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MG MGB Technical - MGB 72-74 FACTORY specs needed

Does anyone have a copy of the 19725, 1973, or 1974 factory service manuals that British Leyland put out for each of those years? I need a copy of the spec's page which deals with the inlet/exhaust valves and the valve guides. Specifically, I'm looking for the specs for those years, the years with the larger inlet valves. The Haynes manual, the Bentley manual, and the BL factory manual that covers most years, all conflict on these specs. I need CONFIRMED numbers for:

1. How much the inlet valve guide sticks up out of the head.
2. How much the exhaust valve guide sticks up out of the head.
3. The angle the valve seat is cut where the valve touches it (45° or 45 1/2°)

Second, what is the consensus on valve stem to valve clearance when using the manganese/bronze valve guide. Moss is saying you should use .003", but others say it should actually be LESS than standard because the m/b guides don't expand as much as the original iron guides. SO much contradictory information out there!

:)
- Paul B.
P. Barnes

I think you need to talk to Peter Burgess!
Dave O'Neill2

I will have to confirm fitted heights of guides when I get to work. We fit in and ex at the same height rather than the two different factory heights.....except for standard race heads.

Are the Moss guides from UK manufacture via Moss USA or are they USA made ones? If UK Moss ones we ream to 11/32" ID same as standard guides. If not UK manufacture then you have to go with what Moss USA tell you.

With regard to clearances please note ex valve guide clearances are usually half to one thou more than the inlets, this is usually achieved by having a smaller stem OD on the ex valve than the inlet stem. That is why it is usually best to set a specific ID rather than valve to guide.

Seat angle is around 45 degrees. Some folk use 46 as the 45 on the valve settles in quicker. As long as the seat blues/laps in ok it doesnt really matter what the exact angles are. Think in terms of grey rather than black and white and all will be well.

I hope this helps.

Peter
Peter Burgess Tuning

Thank you Peter! Yes, the USA Moss catalog notes next to both the inlet and exhaust manganese/bronze guides that you MUST use .003" as the clearance number because those guide expand more than the iron guides. Oddly, (may be not so....) I've seen exactly the opposite, equally strenuously noted, in other places! So what I want to get is the data from one of the one-year-only BL factory service books. I know that the 1972, 1973, and 1974 books will all have the same data in terms of the valves, but I can't seem to find any of those books on the net. I want to walk into my local machine shop with a copy of the "valves and valve guides" section from one of those books so I can simply say, "Here, do it like this!"


- Paul B.
P. Barnes

It's not unkown for bronze guides to nip up on the valve stem, so I certainly wouldn't give them less clearance.
Paul Walbran

Have a look here - it will take a while to download.

http://www.octarine-services.co.uk/opencart/mgb.pdf
Chris at Octarine Services

The worksop manual is irrelevant if you are not fitting OE guides.

Peter
Peter Burgess Tuning

- "The workshop manual is irrelevant if you are not fitting OE guides."

I would agree with that, regarding the valve to valve guide clearance. But the other spec's I'm after, I would still like to get those if possible. I don't understand the reason for the changes in the valve guide height measured above the head ("early cars" - inlet = 5/8", exhaust = "5/8", "later cars" - inlet = 3/4", exhaust = 5/8"). Why the change? The only thing that I can see is that the inlet guide changed to a longer one, so to limit (or, to retain actually) the amount of guide sticking down into the inlet path in the head, they moved the inlet guide up more. Also, the angle of the contact cut of the valve seats gets changed from 45° to 45 1/2°. Why? Especially since the angle on both valves stays at 45°! Hmm...

- Paul B.

P. Barnes

Paul You do see .5deg variation in seat to valve angle in many workshop manuals over the years and it was recommended way back when I did my apprenticeship. It is done to aid the quick bed in of the contact area as it starts off as a line contact. It is done in OE manufacture but is not practical in rebuilding as to be that accurate would be more good luck than skill and in a workshop the valves are usually "lapped in" to be sure of a good seal. Contact seat width is important with inlets about .060" and exhaust about 50% more for heat dissipation. Denis
Denis4

I have been busy but hvae now had time to measure.

we fit in/ex at 0.650"

Factory meant to be 0.750" inlet and 0.625".
OE unused 2709, still in wrapper from factory, in 0.730" and ex 0.610". Pretty close to the book considering these would probably have been fly pressed at the factory and the heads would vary a little against the preset on the presses.

Peter
Peter Burgess Tuning

Thanks you both Denis and Peter! From the factory workshop manuals, it seems the earlier cars had both guide heights at 5/8" (.625"), but later cars got a slightly longer inlet guide, and then that guides height was set to 3/4" (.750"). I THINK that the new guide came in with the 72-74 year/head with the larger inlet valve; maybe they felt it needed more support along it's length. Yet it's set slightly higher in the head to give it an equal amount of intrusion into the exhaust ports as the earlier, shorter guide.
Peter - thank you very much for taking the time to pull that OEM head out and measuring the guides for this clueless American.
:)
- Paul B.
P. Barnes

Just stripped a brand new 1622 Morris oxford head. Guides fitted level and inlet guide length 47.6mm as per later B specs and ex guides 56mm as per standard. Looks like they fittd whatever they had in stock!

Peter
Peter Burgess Tuning

This thread was discussed between 04/07/2013 and 17/07/2013

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