lotustwincam
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| posted on 9/8/07 at 08:58 PM |
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Piston Broke - What caused it?
Maybe the headline should read "Pissed and Broke"!
I blew my XFlow at a recent trackday. Although I have a fair idea of what the root cause of the problem is, I'd like a second opinion.
This is a photo of No. 3 Piston.
You can view a larger pic by CLICKING HERE
Basically the top compression ring is broken and the top two ring lands are also broken.
No. 2 piston is amost identical although only the first ring land is broken. In fact butted together - crown to crown - they are almost a mirror
image.
The engine is a 1600 crossflow, overbored by .060". The pistons are 1100cc (yes really) AP cast items which have been machined to give a CR of
around 11.25:1. Kent 244 cam, big valve ported head, and twin Dellorto DHLA40 carbs.
Like I say, I have a fair idea of the root cause, but don't want to influence anyone. My second question is "Would you replace just the
two pistons or all 4?"
I have examined 1 and 4 under a powerful magnifying glass and can't find any signs of cracking.
Thanks for any opinions offered.
Drew
[Edited on 9/8/07 by lotustwincam]
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omega 24 v6
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| posted on 9/8/07 at 09:17 PM |
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pre ignition (detonation).
Had a second look and wondered if thats rust round the piston and if so is the gasket gone between the 2 cyls allowing hydraulic pressure down the
side of the piston skirt.
[Edited on 9/8/07 by omega 24 v6]
If it looks wrong it probably is wrong.
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mookaloid
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| posted on 9/8/07 at 09:21 PM |
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Looks to me as though the ring might have been broken at assembly and the broken bit gradually broke the ring lands
The rest of the assembly looks pretty new.
Just my thoughts
Cheers
Mark
"That thing you're thinking - it wont be that."
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lotustwincam
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| posted on 9/8/07 at 09:36 PM |
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Its not rust (though I believe that area is the clue), the head gasket was fine and the ring wasn't broken at assembly time.
The engine was a total rebuild just over a year ago and has less than 2000 miles on it.
Thankfully the bores seem to have escaped any serious damage.
[Edited on 9/8/07 by lotustwincam]
[Edited on 9/8/07 by lotustwincam]
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thomas4age
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| posted on 9/8/07 at 09:54 PM |
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scuffing on the top might mean the bores aren't correct for the diameter piston, which would eventually lead to rings going pop aswell.
grtz Thomas
If Lucas made guns, Wars wouldn't start either.
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Chippy
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| posted on 9/8/07 at 10:43 PM |
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It's very hard to get a good feel for the problem from looking at a photo, but it would seem that a partial seizure has happened at some time,
maybe cracked the rings, and then it's just a matter of time before it all goes pear shaped. Well at least that my opinion for what it's
worth. Ray
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lotustwincam
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| posted on 9/8/07 at 11:06 PM |
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Thanks to all for your opinions.
I've just found THIS EXCELLENT ARTICLE which seems to confirm
Omega 24's and my first thoughts - Detonation.
Quote from page 2 "If I were to look at a piston with a second broken ring land, my immediate suspicion would be detonation.
Another thing detonation can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind
of look if detonation occurs."
Well although there are no marks on the top of the piston, the side of the piston above the rings looks decidedly sandblasted.
I still don't know whether I should risk keeping the two undamaged pistons or replace all four?
Either way I think I'll need to pay some attention to carburation and ignition timing.
Drew
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MikeRJ
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| posted on 10/8/07 at 09:33 AM |
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I agree, definately detonation damage. What fuel are you using? Might be worth double checking the CR to make sure enough was machined off the
pistons; 1100 pistons in a 1600 can give massive CRs. Why not 1300 pistons out of interest? They can be used without machining to give about 11:1 CR
IIRC.
If this is for track use I would replace the other pistons, as they aren't terribly expensive anyway. If it was for road then I would very
carefully inspect the others for any sign of detonation related damage before considering to reuse them.
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Dangle_kt
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| posted on 10/8/07 at 11:13 AM |
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quote:
Originally posted by lotustwincam
Thanks to all for your opinions.
I've just found THIS EXCELLENT ARTICLE which seems to confirm
Omega 24's and my first thoughts - Detonation.
Quote from page 2 "If I were to look at a piston with a second broken ring land, my immediate suspicion would be detonation.
Another thing detonation can cause is a sandblasted appearance to the top of the piston. The piston near the perimeter will typically have that kind
of look if detonation occurs."
Well although there are no marks on the top of the piston, the side of the piston above the rings looks decidedly sandblasted.
I still don't know whether I should risk keeping the two undamaged pistons or replace all four?
Either way I think I'll need to pay some attention to carburation and ignition timing.
