Big end knock or blowing manifold?

[MikeNash]

Greetings Comrades!
Advice, please. Had a loud tapping from the engine from 30 mph upward that I feared was a big end. The engine's a circa 1966 Gold Seal job with 120,000+ miles and a few quiet rattles with a max oil pressure of 45 psi when hot, so trouble's been expected for a while. (There was no pressure drop with the onset of the noise.) However, when I pulled off the sump there was no spray of white metal anywhere nor any glittery bits in the bottom. All is delightfully clean. And removing the front big end shows the shell is surprising good shape. ( Yet to do the rest - its teatime.)
But when I removed the inlet/exhaust manifold in preparation to remove the cylinder head (to lift out the pistons) I found the retaining nuts quite loose and the manifold gasket burned at the rear exhaust port. I suspect its been blowing.
Do you think this would be responsible for the "tapping"?
Regards, MikeN.

[MoggyTech]

Yes it could be the manifold and obviously it would be wise to repair that first before taking the engine to bits

45 PSI oil pressure is fair, 60 would be ideal. I'm surprised the engine has done as many miles without more wear!

I would do a compression test after repairing the manifold gasket, and would probably pop in new big ends to be safe, as this can extend the life of the main end bearings.

[bmcecosse]

Probably was the manifold - but have a look at the centre main bearing (it's realtively easy to get off - and takes the most wear) - then decide if you should just box it all back up, or get into new bearing shells. The shells will cost you about £40 (ends + mains) - so not worth it unless the crank shaft is 'perfect'.

[MikeNash]

Thanks MT and BFS (Britain's Finest Scotsman!) Reply held up by the "invalid session" again. I'm about to measure for big end ovality - what do you think is permissable for shell renewal to be worthwhile?
(Incidentally, the engine life is pretty certain 'cos the car's been in the family for over 40 years - but its had magnets for the last 15 years. I'll bore with the story some other time!) MikeN.

[bmcecosse]

Well - inspect the crank more for 'scoring' rather than ovality - although of course it is important. I would say there should be NONE, but up to half a thou might be ok. That centre main will tell all - if it's lightly through to the bronze and NO scores then should be ok (with new shells) - but any scores mean a regrind - and a new oil pump!

[MikeNash]

Just measured the front big end, and I make it 1.585 inch with no detectable ovality. There's no scoring at all and only a light bronze discolouration in parts. I'm really surprised how good it is. I'll pull the others and the centre main tomorrow. If they're as good I'll go for new shells. I assume they're in set sizes - does my size sound standard to you? Do you think that a new oil pump is worth the trouble to fit now? (I know bmc sometimes says so!) Regards, MikeN.

[MikeNash]

I read in the BMC manual that to fit the new shells to the main bearings the crank has to come out. But I know on other engines you can sometimes feed them with a split fitted carefully into the oil hole - can it be done on a moggie? MikeN.

[MikeNash]

Above, that should read "split pin". Sorry MikeN

[MoggyTech]

1.58554 is the minimum crankpin regrind diameter for the engine, so you will most likely need the max oversized shells.

WRT the oil pump. Well you can do the big ends with the engine in situ, so it migh be worth doing the shells, and seeing what the oil pressure does.
If you get up to 55 to 60 PSI I would say the pump is fine. However, if you are pulling the engine out for the work, I would always renew the pump if it's done more than 30,000 miles.

[MikeNash]

Thanks MT for the size and the advice. (I think you mean min undersze!) Is the pump reasonable to fit when you're on your back under the engine? The manual seems to assume the engine's on the bench! MikeN.

[Welung666]

The oil pump is behind the engine back plate so I'd have thought a little hard to change from underneath, but never done it myself so unsure.

[MoggyTech]

I'm pretty sure you need to pull the engine to do the oil pump. Never tried the split pin trick on a Moggy Motor to do the main bearings. When it gets to crankshaft work, it feels like surgery to me, so the block goes onto an engine stand (although I do big ends/rings/pistons with the engine in situ)

[bmcecosse]

Look at the back of the old big-end shell - it will be stamped 'std' or '010' or '020' etc This tells you the size to order for renewal - ie std, or 10 thou oversize etcetc. The shells are made oversize (thicker) - to suit an undersize crank. Standard big-end diameter is 1.625 - I am surprised yours is reading so small - would suggest 40 thou undersize, and so the shells should be marked '040'. It is possible to change the mains shells without taking the crank out - but you will need to take the timing gears off to get the two little screws out of the front main cap lower half before it can be dropped. Same rule applies to mains shells - standard should be 1.750 and regrinds in 10 thou steps. Of course - no way to measure the mains accurately without taking the crank out - so just rely on the backs of the caps for size guide (usually but not always - if b/ends are 40 thou then so will be the mains!) and visual inspection of the main bearing surface for scoring. Yes - oil pump change needs engine out - or gearbox and backplate removed.

