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MG Midget and Sprite Technical - Rear Main Bearing

I am currently assisting someone in building a 1275 engine.
It is a couple of years since I have rebuilt an engine so I had a quick refresh with the Workshop Manual.
The Frogeye 948 Manual says to smear the rear mains joint only with Well-seal before fitting. Must admit this was 'News to me' and have never done so on previous builds.
The Mk2 Workshop Manual says to use Hylomar and the Manual for the 1275 continues this theme. Again this is for rear main bearing only.
Do others follow this practice.
Is the ommission of Wellseal / Hylomar a contrutory factor to the leaking rear mains.
Alan
Alan Anstead

I always took that to mean the joint to the 'eyebrow'. Not the block. You wouldn't want to increase the gap between the block and the cap. I use Hylomar and have even used a very thin piece of gasket paper, but it doesn't show in this picture.



anamnesis

Clearer here.

anamnesis

alan
yes I use hylomar in that location. I have to say I don't have a lot of leakage from the rear main.
Bob Beaumont

From the Manual
'The rear main bearing cap horizontal joint surfaces should be thoroughly cleaned and lightly covered with Wel-Seal sealing compound before the cap is fitted to the cylinder block. This ensures a perfect oil seal when the cap is bolted down to the block".

There is no mention of the vertical main cap 'moustache' that on earlier engines had the bolts wired for security.
Alan Anstead

I recall my dad putting Wel-seal there when rebuilding an engine for an 803cc Morris Minor! I just followed his practice..........
Bob Beaumont

Agreed no mention of the rear scroll cap/ mostache/eyebrow or whatever it should be called.

But referring to my arrow, it's the only bit that isn't protected from leakage by the sump gasket. Any leakagage betwen the cap and block, inside the area of the sump gasket will simply flow back into the sump. So I took/take the view that under the eyebrow is the only place they can be talking about.

anamnesis

I always use Wellseal. The trick is not to remove the eyebrow in the first place, since it's align bored with the caps to ensure the correct diametrical clearance. If it's off then its best put back using a long straight edge,checking radially as you progressively tighten the bolts.Once you're happy with the position remove each bolt in turn and add a dab of Loctite to the threads.
f pollock

Not between the vertical rear face of the block and the eyebrow. Between the HORIZONTAL face of the block and the underside of the eyebrow. Oil is flowing in there, and attempting to obey the command of the scroll, to flow back to the sump. Hence some can 'potentially' flow between the contact faces of the eyebrow; -- both the vertical and the horizontal. Gasket for the vertical, and hylomar or the like for the horizontal.
anamnesis

Indeed! that is where I have always put it, under the rear main. Nothing to do with the 'eyebrow' which generally I don't touch.
Bob Beaumont

AEG240B is the part number for the eyebrow gasket. Once that is bolted on, or never removed, it's the remaining face that MAY need the hylomar. But honestly, I have no idea if it really is needed.

In fact I decided to do it myself long before I saw the BMC manual mention of it, as Haynes doesn't mention it, as far as I have found. It just seems sensible to me.

Here, just to be clear about where I am referring to.

anamnesis

I was hoping a few more might chime in on this subject.
Alan
Alan Anstead

Yes I use Wellseal on both horizontal and vertical surfaces. Measured its film thickness on digital calipers and its under 1/2 thou, so enough to plug oil passing through the horizontal, without unduly increasing clearance to the scroll. Last engine I built I used Plastigauge and it came out at 3 thou - and that motor with a PCV essentially drops no oil. If your clearance is much beyond that due to wear, crank whip or whatever, you might want to consider a JB Weld squeeze cast as described by John Twist.
f pollock

This thread was discussed between 06/10/2020 and 08/10/2020

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