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MG MGB Technical - Which Timing Chain

I recently purchased from a local classic specialist a Low Comp 5 Bearing suposidley reconed B engine to put in my supercharged MGA. After 1400 miles I have had to change the clutch.To do this I found that I practicly had to remove the engine so I thought that I would use this opertunity to change the front seal of the timing cover as it had started to throw oil a bit.
I must say that I was quite supprised to find when I got the cover off that it was fitted with a single (simplex) sprocket and chain set up. As I have no idea what engine I have as the number seams to have nothing to do with BMC. The chain tensioner appears to be the wide type and would definatly stradel two sets of chain.
MY QUESTION. Dose any one know what engines had Simplex set up. are there huge advantages for the Duplex setup. Should I invest in a duplex set or should I go the whole hog and get a vernier set. I must say that apart from the oil leak the engine was running fine producing 131BHP using standard timing and the single chain timing gear.


David
David swaine

I believe the 18V engines have a single row chain.

John
J Tait

and yes, all engines had the double width tensioner - even when fitted with the single row chain.

Look on the nearside of the engine under the front of the manifolds, level with the engine mount - there will be a small flat plate held on with two bolts if it is an 18V engine which originally had the simplex chain.
Chris at Octarine Services

Thanks for the info it looks as though I have an 18v unit. which is what I asked for. It dose seem strange that the later engines only had a single chain and the earlier engins had the duplex It would seem that BMC decided that the single was suffficent. any comment on that.

David
David swaine

David-
In October of 1972 the camshaft drive chain and the drive and the driven sprockets were changed from a Duplex (dual-row) camshaft drive chain system to a Simplex (single row) camshaft drive chain system (Drive Sprocket BMC Part # 12H 4201; Driven Sprocket BMC Part # 12H 4200, Drive Chain 3H 2127) on the 18V-672-Z-L, 18V-673-Z-L, 18V-779-F-H, 18V-780-F-H, 18V-797-AE-L, 18V-798-AE-L, 18V-801AE, 18V-802-A-E, 18V-837-F-H, 18V-846-F-H and 18V-847-F-H, 18V-802-AE-L, 18V-883-AE-L, 18V-884-AE-L, 18V-890-AE-L, 18V-891-AE-L, 18V-892-AE-L, AND 18V-893-AE-L engines.

The Duplex chain has much better wear characteristics, especially when used in conjuction with hot camshafts that result in higher engine speeds. It is a common retro-fit into engines that have the Simplex chain.

Offset woodruff keys are cheaper than a vernier camshaft sprocket, but are a relatively more time-consuming method of maintaining spot-on timing. They're no fun with the engine in the car! A vernier camshaft sprocket is a good idea for easy timing of the camshaft, especially in engines that will wear their camshaft drive chains more quickly due to a hot camshaft. I have one on my car, and I'm glad that I spent the extra money for it.
Stephen Strange

"It would seem that BMC decided that the single was suffficent. any comment on that."

Given the dozens of years, tens of thousands of cars and millions of miles it seems they were right!
Paul Hunt 2010

Steve More confusion! as I am running a standard cam and the engine is supercharged which is low down power rather than revs I guess the single chain will be ok but I have no wish to put everything back into the car and find the chain rattling in 10 thousand miles I tend to take Pauls view.
David swaine

I have two MGBs one has a duplex on a 18v and the other a simplex on the 18v. Quite frankly I've never noticed any difference between the two and the simplex does not seem to have worn as it is completely silent. I would be inclined to just replace the simplex chain which would obviate the need to retime the valve gear because from memory the timing is 4 degrees different with the duplex and you would need to retime with stepped keys or use a much most costly vernier set up.
Iain MacKintosh

Iain

An't this webb site the best. If It was'nt in existance I would have never have found out that I would have had to retime a Duplex arangement. As I have no wish to retime the engine as it is performing fantasticly at present I will rexamine the single chain and teeth for wear and change the oilseal and re assemble.

I do have one more question the pictures in all the parts catalogues show a curved saucer shaped oil thrower. the one I am looking at is flat and has F stamped on. Is that correct for the 18V engine.

David
David swaine

The flat thrower is indeed correct for the 18V and of course the F means FRONT.
Iain MacKintosh

I have never found the 4 degree difference on UK engines - at most 2.5 degrees.

However, when fitting any new cam/sprocket/chain the cam should be timed - not necessarily to the manufacturers timing either, you can shift the power band up and down the rev range to suit your style of driving by advancing or retarding the camshaft.

As a matter of interest, I believe the duplex chain & sprocket absorbs more power than the simplex one - not a lot, but if you are blueprinting an engine to extract the last ounce of power - especially under some racing rules that do not allow modifications then the simplex chain may be the better choice.

For road cars it really doesn't matter a jot.
Chris at Octarine Services

Chris,

“you can shift the power band up and down the rev range to suit your style of driving by advancing or retarding the camshaft.”
Which direction does what, and by how many degrees?

Richard.
RH Davidson

Thank you every one

David
David swaine

RH-
You can consider etarding or advancing the timing of the Original Equipment specification camshaft a very few degrees (4 degrees maximum, beyond that point the gains are increasingly small while the losses become increasingly excessive) in order to respectively move the power up or down the scale as much as 400 RPM.

Ian-
The pitch (3/8”) and number of links (52) in the two chains are the same. The sprockets both have the same even number of teeth, 20 teeth on the camshaft drive sprocket on the crankshaft drive sprocket and 40 teeth on the camshaft driven sprocket. The timing change associated with the simplex chain was the result of a change of the position of the keyway for camshaft driven sprocket.
Stephen Strange

Advancing the cam shifts the power or torque down the rev range, retarding shifts the power up the rev range. Some engine builders will advance the cam to make the car "feel" more responsive, but what you gain at one end you lose at the other and as Stephen said if you overdo it even a little the loss can far outweigh the gain.

Denis
Denis4

This thread was discussed between 14/06/2010 and 19/06/2010

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