[trauschu]

I just received the newly machined crank back, and installed it back into my short block that was decked to my old crank specs (....long story....) The new crank isn't as exact as the old one, but it should work I think.

My deck height varies from -.001 to +.006 out of the hole.

1 - -.001
2 - .000
3 - +.003
4 - +.004
5 - +.005
6 - +.006
7 - +.003
8 - +.003

What thickness head gasket should I target? .045?

What is the optimal clearance between a piston and the head?

I'm running a 400, nodular crank, forged h-beam rods, forged pistons (12.5 cc), and cast "13" heads (75cc).

Thanks.

[quicksilver97ta]

Felpro 1033's crush to advertised .039 and I would be comfortable there for a street/strip deal.

[mechanic17]

I think if you target the proper compression ratio for pump gas on your combo here, you will be okay with piston-to-head clearance or "quench." I would plug an average +.003" deck height into a CR calculator and play with the gasket value until you hit 9.5 SCR (or whatever number you're going for). Hopefully you already have a good idea if the motor in-car was prone to detonation as/was, or if it can tolerate more squeeze.

Many will warn against going over 9.5 to 1 SCR with iron heads. But IMHO that is an "idiot proof" number and it depends how careful a tuner you are.

I have a feeling you'll end up around .050" for gasket thickness, but I haven't run the numbers.

[Matt Meaney]

Originally Posted by mechanic17

Many will warn against going over 9.5 to 1 SCR with iron heads. But IMHO that is an "idiot proof" number and it depends how careful a tuner you are.

"idiots" have no problem getting an engine to ping a way less than 9.5 to 1 SCR. ;^)


i wouldn't sweet the head gasket thickness for a street engine. with the basic fel pro, you still have over .030 clearence. now, if your going to turn it upwards of 7K rpm, that's another story.

[lust4speed]

I'd think that .030" clearance won't be any problem with steel rods. My aluminum BME rods stated .070" minimum piston/head clearance.

Originally Posted by mechanic17

...Many will warn against going over 9.5 to 1 SCR with iron heads. But IMHO that is an "idiot proof" number and it depends how careful a tuner you are...

Many of us certainly wouldn't consider 9.5:1 "idiot proof". Especially with a lousy batch of gas on a hot summer day. Pump gas in our area is 91 octane, and it's all down hill from the posted number. I've seen too many engines that had to be "tuned" by backing the timing down to 28°, and jetting the carb pig rich. Why take the gamble on the half point of compression that might give 2% more horsepower? If the engine has to be backed down on timing there could easily be a 10% horsepower loss.

[steve25]

If its the way the crank was cut that is creating those stroke differences showing up then I would be fretting way more over how the rod and main jurnals where cut and what their run out may be lke!!

[amcmike]

I wouldn't worry about the gasket thickness on SCR, as much as quench. Sacrificing quench for compression you could end up with the opposite affect than desired (more tendancy towards detonation).

With the nodular crank, forged rods and pistons, quality rod bolts, and assuming redline less than 6500rpm, a minimum of .033" clearance should be safe. Since your tightest allowable quench distance is dictated by .006" piston out of the hole, shoot for a .039" to .044" gasket thickness.

The engine may like 1 or 2 degrees less total timing due to a more efficient combustion due to the improved quench.

[trauschu]

Excellent information so far.

The "13" heads were milled approx .010 (...another long story...). Without sounding like a total idiot, if at full valve lift, do the bottom of the vavles come close to extending beyond the head surface if you've milled the head? So, would your head gasket thickness consideration take head milling into consideration, or does the valve stay far enough in the chamber so that it's not a factor?

[TIN TIGR]

wow..just wow..

[lust4speed]

The valves will come down the additional .010" towards the piston. You were probably far from critical before, and that .010" won't make a difference. Doesn't hurt to place some modeling clay in the piston valve reliefs with the heads secured by a couple bolts and the intake and exhaust valve train hooked up on the cylinder with the highest piston. Then just turn it over by hand a couple of revolutions and take everything back apart. Take a razorblade and cut the clay at the most compressed area and take your measurement. I use .080" intake and .100" exhaust clearance as a minimum. I'll take a guess and say that you will still have over .200" clearance even with the head mill.

[amcmike]

Originally Posted by trauschu

Excellent information so far.

