221 cu in 1939 Failed No4 Piston 1

[GOSFAST]

Personally based on the photos it does appear to be "old-fashioned" detonation!

Having that small "indentation" in the center makes that area of the piston a sort of "weak" point?

Most lower level pistons have that "dadum" point to help with the final machining on them. Actually most "cast" pieces usually contain them also! You won't find them on a forged Ross type piston.

Thanks, Gary in N.Y.

P.S. Also by looking closely at the "top" rod brg on the piston with the hole may give a better indication of what happened! On detonated pistons you'll normally find the "upper" inserts down to the "copper" more so than the others that haven't experienced it.

[DocJohn]

Yes, that's the mystery! I'm presuming that you couldn't measure the vacuum on individual cylinders so the 20 inches is the overall vacuum below the carb. Something very close to the port perhaps? Gasket problem? Damage to the manifold face? Has anyone drilled a vacuum port? What did the plugs look like? Did No 4 look as though it had been hotter than the others?
Sorry, this doesn't really help!
John UK

[Kurt in NJ]

Fix your spark plug spacing problem, the xtra washer changes the heat range, the plug could overheat and cause the mixture to fire without spark, and at wrong time

[Phil Gillespie]

Originally Posted by GOSFAST

Personally based on the photos it does appear to be "old-fashioned" detonation!

Having that small "indentation" in the center makes that area of the piston a sort of "weak" point?

Most lower level pistons have that "dadum" point to help with the final machining on them. Actually most "cast" pieces usually contain them also! You won't find them on a forged Ross type piston.

Thanks, Gary in N.Y.

P.S. Also by looking closely at the "top" rod brg on the piston with the hole may give a better indication of what happened! On detonated pistons you'll normally find the "upper" inserts down to the "copper" more so than the others that haven't experienced it.

Thanks also for your assistance gary. will check the top rod brg.
Can you recommend a piston for me as all pistons as removed have scuffing L to R sides of piston in varying degrees.
221cu in 3 1/16 bore 80 thou over.
Phil NZ

[Phil Gillespie]

Originally Posted by Lawrie

That looks like detonation to me,
Lawrie

Lawrie you were correct detonation it was comfirmed by MS Coombes a very long standing engine rebuilders and machinests in Christchurch.
However the cause was not as you may suspect running too lean.


From their observations the following contributed:
The Edelbrock heads(1125 64cc,) the use of dome top pistons and the distributor with too much advance. It would seem these 3 items were the contributors and of late fitting an 11A distributor gave it a little more advance again.
Also noted the scuffing caused by too rich and washing off oil.
So again on a positive note will be taking in block to have bores checked and measured amd proceed from there.
Then once correct bore size is reached after measure & hone can get pistons etc.
Again I thank you all for comments and observations.
Phil NZ

[JSeery]

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Good feedback Phil!

[Lawrie]

Good feed back
Lawrie

[Bored&Stroked]

I agree on detonation - those last pictures you posted (very clear) show signs of heat and detonation on the area around the hole. Given how thin the crowns of the pistons are, it probably didn't take much detonation and probably not for long. I'm not necessarily going to jump on the too rich conclusion as of yet - especially in that cylinder as once there was a hole, there was raw gas washing the cylinder and rings from that point forward.

Questions:
1) Do the other pistons have the same amount of scuffing?

2) Do you know your total advance?

3) Any signs of detonation on the other cylinders?

[Phil Gillespie]

Originally Posted by Bored&Stroked

I agree on detonation - those last pictures you posted (very clear) show signs of heat and detonation on the area around the hole. Given how thin the crowns of the pistons are, it probably didn't take much detonation and probably not for long. I'm not necessarily going to jump on the too rich conclusion as of yet - especially in that cylinder as once there was a hole, there was raw gas washing the cylinder and rings from that point forward.

Questions:
1) Do the other pistons have the same amount of scuffing?

2) Do you know your total advance?

3) Any signs of detonation on the other cylinders?

All pistons had similar amount of scuffing. No piston without scuffing.


Running an 11A divers helmet distributor with Initial timing of 4 degree
and full Mech Advance of 12 degree. Total being 28 degrees.


No signs of detonation at other pistons but the guy at the engine builders said that there would be small radial cracks going out from the centre "hole" at centre of piston crown.


The rich mixture was shown to be at the edge of the rings a sharp edge, and on top top able to detect a ridge with thumb at outter edge. He also commented that ring wear was greater than normal with ring in groove.
I am postive now to move forward with this and with care and attention a good runner back in the 39 Coupe.
Bottom end brgs all look good as do the top wrist pin brgs.
Of note the con rods in use have the tangs at one side for the shell brgs.
But will stick with the existing brg set up. These are 40 thiu oversize, or undersize depends how you want it.
Thanks again to all.
Phil NZ

[Talkwrench]

Ok someone may have to school me on this one .. Normally for detonation issues its a lean condition with a lots of advance [to blow a hole in a piston].. for your situation was the car running ok ..? Im thinking for it to do whats its done didn't you feel something wasn't right.. ? And as far as being rich... for any issues surely the plugs would show much much darker for any issues of 'wash'..?
Just thinkin'...

[cas3]

i had a chain saw once that would "pop" every now and then, and then go rite back to work. i found out later each pop was a piece of the piston blowing off due to lean mix. top of piston had specs missing all over, and one hole. good story phil, hope you get it all good

[john in illinois]

too hot a plug can contribute to detonation. Do any of the plugs have melted edges on center electrode under magnification? As Kurt said spacer rings could add heat.

John

[flatheadmurre]

Check the porcelain on the sparkplugs, if you see any signs of aluminum or black particles fused to the insulator you have detonation issues.

[48fordnut]

This has been a good tutorial from a lot of good guys. Just read the whole blog. Received a set of pistons from speedway today. I think the add said Offenhauser. 3 5/16x4 for a 49 mercury block. Any comments?
thanks,
jim

[Fordors]

I can tell you that they were not manufactured by Offenhauser. Speedway paid Offenhauser for rights to use the name on some of the products they market.

[quickchange34]

After looking at that pistonI would have to say it was a casting flaw . I have seen it before with egge pistons . I would not put them in a lawn mower.

[flatjack9]

I may be jinxing myself, but I have used Egge pistons in many builds without a problem.

[Phil Gillespie]

All plugs looked ok. Definately no sign of heat etc. Once all sorted will go with the plugs where tip does not protrude out so far, thread length is same. Cant remember the name. hope to drop block off early next week for the measure up, final hone and sizing for pistons. These Edelbrock heads with their 64cc do alter the combustion a little and other items as advance etc have now to be match up for that ideal combustion process.
All compressions taken after the mishap were good. Valve clearances checked were also good a couple of exhausts at 0.018 a couple above the 0.016 but will probably settle with that as lifters in use are non adjustable.
Phil NZ

[Phil Gillespie]

Originally Posted by flatjack9

I may be jinxing myself, but I have used Egge pistons in many builds without a problem.

At this stage also I will be using the Egge pistons with Grant rings again. Looking at 0.083 O/sized set. The 0.080 once honed will finish up at 0.083 as Egge have pistons at this size.
Phil NZ

[40cpe]

So judging from your post about ignition advance, can I assume you have the engine back running?