Oddjob

Tore down a motor that blew on track last wknd.

Obvious headgasket rupture on #4.

Coolant got in all 4 cylinders.

#1 and 4 show some scuffing that I have not seen before. Not the typical vertical score lines (longitudinal), but more horizontal lines (circumfirential) and hard scuffs in one spot near TDC. Is that from detonation, or is that from coolant being drawn into the cylinders at high rpm/load?

JET951

how old was that Head gasket Jim? there isnt much carbon build up on those pistons.

Oddjob

3 years, but only about 1000 street miles and 5 track miles on the HG before this happened.

Yeah, when the coolant gets in the cylinders it really cleans up the piston crowns and valves nice...

JET951

We have seen those horizontal lines before on engines that sit for a long time, we had a spare 928engine here that had perfect bores. we lubed up the bores before we let the engine sit for about 6 months. turned it over and there were markes were the rings had been sitting for that period of time

Tom M'Guinn

Same thing can happen when coolant gets in. The coolant picks up carbon from the piston and then stains the bore when it sits puddled up on the piston. Can't tell from the picture if thats a stain or a score mark though -- can you feel anything? You may have had some kind of "event" in there from the looks of that gasket. Scuff marks at cylinder top probably came from too much heat.

Oddjob

Unfortunately not stains. You can feel the marks. More than what shows in the picture and gets worse near the top of the piston/ring travel. Just on two cylinders, 1 and 4. 2 and 3 look normal.

Just curious if the bore marking is a sign of detonation that lead to the HG failure, or if the scuffing is from coolant after the HG failed.

My concern is that if the detonation was severe enough or enough coolant got in the cylinders when the motor was still at high rpm/load - that the rings may be damaged, the rods stress cracked, and possibly the rod bearings.

Chris White

The line half way down was probably from the rings sitting at that point of a long period and corroding a bit.

I don’t see any signs of detonation of the piston crowns – that is where you see it first.

The head gasket did let go in a more spectacular than usual fashion! The scuffs at the top could be from that – the coolant might actually cavitate locally (when at high rpm) and cause some interesting erosion.

Oddjob

I heard from the driver following this car, that the plume of white smoke was pretty spectacular too.

Had coolant blow/sucked into the intake, some in the j-boot. Coolant had been in all 4 cylinders. As mentioned above, am concerned that it may have been enough to hydrolock it, not enough to shatter a rod but enough to stress crack it.

If not detonation, what would tear up a HG like this in under 2 laps? Car is an 89T, stock boost.

Considering: ran lean (FPR, Fuel pump, injector), overboost (bad WG or control), bad gas.

Chris White

Was this in the first 2 laps after a cold start?
Had the car sat for a while?

Oddjob

Car was driven 150 miles to the track the day before. Was the first session in the morning. Not sure how warmed up the car was before the driver got on it, but first lap was a warmup (friend's car, and I was not in it when this occurred).

Chris White

Sounds like a typical ‘squished gasket’ failure.

The ends of the head gasket can get compressed more than the middle due to the differential loading (the end cylinders have two dedicated head studs and only share two head studs while the middle two share all four) and as a result the headgasket can get compressed more on the ends when the engine is hot. This means that the ends of the stock type gasket can get compressed over time. This leads to less sealing when the engine has not fully expanded (due to heat).

Remember that the block / head is aluminum and the head studs are not – unequal expansion rates. Differing clamping loads when hot vs cold.