Cracked (newish) B18 pistons

Sam Glover · Dec 29th, 2017, 09:26

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

Volv1.jpg

Volv2.jpg

Volv3.jpg

Volv4.jpg

Volv5.jpg

Dec 29th, 2017, 09:26	#1
Sam Glover Junior Member Last Online: Dec 15th, 2023 19:12 Join Date: Jul 2007 Location: Cirencester	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 Volv1.jpg Volv2.jpg Volv3.jpg Volv4.jpg Volv5.jpg

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)