2 1/2 crankshaft bearings..

Geoff Douglas
Posts: 355
Joined: Sat Feb 13, 2016 5:08 pm
Location: Helensburgh, Argyll

Re: 2 1/2 crankshaft bearings..

Post by Geoff Douglas » Mon Feb 12, 2018 1:43 pm


Do you know what bearings Barry has been supplying in recent years? My SP engine was rebuilt in 2011/12 using Barry for parts. The answer to the question may be available here, if not I'll ask him. We also fitted a set of brace bars from Barry because of other faults found.

It's worth noting the history of this engine in this topic; it had been messed about by the previous owner who participated in amateur motorsport (sprints and hillclimbs) in the 70s, and it required attention when I bought the car in the early 80s. It was then rebuilt by Oselli around 85, but there was still a problem after that, it felt quite rough beyond 75, although it would cruise fairly happily at 85 to 90 with adequate power. It was difficult at that speed to distinguish engine from road vibration. Work and family matters prevented further attention until after I retired in 08, when a fine vibration at 2000 to 3000 rpm became evident. When dismantled then, some 20,000 miles from the Oselli rebuild, all the bearings were worn through to copper backing, the cause being unbalanced rods. The likely cause of the latter being more likely the previous owner's tinkering than Oselli's errors perhaps, although the latter had clearly, at the very least, not balanced the engine at all.

The engine balancer in 2012 had to remove metal from both ends of several rods, and also had to add heavy metal to some of the crankshaft webs to achieve a good result. I've thought about this a lot; which of the two would be more likely to have had more than one set of con rods to play with at the same time, and mix them up, accidentally or intentionally?? We'll never know the answer now. But apart ftom the engine woes the rest of the car had been very well cared for and that still shows. It is very well put together and feels much more solid and rattle free than most. It also has the later rear axle and had a nearly new gearbox, since replaced.

In my ownership from the 80s onwards the car was never subject to hard use through several years as my sole daily transport in the 80s, followed by purely recreational use with low annual mileage throughout the 90s and 00s.

The significant points worth making from all this are that,
first as Russ says, unbalanced rods cause premature bearing wear within as little as 20k miles, and also
second, that in this case there was no more serious symptom of severe bearing wear than the fine vibration at 2000+ rpm, felt mainly by the bum; no knocking noises or more major vibration at all.

Indeed the last thing I did before the engine came out was to take the car up the old "Rest and Be Thankful" hill climb course, mostly in second gear, for film about the course being made for the new Glasgow transport museum, the Riverside Museum. The oil pressure was "adequate", say 20psi at idle and 35 to 40 hot, IIRC, but, it had the 3/8 inch spacer under the oil pump relief valve spring modification from the time I acquired it. So did the oil pump mod disguise the bearing damage?

The current state is that the crank has been reground, all the bearings replaced, the rods, crank and new pistons all rebalanced plus the brace bars from Barry and the relief valve spring spacer too, and she is now as sweet as a nut at last, although regular readers will recall that the crank vibration damper gave up during the last outing of the 2017 season. It had had more than its fair share of vibration to absorb without doubt. But that is now sorted too and the smooothness is restored, again.

Will stop now!



Posts: 227
Joined: Wed Mar 09, 2016 11:20 am

Re: 2 1/2 crankshaft bearings..

Post by daimlersteve » Tue Feb 13, 2018 2:48 am

The word just in from the machinist is the rear main ( std. bi-metal as all i could find in the length required ) is 1/4 thou different from the cu/lead version ,though both are King Bearings and the spec sheet shows them to be the same. - the increased clearance in the 3 bearing bmc A series is supposed to be in the crank grind size ,not the bearing ---- ???? anyway we are progressing slowly towards the final goal.

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