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MG MGB Technical - Camshaft Differences 3 Main and 5 Main

Does anyone know the difference between the camshaft of a three main and a 5 main engine? Specifically, and realizing that the lift and timing are the same, are there any differences other than the tachometer drive gear and key way for same? is the bearing spacing the same? If everything is the same other than the tach drive gear, can a 5 main cam be used in a three main engine with out the tach gear? Could a 5 main cam be modified to accept the tach drive gear with the addition of the keyway, gear and clip?
James Johanski

Yes, you can use a 5 main cam in a 3 main engine.

And vice versa, althought the machine work to install the tach drive gear would be a bit expensive.
Bill Spohn

Bill, Thank you for the response. So, conversely, could a 5 main engine then be machined to accept the mechanical tach drive? There is some sort of vestigal boss althought at the wrong angle in place of the tack drive casting boss. Don't know what this is for but there appears to be enought meat to provide a landing for the tach drive hardware.
James Johanski

If you are going to fit a camshaft from a five main bearing engine, then there are a few things that you need to know. It is not commonly understood that the B Series engine underwent changes to its valve timing during the course of its career in the MGB. Originally, the engine used a duplex (double row) roller-type camshaft drive chain and sprockets. The specifications of this camshaft (Original Equipment Part # 88G 303) were:

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Intake Valve 16 BTDC 56 ABDC
Exhaust Valve 51 BBDC 21 ATDC

This camshaft had an intake a lobe center of 110 ATDC and an exhaust lobe center of 105 BTDC. The LSA (Lobe Separation Angle) was 107.5. The camshaft was retarded @ 2.5 ATDC.

In October of 1972 the camshaft drive chain and sprockets were changed from a duplex (dual-row) to a Simplex (single row) drive system on the North American Market 18V-672-Z-L and 18V-673-Z-L engines. While the camshaft design was unaltered, the keyway of the sprocket (Original Equipment Part # 12H 4200) was advanced another 2.25 in order to increase midrange torque output.

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Intake Valve 18.25 BTDC 58.25 ABDC
Exhaust Valve 53.25 BBDC 23.25 ATDC

In December 1974, Rubber Bumper cars for the North American Market with 18V-797-AE-L, 18V-798-AE-L, 18V-801AE, and 18V-802AE engines received a new camshaft (Original Equipment Part # CAM 1156). The specifications of this camshaft were:

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Intake Valve 8 BTDC 42 ABDC
Exhaust Valve 54 BBDC 18 ATDC

This revised, shorter-duration camshaft had an intake lobe center of 107 ATDC and an exhaust lobe center of 108. The LSA (Lobe Separation Angle) remained 107.5. The camshaft was advanced to .5 BTDC.

Starting during the month of June of 1977 cars with 18V-846-F-H and 18V-847-F-H engines for the UK Home and Export markets (not North America) received a new camshaft sprocket (Original Equipment Part # ) that further advanced the camshaft timing by another 1. The specifications of this camshaft were:

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Intake Valve 20 BTDC 52 ABDC
Exhaust Valve 55 BBDC 17 ATDC

The intake lobe center was 106 ATDC, the exhaust lobe center was 109 BTDC. The LSA (Lobe Separation Angle) remained 107.5. The camshaft was advanced to 1.5 BTDC.
Steve S.

Steve, Thanks for the very interesting information, but it completely missed the question--Would you know if the MGB block can be machined to take a tach drive?
James Johanski

James: The bosses were still on the 5-main block for the tach drive, but you would need a 3-main block for the machinist to be able to get the correct angles so that the 5-main block could be machined. If your intent is to use a 5-main block in a car that has a mechanical tach, you would be several hundred dollars ahead by converting to an early electric tach.
John Perkins

I agree with Bill and John. I used a 5 main block in a pull handle car years ago and swapped the faces and pointers between my mechanical tach and an electric. There's always the option of using an early Sprite, etc. tach drive generator if you can get the pulley sizes and ratios worked out.

John, There is a boss of sorts in the area of the tach drive on a five main engine but it is vertical on its face as opposed to angled as on the 3 main. It looks like a boss for a tach drive but the angle certainly would not accomodate the three main tach drive housing. John Twist feels that it would be too difficult for a machine shop to machine the correct facings.
David, While the tach faces for the mechanical and electric tach are the same diameter the spread on the two small mounting screws for the eletronic tach is wider. To mount a mechanical tach face would require drilling two additional mound holes in the mechancical tach mount face. Certainly, this is easy enough to do but will leave two small extra holes in the tach face. These would be close to the needle hub and not very noticeable, but still evident.

What I am trying to achieve is a 1800 cc engine with a good lip oil seal and a tach drive--a real tach drive! Perhaps the best route to go would be to use a three main block with the tach drive and install a 5 main crank to get the rear lip seal. John Twist has done this and feels it is a viable solution. I believe also that Steve in Virginia also mentioned this in an archived thread. Twist also refered me to a company in Allendale MI that makes a kit to install a lip seal in a 3 main engine. I am waiting to hear from them. I will keep the BBS informed of my progress.
James Johanski

The boss on the rear of an early 5 main bearing block was actually for a cam driven fuel pump and is not suitable for a tacho drive.

If you really want the early mechanical tacho, find a 1:1 ratio tacho and take a drive off the crankshaft pulley nut (drill, tap and fit an adapter) and make a bracket to carry the cable outer.
Chris at Octarine Services

If you are not dead set on originality, and are going to be racing the car (otherwise this is a lot of fuss to no good end), and you really want a mechanical tach (I agree - I like them better too), I'll tell you how to do it.

Get yourself a TR-6 distributor, which has a tach drive assembly. Remove the 6 cylinder breaker plate and install your breaker polate from the MGB. Get a suitable length of tach cable and away you go.

If you are just worried about the rear crank seal on the street, don't be. The 3 mains don't typically have much of a problem unless the crank has really gone out and beat the return scroll on the block.

I prefer the 3 main MGB engine for most purposes, BTW.
Bill Spohn

Yeah, you can modify a 5 main cam to take the tach drive gear as others have mentioned.
You can probably modify the block to accept the original remainder of the tach drive gear, or worst case make functional equivalents.

It will be VERY expensive to do so unless you have a lathe, mill and various other machine tools at your disposal.

You'll be way ahead to stick with the unmolested 3 main or fit the rear main kit as mentioned. Otherwise fit the 5 main and an electronic tach.

BTW - the TR6 dizzy has an odd tach drive ratio. It's something like 1.33:1 or 1.66:1 so you'd have to make up a converter box besides. It also is setup for vacuum retard vs. vacuum advance. A while ago I machined and mated two bad dizzys to make one good one: a 25D (MGB) with a 22D (TR6) in an attempt to make a tach drive dizzy for an AH 100-6 (original tach drive stand was destroyed in an accident). That's when I found out about the odd drive ratio.

Tom Sotomayor

Can the Five-main block be machined to accept the drive for a mechanical tachometer? Yes, but I have to agree with what Tom said: "It will be VERY expensive to do so unless you have a lathe, mill and various other machine tools at your disposal." On top of that, you'd need to have the machinist skills of a Journeyman Machinist in order to perform the machining operations required.
Steve S.

Steve and Tom,
Your wisdom is appreciated and noted. I hope my wisdom does not exceed my pocketbook.
James Johanski

This thread was discussed between 01/07/2007 and 05/07/2007

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This thread is from the archive. The Live MG MGB Technical BBS is active now.