I took this pic this afternoon as soon as I got my head off. one day its running fine, then for a week or so it felt a slight hair low on power. then during a wot pull it felt awkward for a moment and then THUUUMP! 2 quarts of oil blast out my catch can. it took all of 4 or 5 seconds for this piston to go from good to melted.

the culprit? both exhaust valves in this cyl were at 0.002" cold clearance. all the others were at stock spec of 0.007"-0.008", and the other 3 cyls are totally flawless. that is the exhaust side of the piston by the way

and to those who may try to chime in debating something else, no, there were zero issues with fueling and zero issues with ignition timing. and when I built this motor I specifically made sure this cyl had very slightly less compression than the others due to it being cyl 3, typically the hottest cyl in a Honda motor. and my setup worked well, as this cyl was always the coldest cyl in the motor. also, no, there were zero burs or hotspots on the piston or head chamber

but yeah, boost + exhaust valves hanging open a couple thousandths = catastrophe.

luckily the cyl wall is totally unharmed. the splotchyness is just aluminum stuck to it, and I already was able to scrape it off to reveal a flawless surface still underneath. and the lines that appear to be scoring are not actually scores, they are simply a discoloration on the surface. I got very very lucky; a simple honing will bring this cyl back to brand new, and the only parts I'll need are a new piston of proper size and a set of rings, and a couple gaskets.

 

I don't see how valve lash did that. There aren't any markings on the valve reliefs and I doubt you have a bent valve

looks like the cylinder just ran hot and melted... check the fuel injector on that cylinder for proper operation and make sure it isn't clogged/doesn't have a weak spray pattern/weak flow

 

You can see tiny specs all over that piston top that look like detonation

 

Loose valve lash burns valves not pistons.

those specs is sighs of detonation. Either too much timing bad gas or ran lean

 

you realize your blaming the exhaust valve lash, when its smaller then the stock spec....last i checked 0.002" was less then 0.007"

post a pic of the head

 

he probably means 0.020

 

Tight valve lash burns valves not loose.

 

to hot spark plugs! here is my piston



what spark plugs are you using?

 

ArniFPR, yours looks eerily similar to mine lol. I can assure you though my plugs are fine, read below. I do notice though you have near zero carbon buildup on your piston. notice how mine actually has a nice thin smooth not-chunky layer of carbon on top. that's because these pistons are not polished, but it's also because there is no det and there is not too much heat through the majority of the cylinder like a plug would cause. when there is det or too much heat, as long as afr's are not pig rich, you will almost never see noticeable carbon buildup because the det literally blasts all carbon buildup right off the piston

I knew all you guys would post all that crap, which is specifically why I posted this thread, to display what VALVE LASH can do. As I said, fueling is 100% perfect. ignition timing is 100% perfect. plugs are all perfect. those specs you see on the top of the piston are not det. det causes pits. those specs are actually pieces of the piston stuck to the top, not pits.
I run bkr8eix, and they're actually a little too cold for my setup in this fall new England weather, I'll be going back to 7's shortly.

injector is fully operational, zero issues. afr's were a hair rich, 11.5 across the board at any boost above 2psi (I run 9). ignition timing is 1.5 degrees retarded from ideal. there are zero signs of det on the pistons, in the head, and on the plugs in all cyls including the one pictured.

and yes, I AM blaming the exhaust valve lash for this damage. lash being too tight holds valves open a hair when hot, which in a boosted motor causes extremely hot exhaust gases to flow back into the cylinder during the intake stroke, and can actually ignite the incoming mixture as it comes in, which is exactly what happened here.

no I did not bend any valves, there was zero valve to piston contact. the entire head and valvetrain are still completely flawless.
this cylinder is actually the coldest cyl in the engine, I built it specifically for that because this is the infamous cyl 3. there are no signs of heat on anything in the entire cylinder, except that obvious hole, which happens to be right in between the exhaust valves, also which is the same place the incoming intake air/fuel mixture smashes into.

with it running a 0.002" cold lash spec, that's 0.005" too small, meaning when at full temp under full load it will hold the valves open 0.005". that is nowhere near enough to cause piston contact in my motor, especially with stock cams. it is enough however to cause a decent sized leak of hot gases to re-enter the chamber

 

just trying to learn somthings. But what motor was this and how do you guys know what temperature spark plugs to get?

 

who in the hell would let their lash get out to 0.020"?? I say that while smiling, because I obviously let mine get to 0.002". and yes I did mean 0.002". if my lash was at 0.020", I'd spit a rocker out near redline, and I'd hear major valve noise long before it opened up that far.

too loose of valve lash would not cause piston damage by any means though. it would just be noisy as hell, and cause rocker damage, and a reduction in power

 

this is a 1990 prelude, stock b21a1. I'm using a b20a5 block though bored 2mm over for the larger b21 pistons, because the b21 block is FRM cyls and the b20a block is steel cyls.
(DOHC non-vtec closed deck, near identical setup as an LS or H23 head)

temperature of the plugs is determined by reading the sparkplugs. typically though, boltons will use 6's, 275whp and under will use 7's, 300hp and over will want 8's, and the 275-300 range can often use 7's or 8's but best to just try both and read the plugs.

