Hole in piston

[Colin Cheesman]

Hi all, have a friend new to Austin sevens, he had just got his newly acquired 35 tourer on the road and joined us today on our annual Rally. 20 miles into the run from home, engine noise and some smoke from engine bay, photo below tells the story. Question is, what may have caused it? and is it viable to replace the piston, hone, and new rings all round. 
Any suggestions greatly appreciated.

[Chris KC]

Hi all, have a friend new to Austin sevens, he had just got his newly acquired 35 tourer on the road and joined us today on our annual Rally. 20 miles into the run from home, engine noise and some smoke from engine bay, photo below tells the story. Question is, what may have caused it? and is it viable to replace the piston, hone, and new rings all round. 
Any suggestions greatly appreciated.

Off the top of my head - running with weak mixture / over-advanced?

Replace piston - certainly. Just check nothing else was damaged (where did the bits of piston go?)

[Charles Levien]

Hi all, have a friend new to Austin sevens, he had just got his newly acquired 35 tourer on the road and joined us today on our annual Rally. 20 miles into the run from home, engine noise and some smoke from engine bay, photo below tells the story. Question is, what may have caused it? and is it viable to replace the piston, hone, and new rings all round. 
Any suggestions greatly appreciated.

Off the top of my head - running with weak mixture / over-advanced?

Replace piston - certainly. Just check nothing else was damaged (where did the bits of piston go?)

This happened to me with my first Seven ( a little while ago) only in my case enough mixture got into the crankcase to explode, sending the oil filler cap past my head at some speed. I heard it tinkle halfway down the street but never found it.

Charles

[Bob Culver]

The hole is not in the squish area so not caused by a stone etc. 
When this occurs in pre computer moderns it is attributed to continuous detonation under load, often not audible in a soundproofed car. But with the harshness of Sevens with advance it is difficult to imagine such a conditon escaping notice.
(the only pinging I ever observed was fleeting on flooring the throttle after overrun with a very hot engine).
The bum advice in the early Ruby book, widely repeated in other publications, to set auto advance at 2 inches has led t a lot of troubles.
Perhaps the casting was flawed.

i once spent hours trying to get an outboard motor to run and found a similar hole. With a bolt and nut it ran perfectly!

In the days before Seven owners were wealthy and firms did work cheaply, could contemplate welding. Or a rivet!

PS the head seems to have barely covered the gasket.

[Colin Reed]

Hi,
It would appear looking at the photo that the top land has broken off exposing the top ring and the bits are on top of the valves . A strip down will be necessary to find all the bits

Colin
NZ

[Ian Williams]

Colin could you post some better pictures, is it an isolated piston or are the others showing similar signs?

[Colin Cheesman]

Colin could you post some better pictures, is it an isolated piston or are the others showing similar signs?

Hi Ian, the plan is to pull the engine out and strip down, it appears to be only the one cylinder, I have only been sent the photo, I hope to have a look later today otherwise won’t be until Friday. The oil filler cap was blown off, although it was very loose so not sure how much pressure was in the crankcase. There was a knocking noise we thought was coming from the timing gears, seems we were wrong about that.

[Colin Cheesman]

       

I have had a closer look, the piston appears like it has had some extreme heat and disintegrated, not sure how this has happened as the other three are in good condition. The melted piston has left some aluminium on the bore, i am hoping it will clean up. We will pull the engine out and dismantle later this week, plan is new piston and rings all round with a good hone.  Would like to know everyone’s thoughts as to what may have caused this to happen.
We do not have much history on the car, it has sat unused for around 10 years or even more. We have only had it running for about two hours.

[Ian Williams]

My gut feeling is that the top of the piston has been over skimmed probably in an attempt to correct the deck height after skimming the block, this has left the crown of the piston too thin causing it to overheat and hole. I can not see evidence of it running excessively weak, if it were that and or timing issues I would expect all other cylinders to be effected, it will be most interesting to see further pictures once you have it apart. If I am correct a hone and new pistons should cure, BUT i would place a shim under the block rather than skim excessive metal off the pistons.
Keep us posted please.

[Colin Cheesman]

My gut feeling is that the top of the piston has been over skimmed probably in an attempt to correct the deck height after skimming the block, this has left the crown of the piston too thin causing it to overheat and hole. I can not see evidence of it running excessively weak, if it were that and or timing issues I would expect all other cylinders to be effected, it will be most interesting to see further pictures once you have it apart. If I am correct a hone and new pistons should cure, BUT i would place a shim under the block rather than skim excessive metal off the pistons.
Keep us posted please.

Thanks Ian, what you say makes complete sense, hopefully should have the engine apart by the end of the week and will report back, our club has shims available so should be able to fix.

Colin