Drew
If the cause is detonation then it will be the same with all the pistons, it is just that some have held up better than others (as number 2 piston
held up better than number 1 piston mentioned in your first thread) therefore I'd change all 4, unless there are additional factors which would
cause just two of your cylinders to fire early...
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MikeRJ
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| posted on 10/8/07 at 11:17 AM |
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quote:
Originally posted by Dangle_kt
unless there are additional factors which would cause just two of your cylinders to fire early...
It's quite possible that the two center cylinders run hotter than the outer ones.
I have just rebuilt a Fiat Coupe 20V turbo engine that had a single piston fail (broken and melted ring lands) yet inspection under a microscope
didn't show any evidence whatsoever of detonation on the other 4 cylinders. Of course it is possible that injector for the failed cylinder is
faulty causing it to run weak, they will be tested before being refitted.
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britishtrident
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| posted on 10/8/07 at 12:04 PM |
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I would lower the compression ratio a bit
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lotustwincam
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| posted on 10/8/07 at 07:06 PM |
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quote:
Originally posted by MikeRJ
I agree, definately detonation damage. What fuel are you using? Might be worth double checking the CR to make sure enough was machined off the
pistons; 1100 pistons in a 1600 can give massive CRs. Why not 1300 pistons out of interest? They can be used without machining to give about 11:1 CR
IIRC.
If this is for track use I would replace the other pistons, as they aren't terribly expensive anyway. If it was for road then I would very
carefully inspect the others for any sign of detonation related damage before considering to reuse them.
I've been using Super Unleaded plus an octane booster additive.
The reason that its fitted with 1100 pistons is simply because thats the way I bought it as a short engine. I can only imagine that either the engine
builder had the 1100 pistons lying around and wanted to use them, or possibly more likely it was quite a straightforward machine job to provide valve
clearance and lower the CR without having to machine individual valve pockets.
Here's a pic of the top of the piston before full assembly.
(The brown staining is just some form of preservative wax, which of course particles of dirt have stuck to. But don't worry it was thoroughly
cleaned before final assembly.)
I also have to agree with you Mike, that the centre two cylinders run hotter than the end ones. After all they are each sandwiched between two other
cylinders.
I think the failure was probably caused by a number of factors, but exacerbated by running a bit on the lean side and with borderline cooling. The
temp gauge was reading a tad high. Oh, and did I mention the BMW M3. I'll not try to keep up the next time!!
I've examined the other two pistons again and decided to take a chance with them this time. If I suffer another failure will replace all four.
With 1300 pistons. Not least because 1100 +0.60 are as rare as hens teeth.
Think I also need to invest in an oil cooler and changing to a better rad. Polo? I'm also in the process of upgrading to Megajolt so I think
I'll be keeping the timing on the conservative side.
And of course richer main jets.
Drew
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gazza285
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| posted on 10/8/07 at 07:49 PM |
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quote:
Originally posted by MikeRJ
It's quite possible that the two center cylinders run hotter than the outer ones.
With them being a mirror image, were the broken lands both next to the paired exhaust ports? Fairly common hot spot on Crossflows, which can lead to
detonation. Fueling will be much more critical at 11.25:1 on a Crossflow, I presume you had it set up on a roller?
DO NOT PUT ON KNOB OR BOLLOCKS!
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MikeRJ
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| posted on 10/8/07 at 08:09 PM |
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quote:
Originally posted by lotustwincamIf I suffer another failure will replace all four. With 1300 pistons. Not least because 1100 +0.60 are
as rare as hens teeth.
If it needs a rebore after the next blow up, then I have a brand new set of 1300 +90 pistons that I bought before finding out my block had been
linered 
Was it ever rolling roaded to set the carbs and igntion? Sound investment if your engine is running a bit close to the edge!
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lotustwincam
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| posted on 10/8/07 at 08:58 PM |
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quote:
Originally posted by gazza285
quote:
Originally posted by MikeRJ
It's quite possible that the two center cylinders run hotter than the outer ones.
With them being a mirror image, were the broken lands both next to the paired exhaust ports? Fairly common hot spot on Crossflows, which can lead to
detonation. Fueling will be much more critical at 11.25:1 on a Crossflow, I presume you had it set up on a roller?
The broken lands are on the side of the piston furthest from the spark plugs but closer to the exhaust valves than the inlets.
It was never rolling roaded as I was waiting to get the MegaJolt up and running first. It will however be a priority once I get a few hundred miles on
the new pistons. With the benefit of hindsight, two runs on the rollers would have been less expensive than a rebuild.
Mike, I will keep you in mind for the pistons, but hope I don't need to take you up on the offer.
Drew
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