Engine failure - seizing - what happened?

[AlisoBob]

If it was a 4 stroke engine, and the cam went flat... would you blame the jetting for that too?

[Bomberpilot]

but the spark doesn't like like it was that fat, right? so you think I should just go numbers smaller next time?
I am really curious about a leak down test...

yeah, the head just was milled but the squish was not modified. It was done by a tuner and he said that he does not want to modify the squish because then the head would have to much cc so he just cutted the squish high smaller. he said that this should compensate the raised exhaust port...
I am really afraid of using that head again!

[2T500]

ok, so when did milling the head NOT affect the squish ?

my engine builder said don't go under .060" squish with pump gas and safer to run .080"

I reckon that means .090" for me and someone running race gas .060" or less is fine if you know what your doing, i have a head that looks like yours - its an ML3 head that has been modded to look like an HF1, only problem is it has been milled to about .010" from the head surface at the edge, looks a lot like yours and cc'ed to 57cc, to be conservative i went for .090" squish by stacking steel head gaskets, on an 87 this is two 1mm gaskets leaving an option to remove one or change heads if that was to little squish.

Check your squish but maybe roosty is onto it - so rich no ping or detonation sound.. det is meant to sound like a bunch o ball bearings running around in the engine which is different to a scraping sound... dump the gas - red can be pump gas here ...

Don't (i wouldn't) run .0025 bore clearance either, we always used a rule of .001 per inch of pot so .004" and you'll be much happier running hard or .00325 to .0035 if your going to be nice to it, that rule may be dated by now so listen to the others here with experience on these engines if going under tolerance i.e .0025"

When its running again look for air leaks with ether and add a temp sensor.

[Roostius_Maximus]

true Squish doesnt exist in anything bigger than .060, theres simply not enough velocity.
On my coolhead engine i've run it at .055" on a 63cc dome with pump premium. Because the head is not able to be cut down its done on the cylinder.
Most guys start with the stock setup, piston .015"-.017 down the hole, a head gasket of .012 and an R casting head with .080"step, thats .107"!!!
Dewayne jones and Glen both stay in the .070-.080"range intend to for premium, for race they tun .060-.065"
I've liked the .070" for pump also, in a stock HF,ML3, or ML3-N head, or a re-profiled R head.

The head pictured here looks to have been cut on the squish, is it smooth into the bowl or is there a lip there, looks wierd.

[Roostius_Maximus]

2t500, that cc is less the piston volume correct

[2T500]

Roosty - no I hadn't included the piston dome..

Maybe my technique was not the best, I did 2 heads at the same time, an HF1 with a .032" step and an ML3R with about .007" step - not much..

The ML3 was sold to me as a high comp head, now i know that hi-comp more likely means shaved to shit but sounds better

The ml3 had been modded such that it was almost identical to the HF1 having the same distance from the edge to the bowl and a similar depth from a straight edge to where the bowl starts, i cc'd using plastic bolted to the head with 2 holes and a syringe, came up with 57-58cc on the ML3 and 60-61 on the HF1, both figures sound way low compared to the 63cc dome on your cool head ?

Add to that the volume of the 1mm steel gasket for an 87 and as you say - deduct the piston dome volume, I never got that far just stacked 2 gaskets and ended up with almost identical cranking pressure to the HF1

I wouldn't be using that ML3 on a late model engine with the thin 012" steel gasket, only reason I used it was it had a real nice decomp fitted, the engine still sounds crisp and runs well enough for me.

Bomber Pilot - maybe your right to be wary of that head.

[pstoffers]

My 86 head is @ 50cc We call it a True Hat....

[AlisoBob]

My 86 head is @ 50cc

Blenzall..... Eh Paulie?

[100hp honda]

By and large, problems with piston, connecting rod bearings, crankshaft and seals can be avoided simply by following the recommendations made in the manufacturer's shop manual. The single exception to this is in the fit between piston and wristpin, for the very high temperatures in a modified engine tend to cause a breakdown in the lubrication between pin and piston. Trouble can be avoided in the racing engine if the wristpin is a light, sliding fit through the piston; it should slip through of its own weight, without forcing, for if it is tight enough so that you have to tap it through with a mallet, you eventually may have to remove it with a hydraulic press. Too-tight fits may be corrected by using an old wristpin as a lap, and a dash of some fine, non-imbedding lapping compound to polish out the piston's pin-bore to size.

