Tech / Tech Articles

Ask Away! with Jeff Smith: How Much Oversize is Too Much When Custom-Setting Piston Ring Gap?

I have a 4.030 small block Chevy with 1/16 inch rings. I have a 4.125 inch bore ring set that I’ll never use. Can I use the larger rings and just cut them down to create the proper end gap? How far can I go? I know at some point the ring just won’t be right, but what is the limit?

E.G.

Jeff Smith: This is an interesting question. We posed this to our friend Lake Speed, Jr. at Total Seal. He told us that it’s Total Seal’s policy not to exceed 0.010 inch of ring oversize to custom-set end gaps. This is why oversize rings generally are 0.005 inch larger than the bore size. For a 4.030 inch bore, the oversize ring would be 4.035 inch. Speed said that cutting down larger diameters will oval the ring, which will create a giant leak path that will kill cylinder pressure. Why take the risk?

Looking at this from a larger perspective, a basic set of 1/16 inch piston rings are now very affordable considering the critical nature of their job. For example, Summit Racing sells a non-file fit ring package with a 1/16-top, 1/16- second, and 3/16-inch oil ring for far less than $100. A file-fit ring set will start at around $120. That makes a set of file-fit iron rings for a basic 0.030-over, 4.00 inch bore small block Chevy is a smart investment.

Investing a little more money into a set of ductile iron rings with a moly coating that will not only seal better but last longer is not a bad idea. For one thing, as ring seal improves, the oil will last longer because blow-by, which carries all kinds of nasty combustion chemicals into the oil, is reduced.

But rings are really only part of the issue. You didn’t mention if you’ve invested in proper torque plate honing of the cylinder. Torque plates bolted to the block simulate the stress inducted into the block when the heads are bolted in place. This is an absolute must if you are even mildly concerned with improving ring seal.

Setting a custom ring end gap is critical for any engine. It’s worth the effort to go through the procedure. Here, we’re setting end gap for a big block Chevy using Total Seal’s slick electric ring grinder. There are much less expensive tools for the occasional engine builder, but this is a great unit for the professional.

Most enthusiasts are perhaps most concerned about ring end gap as a leak path. If you improve the bore roundness with a torque plate, you’ve at least attempted to improve the seal between the ring and the bore. But there are two other leak paths: between the ring and cylinder wall and between the ring and the piston groove.

You don’t mention if these pistons are new. Let’s assume for a second that these are used. Pistons with a lot of time on them will tend to wear the ring lands, which will increase their width. This creates more space for combustion gasses to escape around the ring, which reduces sealing efficiency. This is worth looking at when it’s time for a new set of pistons. Some piston companies are now applying a ring land hardening process for the top ring in order to improve its wear characteristics and reduce blowby.

These are just a few things to consider when thinking about a new set of rings. They may be a key to unlocking hidden horsepower.

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