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MG MGB Technical - Crank

Mojn
18gb crank in 18v engine, can i do that ?
hvat is the difrence?

Harry
HL Lagoni

Harry-
Yes, it'll work inside the block OK, but be advised that both the 10.75” (273mm) flywheel (BMC Part # 12H 1474) and ring gear (BMC Part # 1G 2874) of the 18GB engine were unique to it alone, although its lockplate (BMC Part # 12H 1303, Moss Motors Part # 460-715) and its six mounting bolts (BMC Part # 51K 1809, Moss Motors Part # 322-160) are common to the later, larger-diameter 11.5” flywheels. This flywheel also had three locating dowel pins. The engine backplate is also different due to the positioning of the Lucas M418G inertia-type electric starter assembly (BMC Part # 13H 4561, Moss Motors Part # 140-165) is closer to the crankshaft in order to accommodate the smaller-diameter flywheel. It can be readily identified by its 3.95” diameter hole for the rear seal of the crankshaft. The 11.5” (292mm) flywheel (BMC Part # 12H 2184) of the 18GD, 18GF, 18GG, 18GG, 18GH, 18GJ, 18GK, and 18V engines and its ring gear (BMC Part # 12H 2900, Moss Motors Part # 190-050), along with their Lucas 2M100 pre-engaged-type starter assembly (BMC Part # 13H 6130, Moss Motors Part # 131-220), are all interchangeable, although not with those of the earlier 18G, 18GA, and 18GB engines. These engines had two locating dowel pins for positioning the engine backplate. Interestingly, all of the engines used the same locating dowel (BMC Part # 1G 2984, Moss Motors Part # 325-045).
Stephen Strange

Yes. The GB crank is made of forged steel and has large counterweights, while the 18V crank is flat sided, lighter and made of cast iron. The 18V crank is the preferred unit for racing due to its weight and overall layout. However, the GB crank is extremely strong and should suit any task you ask of it. RAY
rjm RAY

Ray-
There were two crankshafts used in the 18V engine, both being cast iron items. The first was indeed the slab-sided crankshaft and may be found in 18V-581-F-H, 18V-581-Y-H/L, 18V-582-F-H, 18V-582-Y-H/L, 18V-583-F-H, 18V-583-Y-H, 18V-584-Z-L, 18V-585-Z-L, 18V-672-Z-L, and 18V-673-Z-L engines. The second 18V crankshaft had flared counterweights and may be found in 18V779-F-H, 18V-780-F-H, 18V-797-AE-L, 18V-798-AE-L, 18V-801-AE-L, 18V-802-AE-L, 18V-836-Z-L, 18V-837-AE-L, 18V846-F-H, L, 18V847-F-H, 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.
Stephen Strange

Little off topic but I am confused.

Were 18v cranks really cast iron? I have two cranks here that are forged. The one above in the pic I beleive to be earlier than the other, both 5 main cranks. Werent there 3 cranks used in 18v engines?

Id be very interested to see a pic of a GB crank if anyone has one.



JS Ahlund

UK engines

1971-1974 18v cranks forged steel.
74-0n cast iron
pre 18v 5 bearing forged steel differenet shape to 18v forged crank.

Best using forged ones for reliability in racing except standard race class sprints/hillclimbs where std cam used and revs not too high so cast iron lighter crank spins up faster.

Peter
Peter Burgess Tuning

Peter-
Is there any real evidence that the cast cranks are less reliable in heavy service?

It is widely believed that forged steel are better, being "stronger" than cast iron, but modern cast is not (necessarily) the mush of the old days, and has many benefits re inherent damping etc. I suppose a lot of this has to do with what steel and what iron, and how they are processed.

So say the Royal Enfield rider who had to listen to a lot of balderdash when the RE twins came with some of the first nodular iron cranks, rest of world trying to catch up!

I did some work with ultrasonics, and discovered first hand a bunch of interesting effects of materials with inherent damping. We used hardened steel, titanium, or hard aluminum for horns precisely because it does not do this, a requirement of the application - and sure enough, after a while they crack, precisely because they do vibrate (tuned to resonance). The same tool steel annealed, or low carbon steel, or soft aluminum, or cast iron, all damp the vibration and turn it into heat.

FRM
FR Millmore

Hi Fletcher

We have seen a few cast iron crank failures in full race app where the rear main snaps we have been told of a few failures from other folk we do head work for too. This is high power high rpm stuff. I have seen one engine snap a forged crank, we discovered this was due to a ring gear moving inder race use.

I think people would be upset if they found a cast iron crank had snapped in a race engine whereas if a forged one snapped it would be..... wow how unusual!

Interesting what you say about the softer materials damping out/ absorbing vibrations, bit like the reed bending with the wind whereas the 'mighty oak' snapped.

Peter

Peter Burgess Tuning

Peter-
The slab-sided crankshaft of the 18V-581-F-H, 18V-581-Y-H/L, 18V-582-F-H, 18V-582-Y-H/L, 18V-583-F-H, 18V-583-Y-H, 18V-584-Z-L, 18V-585-Z-L, 18V-672-Z-L, and 18V-673-Z-L engines found in the 1972 through 1974 models was indeed a forged item. Thanks for correcting me.
Stephen Strange

Interesting to see what turns up in these discussions for amateurs like me.

*Noted* I put in an 18GB cranshaft pic myself...

So the flatsided was Original 71-74 and the differntly shaped throws forged type an 18GB/pre 18v and that there were only 2 cranks in production for the 18V.

Just to be clear, the 18GB crank requires nothing special done to it to use in an 18v engine? They looked identical apart from the shape of the throws.
JS Ahlund

Spot on Jamie

Peter
Peter Burgess Tuning

Peter-
"I think people would be upset if they found a cast iron crank had snapped in a race engine whereas if a forged one snapped it would be..... wow how unusual!"

Yes, just the sort of thinking I'm trying to eliminate!

For some light reading, see following. Note especially the several comments and a section re crankshafts in real testing and use.

http://www.ductile.org/didata/Section4/4intro.htm

FRM
FR Millmore

This thread was discussed between 03/03/2013 and 04/03/2013

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