Max piston to wall clearance?

Posted By: 70440+6bbl

Max piston to wall clearance? - 01/26/12 06:23 PM

I have a stock '69 340 engine that was seized up due to varnish. It took a bit of time, but I finally got the pistons out. Since it has only 30k original miles on it and all of the bearings look great, I just want have a quickie hone done, what is the maximum allowable piston-wall clearance for a stock cast 340 piston? And, what rings will work best? Cast or Moly?

Thanks fellas!

Posted By: fox

Re: Max piston to wall clearance? - 01/26/12 06:50 PM

By .004 p/w clearance you will have piston slap. Stay under .0025

But, you would be much better off to bore it to true it up, I know that isn't what you asked!

Posted By: Streetwize

Re: Max piston to wall clearance? - 01/26/12 07:24 PM

It's not always the bore wear as much as exceeding the allowable bore taper that leads to blow by. Taper is the difference in measurement of the bore at different intervals from the deck to the bottom. "Re-ringing" a bore to the loose side can still seal well (for a time) as long as there's not much appreciable taper.

BTW, Some motors can have (cold) slap and still seal well, btw. Old timers used to flip pistons that had pin offests around so the rod angle was less extreme on the power stroke, they were noisy as heck when you first fired them up but they made a little more power up top from the reduced friction. Lots of 68-69 six packs were set up that way along with cut down (-1/2" to 5/8") pins to save weight.

Posted By: 70440+6bbl

Re: Max piston to wall clearance? - 01/26/12 08:47 PM

Thanks for the replies. I have the block at a machine shop right now and he is going to check the taper and out of roundness and let me know what he finds. I'm not looking for a race motor, just something to get me by a season or two. Working on three project cars right now and spread thin

Posted By: 70440+6bbl

Re: Max piston to wall clearance? - 01/26/12 08:51 PM

BTW- I do plan on having the rods reconditioned with new bearings and ARP bolts since that is cheap and easy. The crank is very nice and just needs a polishing for now.
I typically always go for the full rebuild and use moly rings. This is the first engine I am building as a "re-ringer", so what are the recommended rings?

Posted By: astjp2

Re: Max piston to wall clearance? - 01/26/12 08:56 PM

With forged pistons I am running .012, the type of piston determines what clearance you need. Hypers require the .0025, cast is a little more at around .004-.006, Tim

Posted By: Cab_Burge

Re: Max piston to wall clearance? - 01/27/12 12:13 AM

Quote:

BTW- I do plan on having the rods reconditioned with new bearings and ARP bolts since that is cheap and easy. The crank is very nice and just needs a polishing for now.
I typically always go for the full rebuild and use moly rings. This is the first engine I am building as a "re-ringer", so what are the recommended rings?

If you go with a rering buy the cheapest cast iron rebuilder rings you can find like Grant, Pacific, Hastings, any of the those will work on a cheapy rebuild. If you can find a stock set of those that are 4.045 bore size(.005 over stock) get them and file the rings to fit your bores individually, real race motor technology

Posted By: flatiron

Re: Max piston to wall clearance? - 01/27/12 01:22 AM

Quote:

With forged pistons I am running .012, the type of piston determines what clearance you need. Hypers require the .0025, cast is a little more at around .004-.006, Tim

Your not actually running .012 clearence you mean .006 clearence. Right?

Posted By: RapidRobert

Re: Max piston to wall clearance? - 01/27/12 01:24 AM

Quote:

just something to get me by a season or two. Working on three project cars right now and spread thin

(1) post what he finds. Cast iron rings will tolerate more out of round/taper than Moly but in your situation I'd have him ball hone the right grit for Moly (2) that's the story of my life. Read "breakin secrets" at www.mototuneusa.com

Posted By: hemigod426

Re: Max piston to wall clearance? - 01/27/12 02:04 AM

.007 ive done on many motors,if block is plated and honed right

Posted By: dogdays

Re: Max piston to wall clearance? - 01/27/12 04:14 PM

Guys, it's a 30K engine. It has, from the description, stock pistons as it's only a 30K engine. Taper should be minimal.

I don't see why you'd do anything to the bottom end. Add ARP bolts, maybe. That means you'd resize the rods. If the bearings look good, reuse'em. Again it's a 30K engine.

In terms of honing and rings, again it's a 30K engine so I'd use a ball hone and moly rings. As moly is just a coating on the face of a cast ring there is no reason why moly rings won't tolerate as much taper as a cast ring. Moly rings are easier on the bore than standard or chrome. The ball hone will follow the bore surface meaning you won't lose the roundness when the heads are on and block is warm. Think about it, the engine has been bolted together and run. During that time the rings wore away areas of the cylinder that were higher than their surroundings. This process is what honing with a deck plate is trying to simulate. You have it already done.

One other thing.....cast pistons need LESS clearance because they have a steel strut cast into the piston that doesn't let it expand so much with heat. Hypereutectics and forged pistons do not have that and grow more with temperature. Hypereutectic aluminum/silicon alloy expands less which is why they need less clearance than forged. Some cast piston tolerances were less than 0.001" on the small allowable end.

R.