Thoughts on hole in piston - Alfa Romeo Bulletin Board & Forums
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post #1 of 18 (permalink) Old 07-22-2019, 03:13 PM Thread Starter
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Thoughts on hole in piston

Hi there, as per the attached picture. Is this looking more like detonation or leanness? In a 77 spica spider, all the other pistons were totally ok, as was the head for all cylinders.

On a separate note... The other picture is of my front crank seal, I’m dismayed that sometime in this cars past, someone felt this was an acceptable installation of a seal.... sigh
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post #2 of 18 (permalink) Old 07-22-2019, 04:05 PM
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I did exactly the same on #4 on a race engine many-many years ago (1966?). The engine needed warm up plugs (this was then) due to big piston to liner clearance, and I normally started it with 2 step warmer champion non race plugs, then warm installed 4 N57G fine wire plugs to race.
I had an opportunity for low speed track familiarization at dusk, warmed it up quickly, fitted the cooler plugs and drove off. There was a reverse camber turn running uphill where I was no-good, so drove it harder each lap for three laps. Going up-hill on lap three, my engine became a 3 cylinder.
Back in the pits, it was clear I had removed and reinstalled the street champion in #4 instead of the N57G as 1-2 and 3 had those, but not #4. It looked EXACTLY like yours. I heard no detonation (helmet) and felt nothing unusual until it became a 3 cylinder, under load at high rpm.
So.. Check your plugs, then with a jet wire gauge, check the ACTUAL main jet and air correction jet size. I never use drilled or leaded and drilled jets but some do, and marking can be WRONG. Look for air leaks at the insulator / isolator rubbers between Webers and manifold for deterioration or gasket failure. An air leak will do this at high engine speeds.
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post #3 of 18 (permalink) Old 07-22-2019, 05:49 PM
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How is the surface of the combustion chamber? Detonation can make little divots all over, like it has been poked with a sharp punch. My feeling is like Gordon's, leanness/too hot. Do you know what kind of CO percent you were running? I was going to say were the carbs rubber mounted, but that's irrelevant if injected.
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post #4 of 18 (permalink) Old 07-22-2019, 06:21 PM
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I think it is pre-ignition probably induced by detonation. As Gordon and Andrew have suggested it could be a spark plug that is too hot or lean mixture at high power or a combination of too high CR, too much ignition advance and insufficient octane. Most motors will tolerate some detonation for a while but pre-ignition strikes very quickly.

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post #5 of 18 (permalink) Old 07-22-2019, 06:24 PM Thread Starter
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Hi there, surface of the combustion chamber was totally great. At least after the displaced metal was peeled away. I don’t know CO, sorry. I just set mixture with the 2500 rpm tune method.

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post #6 of 18 (permalink) Old 07-22-2019, 06:26 PM
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What does the plug look like compared to the others?

Ed Prytherch
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76 Suzuki GT500
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A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #7 of 18 (permalink) Old 07-22-2019, 06:36 PM Thread Starter
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I learn through this experience that sometime before our ownership, the engine was rebuilt with 10.4 borgo’s, likely a pre 1995 rebuild. I also found out the head has thirty one thou off it. I put bpr6eix plugs in recently. Has 10548 and I hunt out ethanol free 91 octane. 91 is about the best in these parts that’s commonly available. Compression check numbers are 205-208. I had just put the Marelli dual point back in, just quickly striving to the m mark, to chase my 123 problem so was chalking it up to that. I haven’t seen too many holes in pistons though.

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post #8 of 18 (permalink) Old 07-22-2019, 06:41 PM Thread Starter
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The number one plug is a bit paler than the others.
All the other pictures I’ve seen on here that were lean burn through were not centered in the piston.

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post #9 of 18 (permalink) Old 07-22-2019, 06:59 PM
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I only ever holed one like that and it was on a single cylinder motor cycle with high comp piston, shaved head and too much ignition advance.