I run 8's, and recently made 250tq/252whp on a mustang dyno at 9psi

 

There's more to consider here before announcing a cast in stone root cause.

Of the 4 valves in that cylinder, which exactly were .002 cold...all of them? If they were, how does this necessarily correlate to the damage?

It's possible the cyl temp was so high that the valves distorted or grew from heat damage. Obviously the piston got pushed past it's thermal limit, how do you know the valves did not also? The valves should all have their total length and runout checked.

Only a few thousands of lash is what separate safety from meltdown ? This seems like poor conditions to run an engine under. You should have the engine and tuning strategy setup with some buffer if you want it to last.

It's common for people to get excited and want to find the smoking gun as to why something failed, but take the time and have the patience to study all possibilities.

 

Is it possible the cylinder was overly rich? Says this was the coolest cyl of em all, mabey because it was drenched in fuel. Do you have afr gauges for each cyl? Egt on each? Being too rich can easily heat things up in certain circumstances and can end up being like a cutting torch. We melted a head down on a sbc from a nitrous switch malfunction wich resulted in a pass with 3 stages of extra fuel with no nitrous to add to it.

 

stock spec for the exhaust valves is 0.006-0.008. I usually run 0.007-0.008, but have recently found that 0.008-0.009 makes a hair more power, though does make a little noise at times when cold. both exhaust valves were at 0.002.
stock spec for the intake valves is 0.003-0.005, and this spec seems to work well with my setup. both intake valves were at 0.004-0.005

I lent my valve spring compressor to an out of state friend, he's shipping it back to me tomorrow. soon as I get it back, I will be tearing the head down to inspect everything, as I obviously don't want this to happen again, and I want to make sure the valves didn't get burned either. but from what I can see still assembled, there is zero damage to the head's combustion chamber and zero damage to the valves. also threw some water in the exhaust ports and not a single drop leaked through the valves even after sitting for half an hour.

that's why I'll be running 0.008-0.009 lash on the exhaust from now on instead of 0.007-0.008. yes a couple thousandths is a pretty large margin on our motors. 0.002 is pretty far off from 0.008. keep in mind my exhaust valves are stainless steel from end to end from the factory, and my intake valves have have mild steel heads and lower stem mated to the top half of the stem being SS. I've been told this over the years, but just recently actually threw a magnet to the valves to check and sure enough it's true.
considering the thermal expansion rate of stainless steel, it makes total sense for a cold exhaust valve lashed at 0.002" to hang open when hot.

that's part of my reason for this thread, not just to share, but to also consider other options I may not have thought of already. As stubborn as I come off as at times, I am always yearning to learn more

as of right now though, I can't find a single thing wrong with anything, or any signs of anything else that could have caused this.

even though the plug is oil coated, it's still readable, and it matches the 3 clean plugs exactly; zero det, ideal af, a little cold for heat range, ideal timing both wot and part throttle. the pistons and combustion chambers all also say the same.

the odd part however is even this holed piston shows no signs of det. which theoretically confirms my assumptions that the exhaust vavles hanging open were igniting the incoming air/fuel on the intake stroke, which is when I'm assuming this hole got burned. so I'm thinking it must have burned up enough air/fuel before full compression was reached that it actually prevented det even with a glowing piston.

 

I made this the coolest cyl in the engine by giving it the largest head chamber. when I originally built this head, I smoothed the chambers considerably, and knew some cyls had more metal removed than others to achieve the smoothness I was looking for and in the places I wanted. So I cc'ed the chambers, and matched cyls 1 and 4, gave cyl 2 0.1-0.15cc higher volume, and cyl 3 0.25-0.3cc higher volume. although, I've had the valve seats cut and the surface planed 0.008" since then, so I am due to cc the chambers again just to verify my preset variance is still where I want it.

and no, it's not possible this cyl was overly rich. I check my plugs way more often than necessary, and fueling has always been completely spot on. I also send my injectors out annually for cleaning and testing, and they never see more than 1% variance even when dirty, and have never been reported to have a fair or poor spray pattern (except when I first bought them, used).

I'll be doing a visual spray inspection of my injectors before final reassembly, just to be sure patterns aren't crappy, and to listen for any odd noise in them.

 

the only other abnormality I'm noticing is that the plug in this cyl is indexed opposite the other 3. cyls 1 2 and 4 the gap faces the exhaust side, cyl 3 the gap faces the intake side.

 

that deff. looks like detonation from getting very hot very fast.. to tight of valve lash burns the valve and crystalizes it, making it brittle and will eventually break off and bounce around the chamber, not a melted piston. i have a TON of aircooled VW pistons that look identical to yours- cause- mouse nest in the cooling fins, causing hot spots and subpar cooling- resulting in detonation and melted piston.