Cracked (newish) B18 pistons
 

I removed my Amazon’s head over Christmas to see how its valve seat recession was getting on (quite well, as it happens – see pic 5. This led to an intriguing discovery: all four pistons are cracked.

All have cracks across the tops of their crowns at 90° to their gudgeon pins (pics 1 and 2). On one, the crack has spread down one side, leading to significant melty distress (pic 3).

These were new standard-size items, fitted by a previous owner c20,000 miles ago. The carbon buildup over the melty bit of the really bad one was consistent with that over the rest of the piston, suggesting that whatever caused the cracks did so fairly soon after they were fitted.

The skirts are worn more than one might expect on the thrust side, considering the milage they’ve covered (pic 4). Otherwise, there are no further signs of distress. The bores are in reasonable condition.

I can’t think of an entirely plausible cause. The consistency of damage across all four pistons is particularly vexing. Really advanced ignition timing could’ve done it, I suppose – or a prolonged period of overheating? A dodgy batch of pistons? A combination of these?

Theories invited…

Sam

Attachment 109392

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Incidentally, does anyone recognise the pistons? I'd be interested to know where they came from.

The heads dont normally suffer any VSR when running either unleaded or LPG so would not expect to see any unless you were running lean.

Piston damage is strange. The side of that one looks like it has been dropped

Sam, please check the gap of the rings if you have them in the bore without pistons.
The pistons did overheat.
I'm the same opinion like Dai, it looks like they where dropped before installation.

The seats of the exhaust valves are gone by fare, this head need valve rings fore shure. The right moment to build a unleaded head.

good luck, regards Kay

No – the crack on the side of the worst piston definitely stems from the centre of the crown. The pick-up and deformation around the edge of the crown is a result, not the cause.

I’ll investigate the ring gaps shortly...

I'm drawn to conclude that low-quality pistons are a significant factor, here. Any thoughts on how I can ensure their +20 successors are of suitably Volvo-like quality?

I’ve never owned a car with such distinct valve seat recession. It was significant on the exhaust valves of cylinders one and four. It was occurring at quite a pace – I reset the clearances twice in 3000 miles (of mostly high-speed/high-rpm driving), and they’d closed substantially on these valves. It’s not running lean. It’s suffered a raft of neglect/haplessness in the past, though.

The ring gap’s 0.60mm. The Haynes manual states 0.25-0.50, so it’s not wildly oversize.

I seem to have lost my micrometer. A vernier suggests the bores are standard 84.0mm. The piston skirts are 83.5mm across the gudgeon pin and 84.0mm at 90° to it. These measurements are approximate, obviously.

Measure the bores and pistons carefully. How many miles had the engine done before the pistons were replaced. New standard pistons into worn bores not normally a good thing unless you are positive the clearances are still within limits. That amount of valve recession might indicate that the engine has a LOT of miles on it. As said, an ideal time to get the exhaust seats fitted with inserts for lead free.

Measure your new pistons before boring and then bore to suit.

Sam, your pic 3 shows damage very similar to the previous B18 in our Amazon a few years ago now. The car had pinked a fair bit previously (but not my car so could only comment...) but whilst a passenger again on an ordinary drive lots of fumes began coming up through the gear lever gaiter so we stopped in a layby. Lots of oil everywhere but it seemed to run OK so we limped home.
Next day head off and we find one piston holed, one damaged by bits of valve seat (!) and two with ring-area side damage through what I consider excessive ring gaps. Also all bores damaged, with bits of broken rings everywhere: conclusion economic scrap.
Unleaded engine now in car - that has to be the way to go, but I guess the timing was way-out too (but as I said not my car....).

Paul (glad to see you are still into Volvo's)

The pic's of the valve seats. is that the exhaust valve on the right and if so should it not be recessed? should it be as in the other side. Is this an example of valve recession and if so what would be the likely outcome. would it affect efficiency? Apart from lifting head are there any warnings signs that would be noticed?

Was the block decked to the point where the pistons extended over the top of the block at top dead center? I suppose fore or aft contact with the combustion chamber might crack the pistons assuming the bore has a somewhat different shape than the combustion chamber and a thin enough head gasket.

No personal experience though. Just guessing.