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MG MGB Technical - Crankshaft Pulley

I have a 3" "chip" in the crankshaft pulley, of an 18V engine. This is the pulley with the harmonic balancer.

To avoid the cost of a replacement pulley, can I use a pulley (which I have) from a 1972 engine, which does not have the harmonic balancer?
JR Jim) Ross

You can use the earlier non-damping pulley but you could be looking at vibration at high speed driving.

Used serviceable harmonic balanced pulleys should be plentiful and inexpensive.
Steven Rechter

It's always best to have the harmonic balancer, they are there for a reason. All Bs came out with them, or shall I say that my 1963 B certainly did and every other original B engine I have seen.
Paul Walbran

Later 18v engines - rubber bumper - had a larger diameter pulley, although I'm not sure why.
Dave O'Neill 2

The pulley from the 72 engine should be a harmonic balancer, if it was original. You can tell by the rubber insert.

My understanding is that a balancer was found necessary to stop crankshaft vibration that could lead to breaking.

Herb
H J Adler

Thanks all.

I didn't do a very good job of describing my question. I realize that all the engines had a harmonic balancer, but the one used on the 18V engines had a larger diameter pulley.

So really, I should have asked if it was OK to replace the larger diameter pulley with a smaller diameter one from an older engine. (Gee, it's amazing how much better organized one's thoughts are, if one does the thinking before, rather than after a couple of single malts) :-)
JR Jim) Ross

You'll need to take into account the faster speeds that the water pump and alternator will be turning at. In a perfect world, you should use all of the pulleys from the '72 engine. RAY
rjm RAY

If you reduce the size of the crank pulley, the water pump and alternator will turn more slowly.
Dave O'Neill 2

Dave-
I suspect that the advent of the larger-diameter harmonic balancer (harmonic damper) pulley was due to the simultaneous introduction of the cast EN 16 carbon steel crankshaft with its flared counterweights. It has different damping needs than the earlier forged steel crankshafts.
Stephen Strange

Or the larger pulley made the alternator kick in sooner.

Herb
H J Adler


It must be a belt speed thing--
Maybe the big pulley was introduced to drive air pumps fast enough for polution requirements---??
Just pulled a pair off the shelf and measured them
Surprise the big one is lighter than the little one
Both appear to have the same inner piece but different outers-- the smaller one being chunkier

5" pulley----1393grams
6" pulley----1340grams

Making me think that if there had been a harmonics problem with the cast crank there would have been more weight needed not less

If you use the smaller pulley, I'd be tempted to use the smaller w/pump pulley as well to keep the w/pump up to speed----The alternator should cope ok as is

willy
William Revit

Willy, I haven't delved much into the parameters of harmonic balancers, but would expect the effect to be related to moment of inertia as well as mass and if so, as MOI varies with the 4th power of diameter, there would be a significantly different effect from a larger damper of similar mass.

One thing to watch with the water pump pulley size is cavitation, which has been known sometimes to be an issue at very high speeds. A street engine is unlikely to be worried about it.
Paul Walbran

Usually, cavitation is kept in check by pressurizing the cooling system. It occurs most often at the inlet of the water pump. RAY
rjm RAY

Paul-
Not only that, but the characteristics of the elastomer employed would play into it as well. I doubt that the factory would have complicated its parts inventory and cataloging by employing a different harmonic damper if its use on the cast crankshaft wasn't warranted.
Stephen Strange

All good
But I meant to put the smaller water pump pulley on with the smaller crank pulley to make a pair--
If the little w/pump pulley was used with the larger crank pulley,- yes , in that situation you would expect some cavitation at higher speeds

Paul/Steve
Yes I agree with the theory of larger diameter mass being a factor, but this pulley is significantly thinner on the outside compared to the little one and really, 'just looking' at it there wouldn't be much in it
I'm sticking to the need for speed for the air pump theory--------------------- for now--

willy
William Revit

Or ..... IIRC the larger pulley was also used on the Marina, probably before the B. So there could be an unrelated spurious reason (a clearance issue on the Marina for example) and then the accountants stepped in. These things did happen! Witness the retrograde step to single timing chain at about the same time ...

OK, that's my conspiracy theory session done for the year!

Pay, agreed, but it also depends on impeller speed and local coolant temperature. Most of the time it's OK, but I've certainly seen occasional impellers with cavitation damage on engines that work hard.
Paul Walbran

Paul-
There is an explanation for why the larger-diameter harmonic balancer (harmonic damper) pulley wheel was used on the Marina engine. The cast steel crankshaft with the flared counterweights was also used on the Marina engine! It did use a smaller-diameter crankshaft spigot pilot bushing and a larger, heavier(28 lbs) flywheel that is 1/4" (0.250" / 6.35mm) thinner and so much larger in diameter that it will not fit inside of the bellhousing of the four-synchro transmission that is used in the MGB.
Stephen Strange

This thread was discussed between 21/10/2016 and 23/10/2016

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