Morris Minor Owners Club

Dear All,
After the oil pressure became too low I decided to rebuild the 1098 block. It was rebored to 0,020 inch oversize and the 7 crank bearings to 0,010 inch undersize. New oil pump, pistons, shells, timing chain kit, and reconditioned head. Small ends were fine said the shop.
At 300 km after rebuilding I noticed a slight rumble from the engine when doing around 50 mph and light throttle. At the same speed upon full throttle the rumble gradually becomes a bit less. It disappears when I release the throttle. There is no rumble at start-up. Oil (20W/50) pressure is 70 lbs at speed when hot. But the rumble gets louder every time. Not nice...

After the recon and before rebuilding I checked. Crankshaft bearings, end float, and piston clearance are all spot-on within specification. Only the mains were at the lowest end of the specification tolerance with 1.740 inch (0,010" undersize). Plastigauge showed a clearance of 0,001 inch for the crank journals and 0,002 for the mains, so within spec.

The exhaust is free from the body. It really sounds like bottom end but what else can it be. It is not a pinking sound.

Michiel

I would suggest that from your description you have main bearing rumble.
The only solution is to re-grind the crank and fit new to size bearings or fit a re-con crankshaft assy from the likes of ESM.

Thanks for your reply Phil. Although disappointing outcome. So 1.740 for these 0,010 undersize mains bearings is causing this rumble?

You have a rumble and the engine revs against speed indicates (as described) that the crankshaft main bearing clearances are not correct so causing the rumble.
Whilst plastigauge is good you do need to measure the journals with a micrometre to get the absolute measurement/clearances. The measurements need to be taken at intervals around the crank journal as the journal may not be truly round.

Thanks Phil. My calibrated micrometre shows all bearings to be at the minimum side of the specifications. I compared the caliber with the calibrated micrometre from a nearby garage. There was a 0.008 mm difference. Consequently my crankshaft is just in or just out of specification. Let's see what the reconditioner says.

Certainly the description is likely the crank bearings. I expect an inspection of the main bearing shells would likely confirm the problem, before removing the engine.

A second opinion, from a respected local motor engineer, might be a sensible move to further consolidate the suspected diagnosis.

Main bearing failure is usually heard with the car stationary - obvious by just by revving the engine.

But stationairy there is no rumble at any revs.

Are you sure its not a wheel bearing or gearbox noise. Shouldn't be any bad noises from a recon engine.

No it's not a wheel bearing, they're new. I also overhauled the gearbox.

Michiel wrote: ↑Fri May 20, 2022 7:32 pm No it's not a wheel bearing, they're new. I also overhauled the gearbox.

From your previous reply, it might seem that it may not be engine bearings, either?

Getting that independent local engineer to diagnosis the problem is still my advice.

Many many thanks all. I'll have the crankshaft checked and if the problem lies there and the original reconditioner does not agree to a free regrind I'll go to ESM.
And I'll report back when the rumble is gone.

It may be worth seeing if you can get someone else, a mechanic experienced with minors or a fellow owner perhaps, to drive the car and listen to the noise. It may be that they can tell what it is before going straight to inspecting the crank.

whilst not an A series I had a similar problem on an MGB, Knock /rumble after a short time from engine re build ,Crank grind shells pump etc. Removed sump and crank caps to find wear on one of the shells. Spoke to local re conn firm and they examined the caps to find the suspect one to be out of round(or half round if you know what I mean). Caused ,as they explained by the cap/and con rod being subjected to hammering by a badly worn crank .
They said that every time they rebuild an engine that had been knocking, they as a matter of course replaced that con rod and cap. Not heard of it being common on A series but worth investigating.
Examination of the shells would be my first line of enquiry

I certainly will have a look. Thanks Kennath.

Propshaft? Sounds very like a worn UJ.

Are you certain that the timing is where it should be? On a 1098 I had a few years ago, I got bearing rumble when I'd changed something and inadvertently retarded the timing way too much. It's worth a check as the hot oil pressure figure looks very good.

Thanks Liam, that could be it also! I canged the pulley with one from a Mini that had the TDC Mark in a different place. So I took over the TDC mark from the old one ( if makes any sense).

So I took the crankshaft back to the machine shop/reconditioner and we measured the crank with his micrometer. All seemed done correctly 0,01 undersize. For a second opinion I took the crankshaft to another shop and had it measured. Turned out all on the minimum side of the the specification, but nevertheless correct. I reassembled everything and went for a test drive. Rumble still very much there. Loudest at approx 3000 revs slightly opening the throttle. I like to rule out the conrods as there was no rumble before I had the crankshaft reconditioned. Next I tested with 10 additional degrees advanced and 10 degrees retarded ignition. This resulted only in vibrations bzw lack of power. I looked at the centrifugal advance, was correct at 3000 rpm. Vacuum advance is 14 degrees maximum. TDC mark on the camgear cover is consistant with the dimples on the camgears. Even tried a different brand of 20W50. Pressure is 70 lbs when hot and at speed. I think I drive on untill something gives so I can say: that was the cause.. Anyone a better idea? Totally at a loss now.

The only other thing I can think of is of exhaust resonance if the exhaust is fouling the structure as it leaves the engine bay.

Thanks for trying to help Jowett. Alas, the exhaust is free from the body everywhere, except for two rubber straps.