Unlike detonation which creates high pressure after the spark when the piston is near TDC, pre-ignition occurs before the spark when the piston is still rising. The piston is trying to compress a rapidly expanding gas and the force on the piston is massive and failure is immediate.

In Gordon's case the soft plug became very hot and was able to ignite the mixture without a spark. In my case I must have had detonation which made the plug get very hot with the same result.

In your case your combined CR, softish plugs, questionable advance probably caused detonation and the plugs became very hot and #1 beat the other cylinders to pre-ignition.

I suggest that you refit the head and measure the CR on one of the other cylinders so that you know what you are working with.

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post #10 of 18 (permalink) Old 07-22-2019, 07:41 PM
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Since the head is off, move that holed piston down to look closely for liner scoring. Detonation will sometimes break the top ring and the sharp edges will score or cut grooves in the liner. In the photo, the radius from the crown edge to bore looks pretty good. Detonation will sometimes collapse the very outside edge of the piston.
If you can see what looks like aluminum plating on top of the rod below the hole, you have burned (melted) a hole through the crown. That's what I did. Messy.
I've broken engines in other ways since the holed piston, but I have learned to avoid that one!


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post #11 of 18 (permalink) Old 07-22-2019, 08:03 PM
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I blew a head gasket due to lean detonation when my Spider was on a dyno earlier this year. The only sign of detonation was the damage to the fire ring of the gasket.

Ed Prytherch
79 Spider
85 GTV6 3L
76 Suzuki GT500
2011 Jaguar XKR

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post #12 of 18 (permalink) Old 07-23-2019, 05:46 AM
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I had a single-carb Giulia TI burn a piston this way on #4. That setup was known to run lean on 1 and 4. I assume it was just leanness, but this was in 1977 and I didn't know much then.
I had detonation on a two-stroke Yamaha kart engine, didn't burn a hole but was uniformly eating up the piston and the combustion chamber.

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post #13 of 18 (permalink) Old 07-23-2019, 07:19 AM
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Burned piston

Without excessive spark advance it`s hard to have detonation without a lean fuel mix. You might read my last post at:

https://www.alfabb.com/bb/forums/mot...ml#post8442914

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post #14 of 18 (permalink) Old 07-24-2019, 01:46 PM
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That's a lot of compression (static numbers) for 91 octane. We have 93 at the pump here in NY. I'd take a guess and say you were over 11 to 1 based on the head cut and 10.4 pistons. Detonation is a killer.

If its close to detonation you might be able to get away with it by limiting total advance to no more that 32 degrees, making sure the mixture is never lean, keeping the motor temps down and running a slightly colder plug BP7?

In my race Berlina (11.4 to 1 Compression) I mix in leaded C12 or other race gas with no lead high octane. Because I like to spend my money on beer instead of race fuel. So far no issues except for my belly.

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post #15 of 18 (permalink) Old 07-26-2019, 06:21 AM
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Compression & fuel

Greg is correct, but some elaboration...
91 octane no alcohol fuel has less detonation resistance than 91 octane ethanol added fuel. You would/should be safe (and with a extra margin) on 93 octane 10% ethanol gas.

The old 10.4 pistons cc`d as in a stock head with head gasket were really just under 10.3 to one. .030" off a stock head should result in about 10.7-10.8 to 1 CR. Commonly 10.5-10.6 is OK on 89 octane mid-grade ethanol gas.

Using old Marelli distributors are often the source of ignition advance issues due to the springs on the flyweights design and age. One of the springs doesn`t come into use at lower RPMs due to the long slot where it connects to the flyweight. This causes both too much early advance and mid range advance, and most noticeably the slow return and fast idle due to the flyweights not returning to "rest position" at low rpm. Change that spring to one like the other flyweight has on it.

But the 123 distributor on the "D" curve is the answer if you can get the wiring sorted...

Richard Jemison
RJR Racing

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Last edited by Alfar7; 07-26-2019 at 06:25 AM.
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