The "13" heads were milled approx .010 (...another long story...). Without sounding like a total idiot, if at full valve lift, do the bottom of the vavles come close to extending beyond the head surface if you've milled the head? So, would your head gasket thickness consideration take head milling into consideration, or does the valve stay far enough in the chamber so that it's not a factor?

So far we've only been considering quench. Valve clearance is a function of valve depth in head to head surface, valve relief depth, valve length, rocker arm ratio, distance to piston to head, and camshaft lift/duration. Therefore each application should be checked. For this purpose only, you could check it without the gasket and see how much clearance you have per lust4speed's recommendation, then factor that into final gasket thickness.

[trauschu]

Originally Posted by TIN TIGR

wow..just wow..

Did I miss something obvious that this response was directed towards?

[trauschu]

Getting conflicting info. The local old school Pontiac guy tells me to leave at least .042 between the piston and the head, so he told me I needed to get a .055 gasket.

Is there a reason why I would need to have .042 clearance? Or is that only when you are pushing 6k+ rpm on the track? I trust old Barney with his info, I'm just looking for a reason why he would recommend the gasket that would seem too thick.

My numbers if I used the following gaskets:

.045 Gasket
-----------
9.51 - 9.65 CR
.039 - .046 quench

.055 Gasket
-----------
9.32 - 9.45 CR
.049 - .056 quench

[track73]

I once built a chevy cirlce track engine that had the pistons .005 out of the hole and used a .038 head gastket. When I took it down for a rebuild the piston heads had marks that looked like they were hitting the heads and the pins were extremely loose. You have to compensate for piston rock, rod expansion, and clearance. I put on .046 gaskets and fixed it.

[mechanic17]

Old Barney may be thinking of a standard quench "rule." Another old-timer, Smokey Yunick, recommended .040 as a minimum with steel rods and forged pistons (which do rock in the bore, as track73 said). Now, if all blocks came zero-decked that would be a safe number to "keep the head off the piston." As it is, many (SBCs) come from the factory at .020 to .025" in the hole, so you could actually get away with a .018 steel shim head gasket for optimal quench--which the factory did.

In your particular case, it seems the .045 gasket will put you where you want to be for BOTH compression ratio AND quench (assuming you've calculated right).

My 9.5 as an "idiot proof" iron-head SCR comment above should have included a stipulation to use 93 octane--and they're right, some idiots will have that pinging away regardless. What I mean is, if I were selling iron heads to the public, 9.5 would be the highest I'd allow under warranty, with disclaimers as to engine heat, timing, and about 20 other factors.

BTW, I thought #13 heads had 72cc chambers, and if yours are decked .010, they're even smaller. Any chamber work? CC'd or just a guess?

[trauschu]

I did do some mods to the chambers to open up the lower side to allow for better breathing.

Here is a pic: http://www.1969fb.com/pics/1977Trans...ChamberMod.JPG

From what I read here, the #13 actually have 77-80 chambers, and that is exactly what I had before I had to mill them down.

After milling the heads, I did CC every chamber, and they all come in at 75-76.

I used a drill press and took out .100 between the center valve eyelashes on my forged speedpro pistons to increase the piston dish CC from 6 to 12.5. That was CC'ed too.

So, my compression calculations are based on the 75 head chamber and the 12.5 cc custom dish. I didn't factor in the extra 1 cc on some of the chambers, since that is a lower number and less of a problem.

[Matt Meaney]

i agree with the above said.

just what are your options for head gaskets and at what price? if you can get
reasonabley priced good gaskets that will have a crush thickness to put you at an
optimal quench distance, that's what i'd be shooting for and let actual static compression fall where it may. on the other hand, if those gaskets cost a few hundred dollars by themselves, i'd go with a good quality gasket and be a little tight or loose on the quench.

[Skip Fix]

Speaking of pinging must be the gas formula both my 350 Vortec truck and my wife; Avalanche have the pings bad this week, and they have knock sensors to retard the timing!

I think the "Butler" head gaskets are thicker than the Felpros. You can also get Cometics in various thicknesses.

[goquick]

First, why is the piston height all over the place? Was the block square decked? Not decked, but squared with the crank centerline? The .039" gasket thickness is fine, just use a Victor or another brand that has a boree closer to the engine bore with valve/chamber reliefs. The 4.300" opening on a fel-pro is way too much of a "ping" causer.