But higher compression ratios can also bring about a mechanical disaster: improvements in power gained in this manner are purchased at a disproportionate cost in peak cylinder pressure, leading to reduced bearing life and sometimes to an outright failure of a connecting rod or crankpin. Moreover, because the higher pressures are reflected in a proportionately greater side thrust at the piston, frictional losses are such that net power gains are always less than the improvement one would expect from the calculated air standard efficiency. Finally, heat flow from the combustion gases into the surrounding vessel (piston crown, cylinder head, and cylinder walls) rises increasingly sharply with compression ratio, so that a number of thermal-related problems intrude into the already complicated relationship between compression ratio and power. The worst of these problems is the overheating of the piston crown. A too high compression ratio will raise piston crown temperatures to the point where heating of the mixture below the piston, in the crankcase, reduces the weight of the charge ultimately trapped in the cylinder during the compression stroke to such extent that net power suffers - no matter what Mr. Otto's air standard efficiency formula may say. And if the compression ratio is high enough, heat input into the piston may raise the crown temperature to the point where detonation and then pre-ignition occur. These phenomena will, in turn, very quickly further raise piston crown temperature to such extent that the piston material loses enough of its strength to yield to the gas pressure above - the piston crown then becoming either concave (which drops the compression ratio to a tolerable level) or develops a large hole (and that reduces the compression ratio to zero:zero). Many people have encountered this last effect, and the tuner's one-time favorite ploy of "milling the head" has fallen into disrepute. But it also is possible to encounter trouble without recognizing it: There is a delicate balance between gains from increased compression ratios and losses due to increased temperatures - which appear not only at the piston's interior, but also throughout the crankcase, crankshaft, rod and all the rest of the engine's interior contacted by the air/fuel mixture. When these parts are hotter, the mixture's temperature is also raised, along with its free volume. Thus, the mixture's temperature-induced efforts to expand inevitably force part of it out the exhaust port, and as power is related very closely to the weight of the charge captured in the cylinder, this heating shows up as a power loss. The trick is to balance crankcase heating and compression ratio. There is an optimum combination for every set of conditions, but finding that optimum without heat-sensing equipment and a dynamometer is exceedingly difficult.

[2strokeforever]

the piston crown then becoming either concave (which drops the compression ratio to a tolerable level)

race clearanced compression ratio

[craigf40]

all the talk of the compression but the piston top doesn't show it got that hot nor any sign of ping...could it be possible that your head gasket was maybe leaking and when tapping off the throttle you had some water leak in the jug washing the oil off ....if not an oil issue that looks like a cold seize as in the jug was not warmed up yet but you say it was ,,,,the idea of water leaking in the jug came from the piston top looks like it has clean spots on it like water leaked in and doesn't seem like it got to hot well detonation hot from ping i mean

[Bomberpilot]

so first I wanna do a leak down test. Can you help me with that and perhaps post some pictures of your own, perhaps self-build leak down equipment? And I would need some instructions like which parts keept on the engine while doing the test (reed block etc.) and at which pressure to pump up, how long to hold the pressure and so on...

Then I will check the cc of the head and post it here. I will also check the compression by checking the cc with the cylinder and piston and head assembled.

So do you think I can just bolt-on the new topend or do you think the bottum end is also damaged? I might think that there is nearly no debris from that failure and as the failure wasn't that bad I guess also the crank and rod is ok. What do you think? Just flush the cases?

[Roostius_Maximus]

I dont think it was a wfo lean, i bet it'd ping like unreal when rolling off the throttle tho.

[craigf40]

for a leak down test you need a cylinder leakage tester piston top dead center and jam the engine in that position or it will spin to the bottom plug the compressed air on the tester and it will measure the % air leakage past your rings ...but doing a top end rebuild will require you to first run the rings in or you wont get a good reading coz your rings will leak till they bed in....i think you should hone your jug put a new piston in and dont hone to much you just want to get your x hatch form on your bore so you can run your rings in ....use your old head run it in jet it right and when its run in then you can try your modified head and sort you cc out but first run it in then it wont have any extra heat to deal with while running in. you will also be able to then tune it in right then fitting the compression head on you will have an easier job tuning it in.just rinse out the bottom end with petrol fill it and spin the crank pour out the petrol blow out with compressed air and assemble

[Bomberpilot]

Roosty - no I hadn't included the piston dome..

