yea, call me a tight arse now but this is small budget motorsport. We have had a ring fail and crack on a motor which has now done one season. I have my suspicions why. If when I pull it apart the bearings are in undamaged conditions is there any harm in using them again? Spent the extra to get the ACL race bearings so would be a shame to have to replace it all again.
jmma,
Feb 23, 7:51pm
I would re use, but I guess you can make a better call when they are out.
mechnificent,
Feb 23, 7:51pm
If you can clean them real nice on the back side so there is no "varnish" which can prevent heat dispersal, and plasti-gauge them. and check the nip. sure use them again.
I'd even use them in a new rod as long as they are clean and got the right nip in the new rod.
skin1235,
Feb 23, 7:52pm
as long as they all return to the same place it won't be a problem - note though the rod must stay, even with a new piston on it, if you change the rod you'll need to change the bearings in that rod, you'll no doubt plastigauge them anyway, do a good job of that and it will tell you
mechnificent,
Feb 23, 7:54pm
Why did the ring crack ? Worn groove ?
skin1235,
Feb 23, 7:55pm
and when marking them, use an engraving tool to make light marks the days of using a centre punch and a 20 oz hammer are gone, tolerances and metals today won;t take that sort of brutish treatment
ema1,
Feb 23, 7:57pm
When you take things apart, measure the bearing clearances with plasti-gauge and if the clearances are beyond required spec then bearings/ journals may need to be attended to. If within spec and appear fine then they are serviceable again. Depends on what you find when you strip things down MM. If journals need a regrind then bearings to suit the undersize of the journal grinding are needed. If you go as far as dismantling piston rods from crank and bearings + journals are defective then removing the crank from block could be necessary as no doub't the main bearings & journals could need work too. There's only one way of knowing that I'm afraid, dismantle & measure + check for imperfections. Important to reassemble in the EXACT order everything was originally !
mechnificent,
Feb 23, 7:58pm
Clean, nip, and clearance. That's what they have to have.
morrisman1,
Feb 23, 8:00pm
Not sure yet, but when I had the pistons machined to drop the compression from 13.7:1 down a bit they put the pistons in the lathe. They came back with a couple crimp marks from the jaws and that made the rings tight. I used a file to smooth it back down and the rings moved freely but I wonder if there has been a pressure point. Will know once the piston is out and I can see where it has cracked - that should tell the story. Im just assuming that it has cracked at this stage, theres light vertical scoring around 1/4 of the bore. It lost compression on that cylinder and got heaps of blow-by.
gunhand,
Feb 23, 8:02pm
Sounds like something to do with lady parts.
mechnificent,
Feb 23, 8:05pm
I'd suspect that the groove clean up was ok and it was a weak ring, might not have been warm enough when you were fitting it. They are brittle cold and warming them up a little, even in your hands saves them cracking or breaking. The grooves have to be uneven by a bit before it will break a ring, and it takes time, a lot of cycling-flexing.
mechnificent,
Feb 23, 8:06pm
Lol at the lady parts.
ema1,
Feb 23, 8:15pm
A broken ring will show on the piston lands as areas of damage possibly excess clearance in lands to ring faces. tracks could show up and down cylinder bore affected. Previous incorrect assembly could also be a reason, ring installed wrong way up ( has been known) or lack of clearance on the ring ends once rings are compressed onto piston to allow it to go into bore and when engine heats up rings with no end clearance have no where to expand, thus . snap? eg a bore of 4" requires around .016" end clearance, ring clearance specs to bore size charts are readily available online and elsewhere. Inserting new rings by themselves down the bores and measuring the ring end gap clearance in various positions down the bores for ring end clearance also can denote varying or tapered bore wear by the clearances at the ring ends being different when ring ends measured at different bore positions There are various reasons another common one is fitting new rings onto pistons with a too loose bore fitting, plus bores being tapered or out of round etc. Careful measuring proves these faults often the eye can't see. Inserting pistons with new rings odd times can result in a ring breaking, an imperfect or worn ring compressor can be a cause if a portion of it's body gets caught in a bore, seen that happen too. I'd be looking for another piston and machining it properly to suit your engines specs MM, the shop that did the initial damage you said was on existing piston need to be shot! That's what I call shoddy to the extreme. ?
skin1235,
Feb 23, 8:29pm
at least this, if they have gripped in a lathe chuck suffice to mark the ring lands then they've had it seriously out of alignment, and what shape is the skirt, cos its wider than the lands to start with, who could guess what damage they've done to it
ema1,
Feb 23, 8:30pm
The offending cylinder requires intense multi position inside micrometer measurements to be done to check it's bore as being serviceable. Light honing ( with approx 40 to 50degree cross hatch 45degree optimal)would be in order if bore proves to be in good serviceable condition A crack in bore inspection would be a requirement too.
ema1,
Feb 23, 8:32pm
That's the first thing I'd discard . out of hand before going any further. right there.
skin1235,
Feb 23, 8:35pm
post 4, you can use the old shells in a new rod, its going back on the same main journal, but be super careful re measuring plastiguage, if in doubt don't, you can buy 1/2 sets of bearings ( personally if I needed to buy a new set for just one I'd fit a whole new set)
skin1235,
Feb 23, 8:36pm
yep but theres 3 more of them in there too, and its a small budget, make ya nervous eh
morrisman1,
Feb 23, 8:42pm
Id be tempted to get some brand new pistons. Big investment but will be cheaper than engine trouble. On the plus side the car has done 26 events and this was the first DNF.
I would either get CP 12.5:1 forged pistons, or nissan N1 pistons which give 12.5:1 also but need the skirt notched for the squirter. Cost would be quite similar.
skin1235,
Feb 23, 8:50pm
its a bugga though, you take all the care and have pride in the finished job, and along comes an introduced problem that should never have happened if one piston was nicked on the lands it does throw into question the skill of the operator who had all 4 of them in his lathe this is an eventing engine, it is expected to take some punishment and retain integrity, ie not fail your call re the other 3, but if it failed for the reason you suspect those other 3 would be ashtrays too in my book
ema1,
Feb 23, 8:51pm
Wise decision ultimately chances are the other 3 pistons as skin says would have been done by the same "incompetent" person . numpty whatever?
kazbanz,
Feb 24, 6:54am
MM-my thinking on the subject is that working out the reason for ring failure will help make the decision simpler. Also what are you going to do with the barrels? Push a replacement sleeve in and replace just one piston or bore out all four?
petermcg,
Feb 24, 2:17pm
Re use those bearings, I used to strip down my race engine just to check clearances. Re used several times, Plasti gauge is good. Now those rings should be popped in the bore when assembling and pushed down to the bottom with a blank piston just to check clearance (ring gap). And dont forget to use Dulon when touching up the paint work.
ian86,
May 31, 1:00pm
As has been said it is fine to re use them if they are in good nick - especially if they are copper lead. Years ago when I used to do engine reconditioning - when engines came in and they had undersize bearings already we would keep them and use them for our homers. , Its a different world now though - parts are probably cheaper - finer tolerances.
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