Hi guys my name is Max and this is my first post .
I got a corvette c3 74 here in Sydney. I'm rebuilding the engine from scratch .
Sbc 350
Bored 0.30
Double pumper 650
2701 intake
E street heads
Xe268h comp cams
scorpion rollers
Hp speed pro pistons
Etc

Last night I was about to put the pistons in but when I was checking the ring gap I got .0023 on the top ring and .0020 on the second ring , the oil ring was .0015 .
to be honest I'm a bit confused bc the second and oil ring are within specs but the top one is way to open unless there is something different about this ring kit

I'm using the e251k30 speed pro rings . The top ring is simetrical with a moly face .
is there any chance that the rings are meant to be like that? I've heard that hypereutectic pistons need bigger gap but the fact that just one ring is bigger looks odd.

Thanka guys

 

Your top rings have too much gap. Second rings look good. See SpeedPro recommended guidelines below - this is typical of any ring/piston combo by any manufacturer: The top gaps should be tighter than the 2nd. Every engine I've built with these specs has had leakdown rates in the 5% range. You're bound to get a bunch of conflicting info and responses in this post - trust me: The ring manufacturer's engineers are pretty smart, and they've done more testing than anyone who is going to dispute their facts...

 

Do we assume correctly that you've got the decimal point off by one place (and that the gap isn't actually that tight)?

 

yeah thats right ... my mistake . We use metric and not imperial down here I always do the same

 

JME... I'll never scratch-build another motor with those inefficient, old-fashioned Thick-Wide rings like E251K (5/64,5/64,3/16)
I'd use the more modern, thinner metric ringpak such as 1.5mm, 1.5mm, 3mm or 1mm,1mm,2mm.... of course pistons to match ... ALL of GM's final iron-head sbc have 'em ... for Good reasons.

 

[QUOTE=ebbnflow;1604271755]JME... I'll never scratch-build another motor with those inefficient, old-fashioned Thick-Wide rings like E251K (5/64,5/64,3/16)
I'd use the more modern, thinner metric ringpak such as 1.5mm, 1.5mm, 3mm or 1mm,1mm,2mm.... of course pistons to match ... ALL of GM's final iron-head sbc have 'em ... for Good

Thanks for that... what rings would you recommend for h345dcp pistons ?

It's just weird that all the other 2 bottom rings are within spec and the top one is not . Maybe they got mixed up with a standar pack ? 4 inch bore rings would be around 0.006 bigger gap on a 4.030 . That is why I'm thinking that probably the top ring package is coming from a standar set .

 

No worries. I just wanted to make sure we were all talking about the same measuring units.

 

What rings would you recommend to use with h345dcp pistons ?

it's just funny that all the other rings are within spec but the top ring doesn't.
maybe they got mixed up from a standard set ?

 

I didn't chase the part number...but are those hypereutectic pistons?

If so...pay close attention to the piston mfg recommendations also. It's very common to use a wider top ring gap with Hyper's. Not all..but most. Way too many folks end up with butted rings and popping the top ring land off.

You want to get them in right area...and certainly less is good if they don't butt. But too tight is a LOT worse than too loose. The actual difference of a few .000's won't kill it if a little loose when you do the math of the gap in relation to the total diameter of the bore. Stuff with boost and nitrous even in street engines run looser gaps to handle the heat under full power..but do fine while cruising around under light throttle.

JIM

 

Hey Jim... thanks for your answer.
Yeap they are hypereutectic pistons. I sent the pistons to the machinist and they came back without the paper that has the chart . He told me that .023 should be fine but anyway it takes my attention the difference between the top and the second one ( 0.20 ) .
Maybe I should keep it like that and just file the second ring a few thousand to make it a bit bigger than the top one and just run it ?

Thanks
Max

 

I wouldn't lose any sleep over it. With Hyoereutectic pistons..especially KB's....you want more clearance. And yes I'd open the second ring up to help relieve pressure under the top ring. I've built many engines both ways.....and it will work either way....but there is plenty of evidence to support a looser 2nd ring.

The chart Lars posted is very typical and your gap is in the Nitrous/street category. Meaning it's not going to cause issues in normal street running and provides a safety margin under hard conditions. Those hyper pistons need a cushion.

As you might surmise....I'm not a fan of hyper pistons in anything I'm going to lean on...but your combo is relatively mild and they will be an upgrade over OEM cast...IF they are quality. There are a lot of low cost "claimer" style hyper's out there that frankly I'd use an old OEM stock cast piston before I'd run some of them. The OEM's do well with them because what they get are much higher quality than the general aftermarket (think high end LS and Porsche stuff) and they have computer controls to monitor and adjust to any signs of detonation before the next piston fires.

You don't even want to know what I just put for ring gaps on the 408" Ford engine I'm assembling with forged pistons and planning for the 6-71 blower on top of it.

JIM

 

OP Max
I see No indication OP's motor is/will be boosted.

IIRC, SP Hyper pistons' top ring is farther from crown than troublesome KB Hyper.

Nine year old ring article. None the less; still salient.

https://www.enginebuildermag.com/201...-piston-rings/

if stuck w/ SP H345 you're also stuck w/ that thick width ringpack

GAPS: your SP H345 +30 pistons & your E251K +30 rings .... TOP 0.017" to 0.019" <-> SECOND 0.022" to 0.024"

chamfer-deburr all ring gap edges, ... stuff happens: triple check SECOND rings' UP orientation.

just curious: Scorpion are very good; but if you're in AUS, did you also consider AUS-made Yella Terra ?

 

I just ground some rings for a 4.040 357ci small block......hyper pistons.
Top) .022
2nd) .020

The top runs hot so a little wider.....when running they will be both about the same. Leakdown will be the same as it will only leak as much as the 2nd and oil allow.
I have seen the Hyper piston dreaded butted ring land lift and it happens a lot.......so I make the top a little wider.

Jebby