Last night dad and I were starting to do some work on our 392 shortblock, and noticed something wasnt right with it. After rotating the crank one full revolution, I noticed the pistons dont go up past the deck. These are supposed to be 10.5-1 domed pistons!? According to the blueprint sheet from our machinist the pistons are -.060 below the deck. By the looks of things this setup will be lucku to make 8-1 compression. The crank and rods are Chrysler, and the pistons are jahns fordged, that dad picked up quite some time back on e-bay new.(yea, I know it was a bad idea) What I cant figure out is why a domed piston wouldnt protrude the deck at all. Are these pistons for a stroker, or maybe 354 .090 over? Even if I had a part number for the pistons, I have know way to check to see what application thay are for. At a glance thay look vary close to the original pistons in dome shape. We just recently had the block machined and the rotating assembly installed by a reputable machine shop. Im sure the pistons are the problem, but Im not sure exactly why. Here is a picture of the pistons and rods before instalation.HELP!!
First of all, how do you know you have a 'problem'? They 'look' like stock slugs. Where, exactly, is the 060 measurement taken? Before you get your panties in a wad why not go ahead and actually check the compression? At least you'll then know what you have. FWIW, the 354 actually has more dome volume than the 392... .
The crown of the dome is well below the block deck at top dead center. I personaly didnt take the measurement but can see just by looking at it. I need to lay a straight edge across the deck and double check how far in the hole thay are. The stock flat tops in my '54 331 are closser to the deck than these domed slugs in the 392. If indeed these pistons are for a stroker, my next queation will be where to get a crank welded up.
Before we get to the math, do you have a 250 cc graduated burette ? Can you borrow one? You will need it.... .
I believe the compression dist (pin location) is the same on the 354 and 392 because of the higher deck on the 392. I bored my 354 .060 over and installed 392 std pistons. I think you have stroker pistons. My 2 cents worth.
Granted - I'm going off memory here, but isn't a stock 392 piston something like .200" ABOVE the deck at TDC? .060" BELOW sounds like a "problem" to me if you're looking for 10.5:1 compression. I measured a couple STOCK 392 pistons.... If you measure from the top of the piston to the top of the wrist pin.... ~1.565" (so measuring from the CENTER of the wrist pin ~ 2.060") ~1.650" (so measuring from the CENTER of the wrist pin ~ 2.145") FWIW
I dont have a burett, so checking the dome volume is out for now. I do need to pull a slug out and check the distance from the top of the piston to the pin center. These pistons look like nice pieces, but thay came with no paper work or anything. I couldnt find any identifying marks on them, so thay may not even be jahns. The funy thing is, me and dad talked about doing a stroker until we got these pistons. Thanks for the help guys.
Thanks to HemiRambler for coming up with some compression heights. I've been surfing for half an hour looking for the info. Ross has a replacement 392 piston that makes 10.5:1 with a 97cc chamber and 9.5:1 with a 107cc chamber. The compression height on the piston is 1.955" and it uses a stock 0.984" pin. They list a dome volume on this piston at 22.5cc's. Keith Black has two 392 hyper pistons: http://kb-silvolite.com/performance.php?action=details&P_id=19 http://kb-silvolite.com/performance.php?action=details&P_id=21 Both of them show a 1.940" compression height with a 0.984" pin. The confusion may be with the measurement procedure. Looking at the KB290 piston, you can see a very narrow ledge above the top ring, then you can see how the dome comes up on a slope from there. I think KB is measuring to that very narrow ledge, not to the top of the dome like HemiRambler is measuring. Check on your pistons and see if there is the very narrow ledge before the dome starts. If so, measure for a compression height of 1.940" (1.448" from the ledge to the top of the pin bore).
Fridaynightdrags, I've seen that narrow ledge you're talkign about - that's an aftermarket thing (or at least not a stocker 392 thing) I believe the purpose of that ledge is clearance (or quench if you will) when using non 392 heads - for instance the "triple nickle" heads I have had a slight "corner protrusion" that in my case would have caused ISSUES with a stock type chamfered 392 piston - I don't have any 331 or 354 pistons to measure - just '92 ones - no "ledge" on those. FWIW
looks like I will be pulling one rod/piston to check compression height tomorrow. I still have the stock pistons for a quick eyeball comparison. Hopefully i can find some more info on what thay were made for exactly. Thanks again for all the info.
Just for 'eyeballing'-purposes, here's a stock 392 rod and piston... .020" replacement piston from Egge;
Besides measuring the distance from centreline of pin to top of dome, you can also get a good comparison if you can place a stock piston and one of the 'shortys' together on a common wrist pin, then lay a straight edge across. Your pic makes the new pieces appear to have a very short skirt so they could be for a stroker but also could be for a blower app. What is your bore size? .
You don't need fancy stuff for a quicy&dirt check on compression. This won't be accurate enough for an engine builder, but it will ballprk you: Check medicine cabinet and drugstore...lots of containers in CC's come with cough medicine, I think you can buy various CC'd tools separately in the baby meds area... Tools re less than lab grade and your measurements will be a tad crude, but you will certainly know if you are looking at 3:1 or 11:1! Get a good stainless rule, have someone rotate engine for you. In a few tries you will locate the high spot on ruler againsc cylinder wall...mark it, turn slowly til rule goes down bore 1". Fillitup with multiple dips of trans fluid or mouthwash (gease the piston rings! Ooops!), use the little graduated squirt syringe for baby meds to measure last little bit fairly accurately. Compare that number to volume your cylinder computes to for 1" times bore, what you would see with a flattop down 1" fill up head chamber you now have a set numbers sufficient for computing compression accurately enough for decent estimate of comp. This will not be accurate to the volume of a gnat's eyelash, but will verify the general area of your compression. I like your spirit here...don't wuss out and get the right pistons, stroke the sucker to use the pistons!
pics suck but, these are the best ones that I have loaded in my computer. These are +0.50 stroker pistons for use with a blower They are for a 354, I know they dont show you much upside down in a box but you can kind of see how short the shirt is. And a TDC shot with a measured & calculated 8.5 to compression ratio, but the gasket is in the way. My vote is that you have stroker pistons. When you spin the engine around and the piston is at BDC, do you have a lot of room between the piston and the crank? If so then I'm going to double down on my bet that those are stroker pistons.
Another way to check to see if you're in the ballpark is to check the piston-to-head clearance. Set any one of the pistons to TDC. Set the head (with head gasket) on the deck and snug a couple of head bolts down. Remove the the rod cap from the piston/rod assy that is at TDC. Stick a dial indicator through the spark plug hole and zero it on the dome of the piston that is at TDC. Push up on the rod and read the piston-to-head clearance. You should see a small number like .050. If your seeing numbers like .200, you're way too far down in the hole. Hope that helps.
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