the rocker arm is an indent from hammering on the valve. I took the view that this indent was caused by impaction rather than wear. It therefore followed to me that the hardened material was still there and what I had to do was grind back the surround so that it was flush, thus retaining the hardened material and removing the indent. This pre-supposes that the arm and valve will remain in the exact same alignment, and that I managed to retain the correct shape, of course. However, was this a flawed approach? That is for a car that is not raced, and runs a relatively low mileage, as many do. At least I can set the valve clearances accurately now.
Dave H
Dave Hill

Thank you Tom for your advice!
W_Mueller

I used a whet stone to follow the original shape of the rocker pad until the wear was 99% gone. This was 18 years ago and about 60,000 miles ago. Still looks the same as they did then.
Len Fanelli
Abingdon Performance
Len Fanelli

Len - also a good advice. That way you stay in the hardened material.

Are new rocker arms in a similar good condition?
W_Mueller

R WILSON...

FTFU is www.fromtheframeup.com...primarily for the TA, TB and TC group but engine parts are interchangeable for the most part.

Gene
Gene Gillam

W NEW rocker arms are replacement and usually the pad that hits the valve stem is SO thick that you barely get an adjustment when The adjusters are backed out to the end of the threads. I would source used ones in good condition if needed
Abingdon Performance Ltd.
Len Fanelli

Thank you Len

it is hard to find such used items in Germany. I guess I'll give MOSS Europe a chance. They offer 90 days for return and refund.
W_Mueller

They sometimes turn up on ebay uk, though caveat emptor.
Dave H
Dave Hill

This thread was discussed between 27/11/2018 and 05/12/2018

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