Rookie

iTrader: (4)

I'd run the Fel-Pro 0.039". During dyno I'd also try 87 and see what happens. I run a lot of 87 and 89 octane.
I don't believe quench is a make or break issue on your build. It's good to know but I would not worry about it.

TomZ

iTrader: (2)

Thanks guys! The .039s it is.

articfriends

iTrader: (1)

Late to this conversation but I will add this: On my own boat motor I always used JE pistons, coated and at about .005 to .0055. During one refresh, I used Diamond racing pistons, they appeared identical in everyway and were same alloy (2618). I also had those coated BUT was done in house at Diamond vs swaintech. So my machinist who used quite a few sets of them swore they had to go in at .006 to .0065, I couldnt see WHY they couldnt go in exactly like the JE's (that had NEVER scuffed) but against my better judgement I agreed to the .0065. They rocked measurably more in bores, my quench was in the .037 to .039 range. Motor made 1000 on dyno (blown), leakdown when it came off dyno was less than 5%. In boat I could here a strange fluttering noise sometimes, I figured it was piston slap as do NOT like these big piston to wall clearances and a 1.270 CH 4.25 stroke piston on a 9.800 deck is already fairly short. So ran it a season, did my annnual leakdown test before putting it away for the winter, my back 2 cylinders on each side had ****ty numbers in the 35 to 50% range IF I remember right. Motor barely had 30 hrs on it. I yanked it out, tore it down, the carbon on those back pistons was smacking the head, just enough where you saw shiny spots on edge, it "upset" them and the rock had sorta beveled the face of the rings which in turn knocked out the ring seal. Not enough where the ring lands were even pinched though.
another OSO member "Gimme fuel" asked me to come to his shop a few years ago and help check his cam timing on his boat engine as it was sorta a turd and didnt come close to his expectations. so we verified it was correct, talked things over then it came out he had a strange tapping noise he would sometimes hear. So after discussing it, it came out he used like a .027 head gasket and close to zero or zero decked block WITH big piston to wall. His leakdown was absolutely TERRIBLE and upon teardown he found the top ring was actually pinched by piston kissing the heads, he may chime in as I may have story details off slightly BUT pretty much what he had.
I wont put a marine BBC engine together with less than .039 quench with .005, .042 w .0055 piston to wall, IF the piston to wall is in the .006 oir higher, I wont use less than about .050 as losing a tiny bit better quench vs knocking ring seal out is mmore important, Smitty

fbc25el

Did i understand right that when you used the J&E pistons with the .027 gasket the pistons did not hit the head?

Gimme Fuel

Yes, I was going to weigh in on this, what Articfriends said was true with my 540 and the .027" gasket. Measuring clearance at the pin during assembly made it look like there was plenty of room, however I think I had .0065-.007 wall clearance and expensive Total Seal Gapless AP rings. Pistons were lightly kissing the heads on the corners, little witness marks on pistons and heads viewed with inspection camera told the story. I'll always do .039" gasket from here on out. That little bit of compression isn't worth it and would hardly make a difference on dyno anyways.

mcollinstn

Did you verify the consistency of the center to center lengths of your rods?

Here's what I do during mockup. I put ABCDEFGH on the piston crowns. Then
I put every piston and rod on the #1 crank throw and run it up to TDC and write the down the deck to piston max/min for each.

I note the longest one as "0" and rewrite the numbers designating how much shorter each one is from the longest one.

Then I put the longest one in each of the other 3 corners of the block and write down the deck clearance for #2, #7, and #8 to go along with the earlier #1 measurement.

Now you can see how square your deck is cut to the crank axis. You can also see whether you want some additional decking done on the block.

After all deck machining has been done and the block is clean and ready to assemble, you can start dropping piston/rod assemblies in and juggle them around to minimize deck ht variation. Sometimes moving just a few around will bring the variation in under .003". I try hard to shoot for. 003 max deviation.

and I try to use machine work/decking to make sure I hit my target quench with reasonably-priced gaskets if at all possible.

Also, cc your heads and try to keep the chambers within 2 ccs of each other.

Quench clearance DOES make a difference on being able to run optimum spark advance on more affordable fuel..

fbc25el

What would you consider to be the least piston to head clearance you would use?

TomZ

iTrader: (2)

My experience is pretty much what has been described. At least from the measurement standpoint. I have pistons that rock and measurements at the wrist pins that are consistent across the board. I’d say the variant is right at .003, at the most, at the wrist pin measurements. If similar builds were experiencing the same issues, then the rock must be contributing to the lack of clearance.

So what is the minimum safe distance when considering piston rock?

I’ve already made my decision to go with the .039 gasket. I need this to run reliably for a long while so erring on the side of caution here.

Thanks for all the input. I think this is a great topic to cover. Looking forward to further discussion!