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MG MGB Technical - MGB engine: 3 or 5 main?

For a decade now, I've been sitting on the component parts for a plywood-and-fiberglass-chassised, fiberglass-bodied project. If you're bored enough to want to read more about the project, I've appended a little blurb below.

My main questions at this point are:

1. I have access to both a 3-main and a 5-main block. In my mechanical ignorance, I'm rather partial to the 3-main, mostly because of its wider bearing surfaces, but I'd like to hear the pros and cons of both from those who know. This car will be strictly street use.

2. Since I have a very wide choice of ground clearance, and the MGB engine is very tall in comparison to the height of the body (from the floor-pan to the height of the cowl is only 20" -- a hood bulge very much like the Knobbly Lister adds 3" to 4" at a width of abot 15" in the middle), what can I safely get away with in terms of ground clearance, especially at the sump?

Thanks for reading. Below is a brief explanation of the project, which I figure on finishing in about 2099. The modern-day Dio "Tipo" can be seen here:

http://www.conceptcarz.com/vehicle/z13131/Ambro-Triumph-Special.aspx

Thanks again!

Bart Brown

The body I'm using may be familiar to many of you, it is the Bill AMes-Dewey BROhaugh AMBRO, that was originally laid up as a body for their TR3A chassised "Modified" (formerly skinned in a pretty ungainly all-aluminum body called the Peyote, in which none of the panels had compound curves, as the English wheel was a little beyond them.) Fiberglass proved more suited to their purpose, and copies of the body were sold to other drivers running in the modified classes. One of the well-thought-out features of the AMBRO was its easy adaptability to wheelbases from 88" to 102". The same body -- made from a splash of the original -- is sold by Bill Bonadio (Oklahoma City, OK) as the Dio or Dio Tipo -- the body resembles a mix of Knobbly Lister and Maserati Tipo 61 Birdcage.

The chassis is a plywood semi-monocoque multi-torsion-box design based on a Herb Adams design, and quite similar to the Marcos GT1800 and the Protos FII car. Several of the fiberglass panels of the body will be bonded (with epoxy and 'glass) to the plywood structure. The MGB front cross-member will be used in its entirety, as it handily incorporates the whole of the front suspension and steering rack. I'll also be using the stock non-O/D transmission, propeller shaft, diff, and rear axle. Undecided about stock leaf springs or coils, but whichever it is, there will be a Watts linkage to eliminate transverse axle movement. I was going to use the stock 91" wheelbase, but I may extend that 3 or 4 inches to get the engine farther back in the chassis,
Barton Brown

Bart. Since no one has addressed this subject yet, please allow me to provide my thoughts. First, having owned two MGAs, with the three main bearing engine, and a number of MGBs, with the five main bearing engine, I have found the five main bearing engine to run a little smoother than the three main bearing engines. When I drove my first five main bearing engine, I was surprised at how much smoother it seemed than the three main bearing engines I had previously used. But, beyond that, I found very little difference between the two.

A number of years ago, I discussed, with Kelvin Dodd of Moss Motors, whether to install a three main bearing MGB engine into the MGA I am restoring or whether there might be some advantage to the five main bearing engine. Kelvin's response was that Moss was finding it easier to source parts for the five main bearing engines with some of the parts for the three main bearing engines becoming increasingly difficult to find. Hence, he recommended the use of a five main bearing engine if it was a car which would be driven frequently and kept for a number of years.

Depending on how the various parts would fit up in your chassis, I, if building such a vehicle, would fit a five main bearing engine and a four syncro transmission to it.

Les
Les Bengtson

Bart-
The three-main-bearing crankshaft of the 1800cc engines was a redesign of the crankshaft that was employed in the earlier 1622cc engine. It featured larger-width bearings in order to enable them to absorb greater stress, but the crankshaft had an unfortunate tendency to “whip”. In the end, warranty claims for broken crankshafts caused by abuse forced the redesign to the five-main-bearing version. The bearings were 1.125” (28.58mm) wide for the front, center, and rear crankshaft bearings, and .875” (22.23mm) wide for the intermediate crankshaft bearings of the five-main bearing version of the engine. They all were given diameters of 2.125” (53.98mm), a full .125” (3.18mm) larger than that of the previous 1622cc three main bearing version of the engine. The five-main-bearing crankshaft is heavier than the earlier three-main-bearing version (32 lbs vs. 25 lbs), so the three-main-bearing crankshaft accelerates to high engine speeds more quickly. However, one should be careful not to exceed 5,000 to 5,500 RPM on the three-main-bearing engine. The primary reason for this prohibition is not the crankshaft, but rather the split small end of the connecting rods. These were always the Achilles’ heel of the three-main-bearing engines. The better way to go is to do what the factory race team did: install a set of the connecting rods from the earlier Twincam engine, if you can locate a set of them. These will fit the journals of the three-main-bearing crankshaft, but will require the use of a floating wrist (gudgeon) pin. These floating wrist (gudgeon) pins will in turn fit the later pistons of an 1800cc three-main-bearing engine. The rapid reverse of direction at the end of each stroke of the piston puts a great deal of stress on the gudgeon pin pinch bolt used on the split small end design. Even worse is downshifting to decelerate the car, which develops a great deal of vacuum inside of each cylinder and again subjects the gudgeon pin pinch bolt to tremendous amounts of stress. Every factory racing 1800 MGB used a three-main-bearing engine, even after the five-main-bearing engine was introduced. They preferred the three-main-bearing engine because its lower internal friction was worth about 4 BHP, and its lighter crankshaft and flywheel (10 3/4" diameter as opposed to 11 1/2") made it more willing to quickly attain the high engine speeds demanded by racing circuits.

