And then it went Bang

[Dave Prior]

Good lord.

[Tony Betts]

sad news that one charles.

but thats not bad for a highly tuned 90 year old engine.

i take it you have extra studs in the valve chest, could this be the source.

also do you mind letting us know the bore size, the wall thickness looks a little thin on the valve chest side again.

tony

[Charles P]

Pistons are 57.5mm. It's a mid 30's block. It looked thin to me as well.
There weren't extra studs in the valve chest - it's just broken across there.


Charles

[Ian Williams]

Hmmm, impressive! What sort of CR, valve lift, and valve springs do you run? Very flat tappets and a hell of a lot of load on the the poorly supported valve chest side, possibly casting core misplacement since manufacture or other casting flaw leading to stress fracture???? Will be fascinated to know what you eventually decide is the reason.

[Colin Reed]

Hi Charles
Very unfortunate looking at bores number 3 & 4 there seems to be scoring in the skirt area on the non thrust side suggesting the pistons may have nipped up,
also what was the deck height set at.
all the Best with rebuild

[Charles P]

Hmmm, impressive! What sort of CR, valve lift, and valve springs do you run? Very flat tappets and a hell of a lot of load on the the poorly supported valve chest side, possibly casting core misplacement since manufacture or other casting flaw leading to stress fracture???? Will be fascinated to know what you eventually decide is the reason.

Runnng 5:1, modest valve lift (Paul Bonewell) who also did the tappets to match. Valve springs are doubles, which I may choose to downgrade in the rebuild. 

The mode of failure, as I see it, is either 3 studs break (rear and two rear-most offside), block lifts in that corner, breaks the base flange around the remaining offside stud, block pivots but is firmly held by the through bolting on the near side so breaks under the valve chest. 
The alternative is that the block flange on the off side breaks first, leading to the 3 studs breaking and the same outcome for the valve chest.

I'll run the block through the bandsaw in due course to see whether the casting was weak from the start. Looks thin.

Charles

[Colin Reed]

Hi Charles, I would be looking at why the studs broke you would expect the casting to brake first if it was thin and that does not appear to be the case did any studs pull out of the crankcase.
Colin

[Ian Williams]

In a performance engine normal pattern block studs from some suppliers are not up to the job, you really need to ensure they are high enough grade to take the load or breakages are common in my experience. Charles your engine is blown isn't it? What sort of boost do you run? As we know the factory had this exact problem with their blown engines hence the stiffening gussets and extra studs.

[Charles P]

Stud diagnosis will wait until the bottom half of the engine is out.
Before that I want to prepare the new crankcase and block. In the meantime finding proper high strength BSF studs is on the task list

Charles

[Ian Williams]

Stud diagnosis will wait until the bottom half of the engine is out.
Before that I want to prepare the new crankcase and block.  In the meantime finding proper high strength BSF studs is on the task list

Charles

If all else fails and you can not get the material, I have used long 5/16 unf or unc grade 8 bolt, cut thread off, trim to the length I require and put a bsf thread on. You could I suppose even convert to metric, throws hands up in horror!