GlenYappa

I am building a supercharged 350 for my 84 Z28. I got the crank and cam in today for the final assembly so I was pretty happy. I now need to gap my piston rings. The box is split in the middle by a divider with "top ring"/"second ring" marked on it. In my haste I pulled the rings out without looking at which was the top and which was the second ring. ( I know ... don't do that!) Can someone tell me which one is the top ring and which is the second ring. Each one is dimpled so I know the top side of each. Both are beveled-one more that the other. On one the bevel is on the bottom of the ring and the other ring has a bevel on the top edge. One is marked with 2 white stripes on one side of the gap. The other has 2 white stripes on one side and a red stripe on the other. One ring is dark and one is bright faced. I believe the bright faced ring is the moly ring for this supercharged engine so it would be the compression ring and it also has the red mark that I would gues indicated compression. Can someone verify my logic? I definitely don't want to get this wrong. Also the instructions say gap the rings 5/16":1/16":.043". I am assuming this is compression ring:second ringil rings. Is this true? Later in the instructions it says "Supercharged/injected gasoline engines .022"/.024"" Is this changing the end gap messurement from before?

Many thanks,

Glen Smith

RB83L69

The one that looks like it has a shiny stripe almost all the way across the front, is the top ring. IIRC (these are SpeedPro File-fit Plasma Moly, no?) the 2-white/one red is the top.

I'd suggest setting the gap a little on the wide side, especially if you're planning on really turning up the boost. The reason for end gap is that the rings, esp. the top one, get quite hot and expand. You need to have enough gap so that no matter how hot it gets, the ends don't butt. If that happens it will completely destroy the cyl wall, maybe break the top land off the piston, and otherwise spoil your whole day and possibly part of the next. If these are hypereutectic pistons, increase the gap to about 25% more than the recommended.

FYI: rule of thumb: use about .0045-.005" of gap per inch of bore normally; for blown motors, add 33%; for the hype pistons, add 25%. It's almost impossible to have too much gap, it won't really hurt anything, but too little can be fatal.

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"So many Mustangs, so little time..."
ICON Motorsports

GlenYappa

Thanks for the info. Your rule of thumb falls in line with the .022 - .024 range. Can someone tell me what the 3 values are in the instructions when it says (corrected from first post) RING END GAP 5/64", 1/16", .043" ?

5/64 = .078 - Way to big for the gap. Right?
1/16 = .063
.043 is still to big for a gap.

Do I have to do anything to the top and bottom oil ring rails?

Many thanks

Glen Smith

RB83L69

Ring thickness... those are the thicknesses the Speed Pro rings are available in.

The oil rails and expander are ready to use as they are.

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"So many Mustangs, so little time..."
ICON Motorsports

GlenYappa

Thanks! That was driving me nuts. I'm pretty paranoid since this is my first engine rebuild and I don't want to screw it up.

Thanks again,

Glen Smith

GlenYappa

HELP! Like an idiot I can't find the sheet of paper with the gap recommendations on them. I know what the compression ring is supposed to be gapped at but what about the second ring? These are SpeedPro Pro-moly rings that I am putting on a supercharged 350.

Vader

Glen,

The second compression ring can be a few thou tighter to assure a good secondary seal, but it's safe to gap it the same as the top compression ring. Too tight and you'll cause the top ring seal to fail from excessive pressure under the top ring. WAY too tight and you'll seize the engine or break ring lands.

On forged pistons in NA engines I've had no failures at .016-.018" on the rings. The moly faces on the Speed-Pro file fit rings tend to go away quickly and open the gaps some. I'm tearing down an old street engine build of mine with low miles (need more RPMs) and I was surprised to find over .025" of end gap in bores that measure perfectly straight. Hypereutectic alloy slugs are a different game, and may require a larger gap to accomodate the crush of the piston growth.

The blower factor would indicate more heat and potential blowby, so a little on the loose side would be my GUESS to keep the pressure between the rings low enough. I've got very little experience with gasoline blower builds, so take it for what it's worth.

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Later,
Vader
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GlenYappa

Thanks for the help. Before I posted about the second ring I thought I remembered the gap to be .018 which would fall in line with what you posted. I feel a lot better now.

AJ_92RS

I am doing a similar build up (not S.C.) and was wondering about the comments on hypereutectic pistons. Do they really expand that much? I know that the silicone content is 12-15% as opposed to 10-11% of cast pistons. Does that really cause that much expansion?

I knew that forged pistons expanded quite a bit, but I never knew that hypereutectic pistons did. So the top ring's gap should be enlarged a little, as well as the second ring's? Or should the second ring gap be enlarged at all? And then what about the oil rings? Enlarge them also?

Thanks for all your help guys,
AJ

Vader

AJ,

Sorry if I wasn't clearer on the "crush" from expansion of the hypereutectic alloy pistons. I went back and read the post and understand the confusion.

The whole theory behind hypereutectic silicon alloy jugs is that they DON't expand as much as more common aluminum alloys, but they still expand. Because of the fact that they are designed to expand less, there is generally less room behind the rings (in the lands) for "forgiveness". Also, because the pistons aren't supposed to expand as much, they are made to fit the bores tighter than the old style cam ground pistons, and have less room for the rings. However, the rings themselves will still expand at the same rate as any other cast iron, and will grow just as much with less space behind them for "crush" and "flutter" as they travel along the cylinder walls. Piston rings will move, creep, and bounce as they travel along the walls, and varying pressure from compression and conbustion can amplify the phenomenon. "Plain" pistons generally have several thousandths space behind the rings for high pressure combustion gasses to collect and help seal the rings, and oil to collect and lubricate the rings, and also allowing space for the rings to flex an compress as they travel along the irregular cylinder walls.

High-silicon (hypereutectic) alloy pistons have a little less of this space (from the samples I've seen) and I would tend to run a little more gap as a precaution.

Of course, you should follow the ring manufacturers recommendations for ring gap in your application, but you might want to aim for the higher end of the range.

Then again, with the surprise I recently got with some really low-hour moly-faced Speed-Pros, you might be O.K. since they seems to "lose face" and grow some gap pretty quickly. I may be going back to chrome faces and live with the longer break-in on future projects.

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Later,
Vader
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Vader

Check the ring land minor diameter with a blade mic on an "old fashioned" forged cam-ground piston, and a cast hypereutectic alloy equivalent, and you'll see the difference I noticed.

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Later,
Vader
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"Click - Click - BOOM!"
Adobe Acrobat Reader

RB83L69

Another issue with the hypereutectics is that the material is a far poorer conductor of heat (better insulator) which makes the engine have better thermal efficiency. However, it also make the top of the piston run alot hotter, and the top ring as well. Heat makes the ring expand. The piston itself doesn't touch the ring in such a way as to force it up against the cylinder wall, so it really doesn't make any difference to the ring gap how tight they fit the bore, the rings are a separate issue.

So, since the rings expand more, you need to build the motor with a little extra end gap, so that when the rings expand that extra amount from the extra heat, they have room to "grow" without their ends hitting each other. Instant destruction will result if there isn't enough room, because at that point the ring will be trying in essence to be larger than the bore which is clearly impractical at best. It's far better to have a little bit more blowby in a cold engine (very little at that) in exchange for not grenading it under stress.

The extra heat issue doesn't apply to the second ring nearly as much as the top ring; I'd recommend about 20% more gap with the hypers than the recommendation for regular cast or forged pistons, which both have about the same (higher) heat conductivity compared to the hypers. Plus, a similar safety margin for the power adder much like for the top ring, since that will add to the heat situation.

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"So many Mustangs, so little time..."
ICON Motorsports