Mk 2 Big End End Float

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John.reader
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Mk 2 Big End End Float

Post by John.reader »

Please could I ask for some advice. I have finally got round to attempting to build up a Mk2 Square Four that I bought in a fully dismantled state back in the 1990’s.

My problem relates to the big ends and the amount of side play that they should have. On the front crankshaft, when I put a rod onto either of the crankpins the end play on the bearing is about 6 thou, which is correct. I believe that the recommended clearance is 5-12 thou so perfect. So far so good.
The problems relate to the rear crankshaft. When I bought this bike the deal included the sellers collection of other assorted square four spares which included parts from several engines. In total I have a choice of four rear crankshafts, all in various degrees of rust and decay. Two are standard on the big ends, one is +10 and the other is +20. Two are the early type with the flat on the threaded end, the others are the later type with the split pin hole. Which, if any, were original to the engine I have no idea.

However the problem is that when I fit the same connecting rod that I used on the front crankshaft to any of the rear ones, I have an end float on the journal of 25-35 thou instead of the normal 5-12 thou. This is the same on all four shafts. Now I could understand it if this was only on the shafts that have been reground in the past, but two of the shafts are standard size, so they have never been anywhere near a crankshaft grinder, and yet they still have the same 30thou end float. How can this be?

I am a bit worried about this as going back into the distant past, in the early 1980s I bought my first square four. It was a 1951 MK1 in dire condition, although I did only pay £600 for it. The engine was completely worn out and only running on two cylinders, so I embarked on a complete rebuild. The cause of the poor running was that two of the camshaft lobes were almost completely worn away, but everything else was pretty much worn out as well. I took the cylinder block to the Healey Brothers, who were still in business at the time, and as pistons were not available back then they did a rebore and a conversion job on it that they had developed, using Morris 1100 pistons and fitting custom made bushes to the small ends to take the smaller Morris gudgeon pin. I bought all new bearings and a replacement camshaft and followers from Drags, and I had the crankshafts reground by the local crankshaft grinder. This may have been a mistake as when I went to pick them up he said “Oh by the way we have also cleaned up side faces on the journals”
Anyway when I got it all home I measured the end float on the journals and it was 25-30thou. What it was before I don’t know as I never thought to measure it.

So I put it all back together and it ran fine with good oil pressure until it came under load when it had a really loud knock in it that sounded exactly like a big end had gone. I have lost count of the number of times that I stripped this engine down looking for the cause of the knock but I never did find it.

Eventually I gave up and just ignored it and I drove on for about 1500 miles with it in that condition, until one day when I arrived at one of our local pubs where we used to meet on a Sunday morning, a man whom I knew asked me if I would be interested in buying a spare engine for the bike. Needless to say I jumped at it and did the deal (£200) When I got it home and examined it I was delighted to find that it was in excellent virtually unworn condition, and I only had to fit some new bearings and piston rings to it before putting it in the bike where it ran perfectly and without any nasty knocks. It's still in the bike today, having done many thousands of miles in the intervening forty years. However it was whilst reassembling this engine that I realised that the big ends should only have a very small amount of end float, and not the 25-30 thou that I now had in the first engine. Anyway I just assumed that this end float was the cause of the knock in the first engine, and as I now had a fully working bike to enjoy I didn’t think any more about it.

Now fast forward to today, and you can understand why I am reluctant to fit a crank to an engine knowing that it has 30thou end float on the big ends when it should be 5-12thou. Yet I have four rear crankshafts all with this amount of play on the big ends, and one front crank which has the correct amount of play at 6thou, which I have tested using the same rod on both cranks. How can this be?

Please ignore the rather knocked about state of the rod in the picture, it's only an old rod that I was using to test the bearings. It is however exactly the same width across the big end as the rods that I was intending to use in the engine.

The Test rod and the four rods that I was intending to use in the engine all measure 1.021 across the outer faces on the big ends.
The front crank measures 1.028 across the journals, hence 6thou end float.
All the rear cranks measure 1.051 to 1.058 across the journals, hence 30thou end float.

The rebuilt engine that I mentioned above had 6thou play on all the big ends, and a 1937 4G that I have just finished as a lockdown project was the same. Both these engines run smoothly and quietly.

