Moorron
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 posted on 1/9/08 at 02:22 PM |
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piston breakage
I require some help with my rebuild and the Haynes isn’t helping. Its regarding the positioning of my piston rings. After replacing the old pistons
and rings a short time ago and having the bores honed I asked on here what should I do with the running in. It was suggested that if no work had been
done on the bearings or crank minimal running in would be needed and so after 50 miles I gave it some, but it broke something and after taking it all
apart again it showed the 3 of the 4 pistons had broke between the top and middle rings on the inlet side (all identical) and the part was about one
inch long. The car still ran but was smoking and using oil but i am unsure what caused it and I don’t want it to happen again now I am ready for the
assembly with new pistons (again).
Like anything I do I question myself when it goes wrong and if it wasn’t caused by heavy driving the only other possibility is the piston ring
fitment. The Haynes shows the top ring to have an id mark or ‘R’ or ‘N’ on its upper face, the middle ring shows to be tapered. But both rings look
perpendicular and both have ‘RN’ on one of their faces! They do look different though; the one ring has a 0.5mm or so silver ring around its outer
edge on both faces. I have tried to see if this silver ring is from a machined tapered edge but it looks sharp to me and it can’t be the middle ring
taper as its on both faces. So now i am stuck with what to do with them, did I ‘get it wrong’ and placed the middle ring on top and vise versa for the
top ring and its this that caused the pistons to break or did I just go to fast to early or is there something else.
To top it all off I was running a DIY turbo charger on it, but I had been running with it on the old pistons fine for a while and I only replaced them
because the car has always used oil from when I got it a few years ago.
Sorry about my spelling, im an engineer and only work in numbers.
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gingerprince
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| posted on 1/9/08 at 02:36 PM |
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I'm no expert, but there are schools of thought that suggest the best way to run an engine in is to rev the t1ts off it from the word go. This
apparently best helps the rings bed into the freshly honed cylinders. Have a read here if you haven't already seen it: -
http://www.mototuneusa.com/break_in_secrets.htm
Based on that I'd suggest it's probably not just "giving it some" that broke it. Unless you "gave it some"
cold.
As to why it actually happened, sorry can't help there!
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MikeRJ
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| posted on 1/9/08 at 02:38 PM |
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quote:
Originally posted by Moorron
after 50 miles I gave it some, but it broke something and after taking it all apart again it showed the 3 of the 4 pistons had broke between the top
and middle rings on the inlet side (all identical)
Are you saying the ring lands had broken (i.e. the bits of the piston that support the rings)? If so that is a sign of severe detonation, so you
might want to be checking the fueling and ignition timing when you re-assemble the engine. Are you using standard ring gaps?
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wilkingj
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| posted on 1/9/08 at 03:04 PM |
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 I'm Piston Broke as well
1. The point of a journey is not to arrive.
2. Never take life seriously. Nobody gets out alive anyway.
Best Regards
Geoff
http://www.v8viento.co.uk
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bimbleuk
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| posted on 1/9/08 at 03:19 PM |
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Detonation will destroy piston very quickly. I had a set of four pistons fail in a very similar fashion. Basically I had been running right on the
edge of their physical limit. So when we tried just a few hundred more RPM to make a couple more PSI of boost they failed almost instantly! No signs
of detonation from outside or inside except all four pistons had cracked ring lands.
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Moorron
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| posted on 1/9/08 at 06:06 PM |
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yep it is the ring lands. i have a AFR meter on it and its running rich if anything at 10.5 AFR on boost.
here is an attempt to add some pics
 
piston 1
 
piston 2
 
ring 1
 
ring 2
Which ring would you say is the top ring?
all 3 pistons have broke on the inlet side.
Sorry about my spelling, im an engineer and only work in numbers.
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gazza285
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| posted on 1/9/08 at 06:47 PM |
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Did you machine off the wear ridges completely before honing? And how did you check that you had?
DO NOT PUT ON KNOB OR BOLLOCKS!
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AndyGT
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| posted on 1/9/08 at 08:50 PM |
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"the best way to run an engine in is to rev the t1ts off it from the word go"
my grandad used to say that "paper doesn't refuse ink". Looks like hypertext is the same...
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MikeRJ
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| posted on 1/9/08 at 11:33 PM |
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Usually the ring with the tapered inner edge would be the top ring.
Have all the ring lands broken in the same place? Is there any evidence of detonation on the piston crown nearest the broken ring land?
As mentioned, broken ring lands could also be cause by a wear ridge in the cylinder if it hasn't been removed prior to rebuild.
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Moorron
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| posted on 2/9/08 at 08:51 AM |
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Cheers for the help.
When the bores were honed by a local engine specialist (The guy builds all the engines for severn vally motorsport) he said it should be fine to carry
on using standard piston and ring sizes so that what i have used, I assume this meant the 'lip' at the top of the bores is fine. I couldnt
detect any ridge even before the honing but i did check last week the piston ring end gap at the top and bottom of the bore to double check this and
found its even but at top tolerance using the haynes manual but no where near the service limit.
There are no signs of pinking on the pistons or head and i didnt hear it either.
A friend at work has had a look at the rings to see which one is top and we cant see this outer tapered edge that the haynes shows as the middle ring,
but the one ring has an inner tapered edge and from what you have just said that its the top ring means i have fitted them the wrong way round.
Can i confirm that the ring with a tappered edge on its INNER edge is the top ring? Does the edge face upwards or downwards?
Sorry about my spelling, im an engineer and only work in numbers.
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Canada EH!
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| posted on 2/9/08 at 04:42 PM |
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piston breakage
the tapered side faces up, so that compression pressure pushes the top ring against the bore during the combustion stroke. I noticed some damage at
the very top edge of the piston, I believe this is why others are asking about the bore ridge.
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