Cylinder Size-Taper
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Senior User
Joined: Sep 2004
Posts: 223
Likes: 1
From: Rexburg,Id
Cylinder Size-Taper
I bought a 73 390 to rering. When I tore it down, the pistons were all stamped 060, which I assume meens it's been bored 60 thousandths. There was no ridge in the cylinder walls at all! I pulled the pistons and a couple of them have some scarring on the skirts--nothing much, just scrape marks that you can't feel with your fingernail. We used to have the machine-shop cross-hatch these and fit them to the hole by filing carefully. Nowdays; however it is as cheap to buy a rebuild kit with new pistons as it is to buy rings, rod and main bearings, an oil pump, timing gears and chain, and a gasket set! My question; if I install new pistons, will that take care of this problem, or will I have to have them cross-hatched, too? The only way I have of measuring bore taper is to measyre ring end-gap. It was .030 at the very bottom, and .032 near the top of ring travel. I tend to think the new pistons will take care of this problem, but would like to hear some other views. I value you guys' opinion! Thanks a lot, Brett
Elder User
Joined: Oct 2004
Posts: 556
Likes: 0
From: Ridgeway, Virginia
First it isnt cheaper to buy new pistons to do this job. I am in parts so believe me. You can order a re-ring kit with mains for much cheaper. From what you describe you dont need pistons or knurling. Just clean up the old pistons and install new rings. You may want to have a machine shop check wrist pin wear. Also the cylinders will need honing and you might has well have the crank pollished. If you are going as far as hot tanking the engine MAKE SURE they run bore brushes and cleaning solution through the camshaft and lifter oil galleries. I almost lost a nice 390/330 because the idiot at the machine shop gave me back a block that was completely sludged up. Luckily I had him intall pipe plugs at the back of the block where the old knock ins had been. I just happened to remove one to check his work. To think he stood there and cut threads into the end of the oil gallerie hole and didnt see the muck is almost unbelievable if no plain scarry. I recommend having the pipe plugs installed because they will never leak behind the flywheel plus you can access and inspect the oil gallerie and check your machinist's work.
Hope this helps
Hope this helps
Logistics Pro
Joined: Feb 2000
Posts: 3,565
Likes: 9
From: Sun River St. George
Yup! All true! I will never use a set of used pistons unless I installed them and know how long they have been in there. No taper is a real good sign! I'd pop for a set of good pistons and moly rings. Have the rods resized follow and all the advice Gregtruck provided.