So what to do? Should I just get the best one of the rear shafts that I have reground and put it in the engine and hope for the best, or should I carry on searching in the hope of finding a usable rear shaft with the narrower journals to match the front one that I already have. Or am I worrying too much and does it matter anyway.

As usual any help and advice would be much appreciated.

( You can see the gap at the side of the big end in the picture. A 30thou feeler gauge easily fits in this gap.)
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Last edited by John.reader on Fri May 27, 2022 9:34 am, edited 1 time in total.
allan.walker
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Re: Mk 2 Crankshaft End Float

Post by allan.walker »

Snap!!
I have had a similar problem. The rear crank in the engine of a bike I bought as 'having been rebuilt' has about 30 thou clearance on the big end journals. It developed a knock and I took it apart and that is when I found the side play.
I, too, have a selection of crankshafts. One pair have been reground and came with new shells. Great, I thought, but the end float on all the big ends is about 35 thou! Doubtless victims of careless grinding...
I shall be taking the two best pairs of spare cranks for regrinding shortly. I've already talked to the engineers about the need to avoid the thrust faces on the crank cheeks and to maintain the radii between the crankpins and the webs. Fingers crossed!
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Re: Mk 2 Crankshaft End Float

Post by paul.jameson »

I have just looked at the crankshaft drawing and found a note dated 4-5-51 reducing the width of the big end journals by 25 thou.
Paul Jameson
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Re: Mk 2 Crankshaft End Float

Post by nevhunter »

If the rods are straight and no twist the extra end float is no problem. Some chev 6 high performance engines used GMC rods and 3 times that end float with no issues. Nev
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Re: Mk 2 Crankshaft End Float

Post by allan.walker »

paul.jameson wrote: Thu May 26, 2022 8:46 pm I have just looked at the crankshaft drawing and found a note dated 4-5-51 reducing the width of the big end journals by 25 thou.
I wonder if this coincides with the change from white-metalled to shell bearing rods?

I had a set of early rods and it was noticeable how thick the white metal was on the sides of the conrod. They've gone now, so I can't measure them.
I do have the crankshafts that they were fitted to, though, so I'll measure the side clearance on those using a shell bearing rod.
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Re: Mk 2 Crankshaft End Float

Post by allan.walker »

nevhunter wrote: Fri May 27, 2022 4:25 am If the rods are straight and no twist the extra end float is no problem. Some chev 6 high performance engines used GMC rods and 3 times that end float with no issues. Nev
Are these the engines built for short-burst, high-speed runs, such as drag racing?
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Re: Mk 2 Big End End Float

Post by Simon.Gardiner »

I'll just throw in that I've come across some very serious discussions of crankshaft lubrication mods and racing preparation for BMC 'B' series engines (as in MGB) and the con-rod side clearance is considered a key factor in the oil flow through the journal.

I've not given any thought to how that might relate to 'everyday' use of a Square 4 (not having one even on the horizon) but I'm certainly going to have a review of the FH.

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Re: Mk 2 Big End End Float

Post by allan.walker »

I've just measured the side clearance on a pair of early crankshafts (those with a flat on the end of the rear shaft for the shock absorber nut and with non-split bobweights.) These came to me with white-metalled rods attached.

Using a Mk2-type con rod, the side clearance on all the journals is between 5 and 6 thou. Perfect! Let's hope that when I get the journals reground, that clearance can be maintained!
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Re: Mk 2 Big End End Float

Post by paul.jameson »

The change from white-metalled rods to shell bearings took place for the 1948 season so no, the dates don't tie up.
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Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
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Re: Mk 2 Big End End Float

Post by nevhunter »

No the Chevs were not for drag racing but the engines crankshafts were drilled for full pressure lubrication and the chevs used splash with dippers originally and hence the conrod change to GM with slipper bearings. Metalled bearings are radiused like the cranks because it supports the babbit metal because it's usually quite thick and more susceptable to fatigue and cracking. . I've built a lot of normal and high performance engines,and bike ones. Some roller big ends don't like too much end float as cages etc are involved but usually plain ones are NOT critical as long as the rod is free to float without alignment issues. End float is not involved with oil retention in normal designs. OFFSET rods often thrust one way and they are poor design and tend to wear the crankpins tapered and tilt pistons sideways. Nev
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