Engine Failure

[Westfaling]

Has anyone experience an engine failure similar to this? I'm not entirely sure of the cause.
No.2 piston or whats left of it, remained in the bore, the majority of it ended up in the sump.The gudgeon pin remained on the conrod- just.
The crankshaft main bearing caps all broke some parts also joining the piston in the sump.
The noise when cruising along at about 50mph was like I'd run over a load of scrap iron. I never want to hear that again.
The other pistons and bores are fine but some sharpnel made it into three other combustion chambers.

[mole42]

I’d say No2 piston broke apart and jammed between the conrod and the head. That would explain the main bearing cap failure. Not a happy engine at all.

[jbuckl]

Cap failure can be due to a previous incorrect assembly…. Which causes a crack to start… quite common on stag engines built at home…. If the cap has been tried the wrong way around…..

Same with cam bearing caps too.

Have never seen a stag piston failure like that though…. Plenty like that on other engines though…..

can happen due to gudgeon pin circlip not staying in.

[flying farmer]

I once had a 1300 Toledo engine do this but I was doing about 80mph at the time so it would be over 5000 revs.

Main bearing caps on a Stag are not very strong so I am not surprised they all broke when the No2 piston collapsed, the others furthest from no 2 probably broke when the pistons tried to compress the shrapnel in the other cylinders.

Can't imagine what caused the piston to break up unless it had a casting flaw in it, 50 mph isn't a lot of revs and standard pistons are good for 7000 rpm

[Westfaling]

Engine was re-built 13 years or about 10,000 miles ago seems strange it waited till now to fail. I've now found that three of the crankshaft main bearings in the block have cracked so the block is toast.

[jbuckl]

Personally I don’t see that as strange….

cap crack initiation upon (initial) incorrect reassembly.
not apparent or visible though.

took 10000 miles to manifest failure after propagation and growth of the crack.

see similar sorts of mis-assembly / early life failures frequently….. this stuff is part of my day job for the last 40 + years now.

these days it’s mostly from LCC sources….. previously it was from new intake labour.

Designers learnt that the design must include features to prevent mis assembly.

the stag engine lacks those features…. It’s almost prototype level…. Requires (very?) skilled labour to assemble correctly.

the stag main bearing caps are not properly’poke-yoke’…..

meaning it’s possible to try them the wrong way around and cause damage.

a tiny design change would have prevented this from happening. Small offset from bolt hole to side face….. 4 caps are almost symmetrical apart from dowels.

Larger or more significant asymmetry was needed to prevent bolt engagement when fitted the wrong way around.

I had a few people ask to buy blocks and caps for their stag engines after they broke main bearing caps.

from a width perspective the caps are narrow (compromised oil film integrity)

from a structural perspective they are sound.

from an assembly perspective they are lacking poke-yoke features….. its usually too late when (if) you got them the wrong way around at first.

[STypeR]

That's eye watering damage.. I think that's now made a few of us paranoid; myself for sure.

I at least paid a local Stag engine expert (who's now well retired) a few hundred in cash to assemble my spare engine, that I put in the car but I still have the cars original engine to go; stripped and sitting in my garage that I thought I may have a go at myself since I had the cars original engine's heads done by another (much younger) head expert.
My cars original engine's heads are wrapped in clear plastic and ready to go but I need to double check that they've been assembled the right way.
I did damage one heads trying to remove it off the block, which he expertly rewelded before reassembling.

jbuckl, how can we effectively check that the caps are the correct way around? Just so you can scare me a bit more, what other common mistakes have you seen?

[Westfaling]

The main bearing caps all have numbers cast into them which all need to be on the same side (number opposite side to the oil pick up). Number 3 cap is the only one that's really obvious which way it goes, as it has a recess machined in for the end float thrust washers.

[Westfaling]

I've got access to a used short engine that has been fitted with Toyota Corona pistons. A common mod down here in the past when new pistons were hard to come by. Anyone had any experience with this?

[STypeR]

Yes, thanks for reminding me. I do now remember the numbers on the caps.
Were you able to ascertain if the caps on your failed engine were assembled all on the same side, that is opposite side to the oil pick up?

[Westfaling]

I couldn't tell! 1 and 2 caps were in the sump. 3,4 and 5 were right.

[richardthestag]

Looks a very similar failure to that on a customer 3.9 Rover v8 which we reckoned the piston hung up on cylinder #1, broke the top of the piston from the skirt, the conrod flailing about reduced the remains of the piston to bite sized lumps and then decided to punch a hole through the block and snap off. Whole process maybe a few seconds, he was giving it some beans getting onto a busy roundabout. Incredibly the engine still ran but badly of course now being a v7 with a stubby conrod from #1 flailing about.

However, my current Stag original v8 was struggling with low oil pressure, I noted that when accelerating the oil pressure would dip and then rise as I lifted on the accelerator. shortly after there was a dreaded knock, I was 50 miles from home, it was flippin freezing and I opted to drive it home with like 15psi oil pressure. Naturally I was very careful but also stupid in equal measure.

When I pulled the engine with the aim to replace crank and bearings I found main caps 2, 3 and 4 all broken. The stripped down block was found to be littered with cracks around the main journals. Just wondering if the main bearing cap failure was the root cause on your poor old engine. and brand new boat anchor

[flying farmer]

My very first Stag engine was fitted to my TR. After a track day it lost about 5 psi of oil pressure, and a few thousand miles later suddenly lost another 5 psi.

I dropped the sump to find no 2 and no3 main bearing caps came off in two pieces when I decided to check the bearings.

I took the cracked caps to my local TR club meeting, and it just so happened that one guy who was there had experience in metalurgy. He reckoned it was showing classic symptoms of a fatigue fracture.

What I did realise a while later while braking down a hill was that the oil pressure had disappeared. I then found this always happens when you brake going downhill in a Stag. It is due to the design of the sump, rear pan and rear pickup. Brake hard and the oil surges to the front, even on the flat. This is why I always baffle my Stag sumps. I suspect the crank was flailing around in its bearings with no oil pressure and that this initiated the crack. Strangely though the bearings were not scuffed due to lack of lubrication, only where the crack made them out of round

It is possible that a previous engine blow up might have been responsible, I bought the engine as a Recon short block from Rimmers, then had to fully strip it as the plastic wrap it came in was full of casting sand. I know it was ok at this point

I was very lucky with the engine with the cracked caps, I had 5 spare caps in a box of bits. Bolting the cracked halves to the spare halves formed a perfect circle, but the caps were offset by about 6 thou from memory. I ground down one side of the spare caps and built up the other with weld before filing it to a push fit in the block. This was all done from under the car. About 70,000 miles later the head gaskets were failing, so I totally stripped the engine. All the crank bearings were perfect, as was everything else in the engine so it was fitted with new gaskets and reassembled.

I don't think your main caps were the source of the failure, that would be the piston exploding and putting massive shock loads through the block

[Westfaling]

my gut feeling was that the main cap failures were secondary to a piston/conrod issue. If the main caps failed first I'm not sure why the damage would have been so concentrated on just No2 cylinder.

[Westfaling]

Hi again. I've located a replacement short engine. Does anyone know approximately what a short engine weighs??