New SBC 350 Build 14-18% Leakdown

[68maliblue]

I just put together a 350 SBC bored .030 over and honed with torque plates. Sealed power E251K-030 Moly rings and Silvolite Hypereutectic pistons. Broke in the cam for 30 min and all went well. Engine doesn't smoke and there is hardly any blowby when I pull the breathers and PCV valve off. I do a leakdown test at this point just to see how things are going on my builds and i usually see 5% or less leakdown. I was surprised to find 14-18% leakdown with #8 piston the worst. I don't hear any air coming from exhaust or carb. Just the breather. Spark plugs are not oily. I'm not sure what is going on. Stuck a borescope down #8 and don't see any evidence of scuffing. Everything looks good. The only thing I can think of is I opened the top ring gap up to Silvolite's reccomendation of .022. 2nd ring was set to .024. Could this be the problem? Haven't ran it on the road yet so maybe I just need to seat the rings. Looking for ideas. It runs very well so I'm a bit surprised at the leakdown values.

[PRH]

I’m not trying to be condescending here........ but what were the in/out pressure readings on the gauge?

Are both of the gauges 0-100, or is the second gauge 100-0 in %?

[68maliblue]

It’s a sun electric leak down tester probably over 40 years old but seems to give consistent readings. It only reads in % and has a knob to zero out the gauge with input pressure. . I’ve used it for years and I am comparing my readings with it now against engines I did last year with the same gauge. Always test at 100 psi.

[PRH]

I’ve never used that type before.
The reason I asked is that I have run into instances where someone is using the dual gauge style, where the second gauge reads 100-0 in %, and doesn’t understand that the first gauge has to be right at 100 for the percentage reading on the second gauge to be correct.

“What I would do” is try and borrow another gauge from someone to see if the results are the same/similar, before I went very far troubleshooting possible mechanical issues.

[MadBill]

If another gauge is available, by all means use it. However, since the gauge reads 14-18% and you hear leakage from the rocker cover vent, chances are it's real. Flat tappet cam break in and ring break in processes are not identical; the latter requires load, not just RPM. I'd get it on the road and romp on it in short bursts at different revs for 30 min or more, then re-test.

[1972ho]

I agree with madbill the rings need to be loaded and seeing how it’s all the cylinders yes put it on the road.

[Schurkey]

A fleck of rust or dirt that gets through the leakdown tester regulator, and lodges in the orifice between the two gauges, can make the leakdown tester unusually sensitive.

I have a Snap-On single-gauge tester that was hyper sensitive to cylinder leakage for decades. Eventually, I pulled it apart, cleaned the debris out of the orifice, and now it's back to agreeing with my Aviation leakdown tester.

[DCal]

It's not the end gap. Your bore (4.030) if measured in a straight line would be 12.66 inches on ruler. a couple of thousandths bigger gap is a pretty insignificant amount.

[houser45]

How smooth did you finish the cylinders?

[mag2555]

Had the ring groove clearance been checked ?

[68maliblue]

Told the machine shop to bore and hone with torque plates for moly rings. This is listed on the receipt. Pistons are brand new and ring grooves checked out. Going to drive it and see if the seal improves with load.

[rcparker]

I've seen some new motors come to me with fresh bore,torque plate hone jobs with well over a .001" of taper and egged cylinders from a couple of our local butcher shops. The RVK and Rk from the hone job was equally as bad. Hopefully it's not your machine work

[68maliblue]

I hope the machine work is good. I don't have the tools to check the bores. The hone finish looked OK but that is just my eyes, no gauges. I build a couple engines a year so I have been relying on the quality of the machine shop. I have had good luck with them in the past and they have been around for over 30 years. Hopefully their work is not going downhill. Still have some details to sort out before I get it on the road. Thanks for all the input.