Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Piston ring choices - Iron Vs Moly Vs Chrome
View Single Post
  #15 (permalink)  
Old 02-22-2013, 10:45 AM
oldbogie oldbogie is offline
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 6,437
Wiki Edits: 0

Thanks: 3
Thanked 331 Times in 288 Posts
Quote:
Originally Posted by Torque454 View Post
So what would you recommend for a Std OEM bore with Std OEM pistons and little wear on both? There is no ridge in the cylinder wall but there is some minor scratches that you cant feel with your fingernail.

I'm gathering from all the posts that I should use either Iron or Moly, but not sure exactly which. I may decide to just leave the rings alone as good as the cylinders are (no ridge, no smoke, plugs are light brown in color, compression was 165-175psi across the board)

Thanks for the info!
I would do a very light hone although not to the point of a final bore finish that comes following a rebore which usually removes about .005; just enough put some tooth on the wall for new rings to seat into. Any ring new or old will have trouble getting a bite on a wear smoothed wall. Keep in mind that honing will increase the piston to wall clearance similar to what happens when the bore wears. This isn't quite that bad in that the bore is round and not tapered as happens with wear. But the pistons may be a bit noisy till the engine warms up. You want to use an abrasive ball brush hone with about a 320 grit pass it through the bore about 5 to 8 times, use plenty of light oil do not ever let it go dry. Keep moving be careful not to dwell at the ends or middle of the stroke or the wall will develop tapers, or dips these cause the rings to flex in and out of their lands as they track the wall which will result in lost compression to blow by and increased oil consumption. If youíre not sure of your abilities take the block to a pro, this is very easy to screw up and the only recovery is to rebore with new pistons. For the average guy this job has a high risk of failure combined with a lot of expense to repair. If using a coarse grit or a diamond hone it is necessary to go back with a finer hone of 400 to maybe 600 grit to smooth the rough cut that coarse grits produce and especially any diamond cutter will make. Moly and straight cast iron rings need a better finish than does chrome. The wrong finish will kill moly or uncoated iron almost instantly. When the honing is done wash the bores first with solvent then with hot water and detergent when you think they're clean wash 'em again. Dry the bores and the block with compressed air and apply WD40 on the cylinder and bearing bores, spray into the oil passages and onto the gasket surfaces.

I'd use a moly ring, they last much longer than simple cast iron, they bed in quickly and since they are hard but porous they reduce wall wear by carrying an oil film way up the contact portion of the cylinder wall. Wall wear that's seen as taper and ridge at the top of the bore is largely the result of cylinder pressure behind the ring pushing it into the wall with little to no lubrication. The lack of lubrication causes similar metals to micro weld themselves together. The motion of the piston forces these welds to fracture tearing out microscopic pieces of ring and wall, which of course now circulate with the lubricating oil becoming an abrasive between every other moving part. Moly and chrome are very hard materials that innately do not form micro welds with the iron cylinder wall. This greatly improves both ring and wall life by preventing the formation of all that micro-wreckage (some not so micro) that circulates with the oil. Moly, and to a lesser extent chrome, have pores and channels that trap oil which then carries a lubrication into the upper bore area where itís really hot, the forces are high and the lubrication is thin at best, if at all. In this context chrome is probably more the brute force solution being very resistant to wear because of its harness but not as well lubricated by the oil as moly. Chrome unless treated chemically or physically actually sheds oil not unlike water beading on a plastic surface. The wear characteristics of the ring material is really important with modern engines where the oil circulation is much reduced compared to engines of 50 years ago, which may still have been the same casting as today like the SBC Gen I and II, SBF W and C, and the SBCh. The OEMs changed from plain iron rings to chrome in the 1960s and moly in the 1970's. Given their cost consciousness I'd take those changes as really big technical message.

Bogie
Reply With Quote