[MikeNash]

Thanks for all your advice.
I find that all big ends are exactly 1.585 inch dia without detectable ovality. This has been done with several measurements per bearing with a good micrometer (but I don't expect to go to under half thous lying on my back with the subjects only 6 inches from my face! The vary focal bit you want to see through always seems to be in the wrong place.) The surface of the bearings are dead smooth without scores so I'm going to replace the shells in the rods as you suggest. (The old shells look nice matt grey.)
The centre bearing shell, however, shows a large patch of copper which I take to indicate its worn through the lead-indium bearing material and so needs renewing. Trying to measure the bearing itself with a vernier (not easy to do and liable to be unreliable 'cos its done just with the tips of the jaws) I get 1.709, 1.7105, 1.7095 inches dia while my Haynes says it should be 1.7105. So I'm at the minimum grind for the mains as well and have some 0.001 to 0.0015 wear. No scores, though. I'll just put that cap back till another time.
I'm astonished on this engine's good condition for its mileage. I've checked the logs and the speedos that have been fitted and I estimate that its done at least 100k (of which 56k have been with me. Since the car's been in the family for 35 years I'm fairly confident of these figures.)
I suspect that the previous owner, my favourite old uncle, a great fiddler and bodger in the best meaning, has replaced the big end shells before, just as the Haynes manual I've inherited from him recommend. Thus "Very long engine life can be achieved by changing the bearings at intervals of 30k and mains at intervals of 50k . . . [and] will ensure mileages of btween 100k and 150k before crankshaft regringing becomes necessary." Plus the magnets, of course. (The oil's been Halford's cheapest, but replaced often.) Of course, he couldn't replace the mains.
Thanks and regards to all, MikeN.

[MoggyTech]

That's great to see just how strong the A Series engine is. Probably my all time favourite British engine, and all on a 3 bearing crank, amazing.

[bmcecosse]

You really MUST renew these mains shells if they worn like that - madness to leave them. Again - have you inspected the backs of the shells to see what undersize is stamped there ? Your crank sounds fine - just fit new shells all round. The centre and rear mains are easy - the front a wee bit more difficult because you need to get the two little screws out from the front plate first - so if you must - leave the front main as is. The ecntre main is usually the one to show the most wear - new shells and it will good for another 100K !

[MikeNash]

Sorry, Bmc, I think I misunderstood you.
Firstly, the B/E shells are stamped "40" - and I've ordered replacements. But for the mains, I'd understood you (in plural) to say that replacing their shells from beneath was not on. That is so, in't it? If I've misunderstood please correct me! I suppose I could replace just the lower shell, which I expect shows the maximum wear.
I intend to do the full work next spring cos - as usual - the old Traveller is down for a load of work NOW! And I was surprised to find that a full replacement crank and new bearings throughout is only £75 (plus VAT, I suppose) . And they'll accept a crank already ground to the limit, won't they? Regards, MikeN.

[bmcecosse]

Easy to change the centre and rear mains - both shells - just rotate the top one out in the direction of the little 'tab'. Well worth doing - if you leave them worn the oil escapes there and the b-ends are starved. Sounds cheap for a crank + shells - but yours is not badly worn and new shells will make it like new! They will usually NOT take a crank at 40 thou. There used to be 60 thou shells at one time - but not sure now.

[MikeNash]

Right bmc,
I'll give it a go. Will order a set of mains shells straight away. I'll try to fit the centre and rear shells by rotation and then the front set later when I inspect the timimg chain - cos I understand from you all I need to remove a couple of screws from the front. That should do it, bar a pump replacement.
(And bmc, if it all goes pearshaped just remember I've a long reach. My son-in-law's ex-para corporal, a debt collector for a rough firm and his area includes CENTRAL SCOTLAND!!!) Regards and thanks to all, MikeN.

[bmcecosse]

Agggghhhhhh - but - you don't know where I live (actually - I think you maybe do!!) - and I'm moving any way - later today, to a far away place, not even in this country.
For pump replacement you do need engine out (or at least shuffled forward within the engine bay) or gearbox off.