Maybe my technique was not the best, I did 2 heads at the same time, an HF1 with a .032" step and an ML3R with about .007" step - not much..

The ML3 was sold to me as a high comp head, now i know that hi-comp more likely means shaved to shit but sounds better

The ml3 had been modded such that it was almost identical to the HF1 having the same distance from the edge to the bowl and a similar depth from a straight edge to where the bowl starts, i cc'd using plastic bolted to the head with 2 holes and a syringe, came up with 57-58cc on the ML3 and 60-61 on the HF1, both figures sound way low compared to the 63cc dome on your cool head ?

Add to that the volume of the 1mm steel gasket for an 87 and as you say - deduct the piston dome volume, I never got that far just stacked 2 gaskets and ended up with almost identical cranking pressure to the HF1

I wouldn't be using that ML3 on a late model engine with the thin 012" steel gasket, only reason I used it was it had a real nice decomp fitted, the engine still sounds crisp and runs well enough for me.

Bomber Pilot - maybe your right to be wary of that head.

so I measured my both heads that I have - the one which was on the engine and one that has a decomp. valve and has also a shaved squish. I cc'd both the same way like you did and the following came out:

Both heads are ML3R.
Failure Head: 58ccm, diameter 89mm (was on a 90.5 engine, tuner said its alright), inner diameter that the dome has: 54.5mm, therefore squish band 17.25mm wide, squish height 0.8mm-0.9mm which is 0.0315-0.0354".
Other Head: 62ccm, diameter 90.7mm (was original on the 90.5 engine), inner diameter 61mm, therefore squish band 14.85mm wide, squish height 0.7mm which is 0.00276".

I used both heads with the stock steel gasket for the new models (I got a 1997 engine) - I measured it with 0.25-0.3mm which equals 0.01-0.0118".

Do you think I could use the 90.7mm diameter head on a 90.00 engine? Some guys say that you can use a smaller head diameter on a bigger piston diameter, but is it also possible the other way round?

So what do you guys think? Was there too much compression?
Still have to wait on the gasket kit before I could do a leak down test. And I would have to build my own kit. If you got pics of your leak down test tools, please shown em!

[blownbillybob]

The squinch on the failure head is to wide

Buy a stock head from eBay and call it a day!!!

[Roostius_Maximus]

true, or you can machine that head different. I've cut some as much as .150" into the chamber, i always sonic check the castings but theres plenty of meat there.

[iggys-amsoil]

I'd put my money on the PWK being jetted a little leaner than the other carb.

I'm with Brit, I'm going with too lean...when these things ping it can be pretty dam loud!

Plus it sounds like a bunch of small rocks hitting the skid / glide plate.

I'd ditch the K needle and try DGJ put it on the fourth notch to start. Leave the others.

[nmdesertrider]

You need a bigger ring gap with high compression too, at least according to vertex manual.

[Roostius_Maximus]

vertex will come some where in the .018" range

[Bomberpilot]

my ring gap was 0.4mm, 0.0157".

http://cr500riders.com/cgi/yabb/YaBB.pl?num=1293957443

this thread her tells that many (also Roosty) use heads like 54cc or less! I just had 58cc but somehow I guess the higher compression that lead to that failure...
So how can you run a 54cc head without race gas and without any failures?
Somehow I am getting very frustrated because I can't find the real reason for the failure - even though I think it is related to the new head...

I also still got this question:
"Do you think I could use the 90.7mm diameter head on a 90.00 engine? Some guys say that you can use a smaller head diameter on a bigger piston diameter, but is it also possible the other way round? "

And I would still need some more infos and pictures on the leak down tester please.

[100hp honda]

you have .031 clearance between head and piston ? that seems way too close to me. another thing nobody mentioned is when you cut so much from that head you also moved the spark plug proportionatly the same distance closer to the piston crown. in the pic it looks to me theres a round heat mark on top the piston. hard to tell but thats what it looks like