[455-4+1]

Hi all, this is for a street engine, but thought I get a better response here.

Got a 350P that's a budget rebuild as a temporary engine. Purchased as a core.
Has a set of nice #18 heads at 66cc, and is already 040 over which will help with compression. Pistons are a stock type cast replacement, and bore is in great shape, just in need of a hone. It was a recent rebuild, but had a few problems which are largely sorted

Over here in NZ, parts are a little hard to come by, and sourcing 350P 040 rings was not looking good. Not a very common size. Bore is 3.875 + 040 = 3.915".

I found a set of Chevy (gasp) 040 rings locally for a 305 or 283 (cant remember) that share the same std ring pack dimension, and overbore size. Bingo...... or so I thought. Tried them in the bore and ring gaps were spot on (old ring had 0.030"gaps !!!!).

But.....When I checked the replacement Chebby ring against the take out, the radial depth was a lot less (see pics). The old ring was 0.176" and the Chebby for same bore size was 0.145". Radial groove depth in this piston was deep at 0.225" (comparison of a stock set on hand had a depth of 0.205").

So the old ring had a 0.049" back clearance, and the new will have 0.080".
I have searched and come up with varying opinions. Some say increased backspace can affect ring sealing as the volume is increased, other manufacturers such as Mahle now run rings with a reduced radial width in steel to reduce weight and have seen no difference in ring seal with increased back space.

Maybe I have inadvertently stumbled across a stock / Hobby stock class power secret !!!!

Has anyone successfully run a thinner (radial) ring with success in a piston not specifically designed for it ?

Thanks

[25stevem]

Here is what I believe you are asking about right out of the pages of a Speed Pro catalog.

[PAUL K]

We came across this same issue rebuilding a 327 Chevy. Somewhere along the lines the radial depth of the rings changed drastically and left a whole bunch of additional clearance between the back side of the ring and the back side of the ring groove. After consulting with Hastings, Speed Pro, my Total Seal guy and the buddy across town that builds more Chevrolet engines than me, we decided to build the engine and had no issues.

It didn't make sense to me that the performance industry makes a big deal out of that "critical" clearance but then when it's off by a mile they says it's no big deal.

[25stevem]

I think it has to be looked at from the point of compression and sealing life.

The added back clearance add’s to the CC volumes and lowers the level of power that could be made.
A pure street motor that due to excess clearance and is down 2 to 3 percent will not be noticed, when dealing with a competition motor that percentage is the winning or loosing edge!

Next as the rings drop tension due to usage the piston is going to rock more and this will cause a drop in sealing also as the piston rocks the ring groove itself will ware faster.

Rocking may not be much of a issue for a low rpm motor with tight piston to wall clearance due to running cast pistons, but the added clearance of a forged piston along with far higher rpmsI think is a different story.

For me I will follow results from companies that have done the A/B empirical work to find the differences.

I will refrain from even getting into oil contamination.

[455-4+1]

Thanks for the comments, an interesting topic for sure. I was surprised by the comments from Mahle who had carried out testing and saw no detrement. Certainly disagrees with the Speed Pro catalogue posted to a degree.
To be fair the catalogue section was not related to passenger car applications.

[darbikrash]

Just now putting together a 350P with custom pistons using Mahle 3.905 Chevy LS ring pack (3.908 bore). Top ring 1mm, 2nd ring 1 mm, oil ring 2 mm thick. Mahle uses these thin rings and they have pretty much rewritten the book on ring groove design based on tens of thousands of OE applications. Not much in common with the old Speed Pro/Total seal thick ring stuff.

[455-4+1]

Nice, Just out of curiosity, whats the ring radial depth, back space clearance, and ring material (assume steel at that thickness ?).
Cheers

[darbikrash]

Ring width is .140", groove depth (land width) is .275", so back space clearance then is .135".

Mahle uses 9254 Alloy steel as the ring base metal, coating is HV385.

[455-4+1]

Thanks for that, hmmmmm interesting, so the back space dimension is considerably more than my example at 0.080".
Your ring thickness is obviously less, reducing the volume there, maybe there us nothing to worry about?