Did they polish the crank bearing surfaces?
Whoa! That sounds like the "smoking gun" to me. The #2 and 4 main bearings only receive oil pressure through the crankshaft oil passages. And, of course, all of the rod bearings are oiled the same way. It sounds like the STP created an embolism in a crankshaft oil passage that starved #2 and/or #4, leading to the bearing failure.I applied a little 'control' and 'variable' to this experiment:
I wiped off the bearings and crank, applied some oil, installed the crank and all caps...no turn. So that was 'control'.
I removed all the above, removed the #2 and #4 bearings, reinstalled the crank and caps...it turns.
That is unnecessary on an engine that will not see lots of action above 7000 rpm. I have been running a 170+ HP motor for years, regularly shifting at 7000 rpm and there have been no bearing lubrication issues (just sheared flywheel bolts!)The best fix is to cross-drill the #2 and #4 block and main bearings for direct oiling.
I will agree that the factory oiling system is adequate for normal use when everything is working properly. And I would never disagree with RJ.That is unnecessary on an engine that will not see lots of action above 7000 rpm.
My 'Alfa' mentor, Alex Bardi, always added a can of STP to his 1750 engine after an oil change. He never had any issues with his Alfa engines. I followed his lead for a few years but grew tired of adding the 'engine honey' as Alex called it. His father owned 'European Auto Imports' in the Marietta/Smyrna area of Georgia. They had some top notch mechanics, so maybe he learned it from them.Maybe sludge + STP was just too much to allow good oil flow? Mongo dunno.
Piston and rings look good; piston not the most carboned-up of the four. I'm replacing the rings, while I have everything apart.Any time one is different you should find out why. Have you looked at the pistons and rings? Sometimes the spring behind one of the rings has broken.
that blotchiness on the top of liner #2 ('piston not so carboned up') could just be a coolant leak into that cylinder.Piston and rings look good; piston not the most carboned-up of the four....
The hone grit needs to match what type of rings you get. For example chromed rings need the cylinder honed to a different grit then cast iron rings need. If the cylinders are not honed to the correct grit for the rings. They will either never seat in.Get the best Hastings rings. A bit more expensive, but very good and worth it.
To install new rings you will need to hone the cylinders. The V-shaped scratch pattern is key to getting the rings to seal. They wear and the cylinders wear together for a perfect fit. If you don't know about this, get your engine shop to hone the cylinders.
Do you choose the grit based upon the top compression ring? When I talked to the Hastings rep I was offered a choice of both top and second rings in cast iron or the top one chrome and the second iron. I am under the impression that the second ring is usually CI in the Hastings set.The hone grit needs to match what type of rings you get. For example chromed rings need the cylinder honed to a different grit then cast iron rings need. If the cylinders are not honed to the correct grit for the rings. They will either never seat in.