It is not commonly known that the five-main-bearing B Series engines used in the MGB actually made use of a succession of four crankshafts. The first was peculiar to the 18GB engine, being of machined from a forged steel blank, which used a 10.75” (273.050mm) flywheel (BMC Part # 12H 1474) and ring gear (BMC Part # 1G 2874, Moss Motors Part # 190-040), neither of which are interchangeable with the later flywheels and ring gears used in the later five-main-bearing engines. The second crankshaft was a forged EN 16 carbon steel design that had a wedge-shaped taper to its counterweights. It may be found in 18GD, 18GF, 18GG, 18GG, 18GH, 18GJ, and 18GK engines. Both of these first two crankshafts had shoulders on their throws near the crankpins that were used for mounting during the machining process. New production techniques permitted the elimination of the shoulders, and made possible the third crankshaft, which was a slab-sided cast iron design. It 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 fourth was a forged EN 16 carbon steel design, easily recognized by its flared counterweights. It may be found in 18V779-F-H, 18V-780-F-H, 18V797-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.

The crankshaft with the best balance and wear characteristics is the slab-sided five-main-bearing cast iron version found in the early versions of the 18V engines (18V-584-Z-L, 18V-585-Z-L, 18V-672-Z-L, and 18V-673-Z-L). Although technically slightly weaker than the alternate forged EN 16 carbon steel crankshafts used in other versions of the five-main-bearing engine and seven pounds heavier than the earlier three-main-bearing steel crankshafts, it is strong enough for a streetable enhanced-performance engine. Do not succumb to the temptation to use the less expensive crankshaft from a Morris Marina. I do not say this because of its too-small spigot pilot bushing. The spigot bore can be easily machined to accept the larger-diameter spigot pilot bushing of the MGB. I give this warning because this particular crankshaft is made of flow-cast spheroidal graphite iron and was intended for use in sedate family cars. It is simply not strong enough for use in an enhanced-performance engine. In addition, it uses a completely different, much heavier (28 lbs) flywheel that is ¼” (.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

Stephen, I've owned my '67 B since '72. I converted the 18GB engine/3 synchro trans to the 4 synchro/od trans using the backing plate, flywheel and starter from a '68 B. According to your posting, my crankshaft should not have accepted the later flywheel but it bolted right up. It's been performing flawlessly for over 20 years now. RAY
rjm RAY

Ray-
They are non-interchangeable because their diameters are different. The electric starters of the 3-synchro and 4-synchro transmissions that they are used with are in different locations, and thus will not align. You have to change everything, as you did.
Stephen Strange

Oh, I thought that you meant that the later ('68 and on) flywheel would not line up with the bolt holes of the GB crankshaft in the '67 B. Since I used everything from a '68 or later car, sans the engine, I had no difficulty in fitting any of the components all. RAY
rjm RAY

Lots of good information in Stephen's post, but I am going to disagree about one thing. The clamped small end was not necessarily a weak spot and I have run many 3 main engines to 7000 without problems. I even have an MGA con rod I use as a paperweight. The crank broke in half (something the 1500/1600 version was prone to do) and turned the rod into a nice 'S' shape, shatterng the piston, but leaving the small end gamely clamped in place.

I prefer the 3 main engine, and have used it extensively for racing. The crank will flex more than the 5 main, so has a reduced life, but it runs more freely. For the street I wouldn't even worry about whether it was a 3 or 5 main engine. For racing I'd go 3 main if possible and lay in a couple of extra cranks.

I must also disagree with the Twin Cam wrist pins fitting ANY MGB pistons (as I understand Stephen to be saying). They didn't as the 5 main pins were a smaller diameter than the Twin Cam used.

Great account of the various crankshafts, Stephen!
Bill Spohn

Bill-
Which Twincam connecting rods are you referring to? I believe that there was originally an obliquely-split connecting rod that was later suceeded by a horizontaly-split connecting rod. They used different diameter wrist (gudgeon) pins and pistons. The Twincam went through such a sucession of changes that keeping everything in order is a monumental task!
Stephen Strange

No, Stephen, all of the Twin Cam rods had an oblique split - there was never a horizontal split rod on the B series until the late MGB.

All Twin Cam rods also had the same diameter of full floating wrist pin, 7 years before the MGB adopted them, albeit in a smaller size.

The only changes the Twin Cam rods went through were details such as the size of the balancing lugs near the small ends and the slightly heavier caps, both used on the second iteration of rod.
Bill Spohn

Bill-
Well, that's a new one on me! I thought that there was a horizontally-split connecting rod that was developed for the factory racers late in production. What was the diameter of the Twincam wrist (gudgeon) pin?
Stephen Strange

Stephen Strange of Virginia:

BRAVO! Your technical information is remarkable.Thanks for educating the rest of us MG wing nuts out there.

Just plain ol' horse sense makes me prefer the 5 main bearing over the 3 main bearing power plant. You have to admit that it was an engineering refinement. (Dare we take much liberty to use such an adjective for an Abingdon motor car engine?

RM
R Murray

This thread was discussed between 04/03/2010 and 10/04/2010

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