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MG Midget and Sprite Technical - Cracked Main Cap

All the swearing!

Got my "new" block back from the machine shop on Friday. I was cleaning up and inspecting some of the other bits and bobs today and spotted something rather worrying.

No. 2 main bearing cap is cracked (see pic). It also appears to have a bit of old bearing tang lodged in it, whilst this is less worrying, it does perhaps indicate some sort of cataclysmic failure at some point?

Any thoughts on the best way to proceed? Could this be repaired some how?


Malcolm Le Chevalier

In fact, the more I look at it, the more goosed it is!

Malcolm Le Chevalier

Well spotted Malc. Proper goosed indeed. I've never seen one cracked across the lug in that direction before, only parallel to the line of the crank.
Speculatively I'd surmise that the bit of old bearing tang had something to do with the failure, since it's offset from the centre of the bolt hole and should have caused unequal stress across the the lug, made worse by it being the centre cap and exposed to the worst of the rocking couple stress from the crank.
I'd suppose in theory it's repairable but I sure wouldn't chance it. The catastrophic failure is not far away - if that thing lets go it's bye-bye crankshaft at least I'd imagine.
I wonder how the engine went together with even a semblance of normality. I'd have expected that bearing to be dragging a lot.
I think your only sensible option is replacement, but I don't know what availability is like and I assume it would need line boring to match the block. Nasty. What a bummer.

I wish I had inspected things more thoroughly before getting it re-bored. I am really bummed out.

Investigating further, the cap is a bit loose in its seat in the block and it doesn't line up quite right. I wonder if it has been switched/mixed up with another block at some point as well as the rebuild bodgery.

I kept the main bearings, although I didn't label them I can probably safely assume the attached pic was the no. 2 pair.

One step forwards five steps back.

Malcolm Le Chevalier

With an A-series, you can get a steel centre cap. I don't know about availability for the Triumph engine.

As Grey said, it would need line-boring, assuming you could get one.
Dave O'Neill 2

While not ideal I don't think it likely that it would fail catastrophically as in use that section of the cap should always be under compressive stress due to the tension in the main cap bolt/stud. I would suspect something failed serious to result in the damage though and would question whether it was still round. I would replace it.
David Billington

Now that's a thought Mike. I'd forgotten about the steel caps, but I haven't heard of them for 1500s. There used to be an outfit called Burton's (?) that made them, but I have an idea they were Ford specialists.
But it's not a complicated component - maybe gettng one made and bored would be economic after all.
Malc the shells are worn, but I've seen much worse. It looks like both halves have been out of line radially wrt the crank.

Edit: David I was more concerned about the crack across the root of the lug in Malc's second picture.

Would it be possible to line bore a used cap from another block? (I have access to another). Or does it have to be a "new" cap?
Malcolm Le Chevalier

A used cap could be used but it would need to be ground down a bit to tighten the hole & then line bore the block. I absolutely would NOT take it to the shop you just had the work done at. If they can't see something as obvious as that crack, they cannot be trusted! Look elsewhere.
Stan Kowznofski

The shop didn't have the caps. Just the block.
Malcolm Le Chevalier


Have you seen info like this?

M Wood

Also see:
M Wood

If the shop didn't have the caps, that's a big red flag. When I was working at a speed shop here in the states we repeatedly documented and showed many disbelieving customers that honing cylinders without properly torqued main caps was a waste of time & money. Properly torqued main caps definitely causes movement in the cylinder walls. You should also use a block plate to replicate the effects of the torqued cylinder head, anything less is false economy IMHO!
Stan Kowznofski

I agree with stan on this one.

Get the used caps find a pro machine shop and have it re line bored so it's all a match set

Otherwise its 8 steps back and 1 forward

1 Paper

I think I'd skip getting used caps. Along with not knowing their history (they could be a great find or another piece of junk) they may not register well in the block. I've staked blocks to tighten the fit & even offset the bore slightly but it takes a lot of work. The machining costs will be almost the same for new steel as used cast but the finished product will be immensely better. This is one spot where skimping can have disastrous results. 'Penny wise & dollar foolish' is not where you want to go with your bottom end!
Stan Kowznofski

Well yeah... if you can get new then skip the used. I wasn't aware new was an option

Looking at those cracks, I wonder if it was pinging, thwtd some serious damage, if it was just worn shells then I'd think they would have worn down to dust for those cracks to have occurred

I'd have the crank shaft examined as well.. some kind of sound sonic or X ray test to see if any of the journal's are damaged as well

That took alot of pounding to crack those mains that badly

1 Paper

Thanks everyone. I am going to get the old block out of the car and go from there. Maybe I will win the lottery and one of the caps will swap over (unlikely, I know) so I also have feelers out for another block. More . :-(

Prop: I am not using the crank from this engine and I believe it has cracked due to some ape replacing the bearings and not noticing the old broken tang in the cap. Tighten everything down and something had to give.

Malcolm Le Chevalier

I don't know male

At this point I'd be tempted to rethink a duratec/ecoteck 2.0 engine and ... stick 4 carbs on the intake ports plus a crank trigger ignition and avoid EFI/ and modern ignition

Your getting into some money doing another triumph engine... good news with the part Bing pricey for 1500s ... you could maybe break even on the engine and free you up on the duretec to start fresh back to zero
1 Paper

Malc, if you are looking for a replacement engine how about this:

Claims it as a "low mileage" but whatever, it could provide the basis for reworking and if you sell on the rest you could be in profit!

Why is everything good for sale always in Devon, Kent? Never in middle earth!

I am not getting involved in that. I am already getting heat about the B festering in my garage!
Malcolm Le Chevalier

B festering in the garage ?

As in MGB ? Or honey bee ?

1 Paper

MGB Prop - the next project.
Malcolm Le Chevalier

Good grief malc... you must be one of those people that love pain

What's the plan for the B... hot rod, or classic resoration... If a hot rod you certainly have some options... here they drop in Buick aluminum v8 and v6 motors all the time

Between you and the new guy in germqny... the 2 of May have a complete working engine.

1 Paper

Dunno what the plan is. Might just sell it. Dunno if I want to get into it, whilst leaving the midget to rust outside.
Malcolm Le Chevalier

Prop, he should have just sold it to me when I offered to take off his hands last year ;-)

It did look like an "easy project" but you know how those go.... once you start you end up doing a full body strip restore...

Now he's moved it too far from me to make it worth while to move it back :-(

Malc, just get your head down, ar*e up and get on with it lol

Best of....
M McAndrew

An update on this:

I pulled the other engine out of the car yesterday. It seems that no. 1 cap from that engine fits well in no. 2 position in the new block (no.1 and 2 caps are the same). It locates in the block well with no discernable side to side movement. The bore seems to match well, the bolts go in easily without binding, using plastigauge the clearance seems within spec and it turns freely by hand when all the caps are torqued up.

Seems to me like a winner.

Malcolm Le Chevalier

This thread was discussed between 14/01/2018 and 29/01/2018

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