How much can I mill from my block?

[akadeutsch]

I have just installed my pistons and I am measuring deck clearance. I keep getting numbers between .032 and .039. I am using the Sealed Power H802CP Pistons and stock 258 rods. Can this be correct? Do I really want to mill .030 off this block? I have attached a picture of how I took the measurements. Should I have used a straight edge and feeler gauges instead? I will double check my numbers with a feeler gauge when I get off work.

[akadeutsch]

I decided that this was a little better way to not only take my measurement, but also to ensure that I was at TDC. I got roughly the same numbers though....035-.040. I think that I should deck down at least .020-.030. This will take my C/R somewhere into the mid 9s static. And with the Comp 235 cam my dynamic C/R should be just about 8:1. What say you

[the grey ghost]

Make sure you take your measurement centered front to back, along the piston pin, and allow just a couple thousandths for stretch

[akadeutsch]

I will have to measure that again tonight Ghost. Measuring alone the pin line makes a lot of sense, I did not even consider that my pistons would be rocking in their bores. How much play should I account for? Ive got .001 at the main, .002 at the big end con rod, another .001 at the pin. That makes .004 slop. Add another .001 for expansion. Even my tallest piston is still .035 down the bore so if I deck .030 I think I should be right in the tight quench zone. Plus there is still a head gasket to consider so I think I will be fine. I have just never heard of anyone cutting more that .020.

[SIXPAK]

I have decked my blocks .080 for many many years without issue. Comp was over 14 to one on those blocks

[Cheromaniac]

I have just installed my pistons and I am measuring deck clearance. I keep getting numbers between .032 and .039. I am using the Sealed Power H802CP Pistons and stock 258 rods. Can this be correct?

Yes. Those numbers are within production tolerances when using the 4.0 block, 258 crank, 258 rods, and H802CP pistons.
Assuming a chamber size of 57cc, the SCR is going to be around 9.7:1 even without decking the block. If you want to deck 0.030" from the block to reduce the quench height, you'll need to find an extra 6cc from the combustion chambers/piston dishes to maintain the same SCR (unless you don't mind it being over 10:1). You'll also need shorter pushrods to correct the lifter preload.

[akadeutsch]

The volume of the combustion chambers of my head are 53.6 CC average. I had the head milled .030 to lower the volume from 59cc to ~53.6cc. When I run the numbers with all of the milling I have done and plan to do, I get numbers closer to 9.6:1 static and 8.12:1 dynamic. Where are you getting 10:1? I don't think you knew that I was at 5,500 feet in elevation here in Denver.
here are the numbers that I used...
6 cyl
4.6 liter
53.6 CC volume average
.005 deck clearance
.043 gasket thickness
4" gasket bore
15.5cc piston dish
5,500 feet elevation
=9.58:1 static, 8.12: dynamic (with the comp 235 cam; 254 intake duration, 111 LSA, 4 degrees advance)

[the grey ghost]

Sounds like you got it, I would personally go with more cc volume/piston dish, but if you plan on staying n/a with atleast mid to premium fuel it should work fine

[akadeutsch]

Yeah I doubt it will ever see sea level so I think I'll be ok on premium.

[SilverXJ]

Make sure you take your measurement centered front to back, along the piston pin, and allow just a couple thousandths for stretch

I am not sure how a machinist would measure this, but I would measure with piston rock figured in. E.G. Push down one side to the piston so the other side is up, measure the up side. However, I may be wrong on that. Piston rock will be greater than rod stretch. Get that number then decide what quench you want. Subtract the head gasket from quench then that from the clearance. That is how much you need to cut. IIRC, the MLS gasket is ether .043 or .044" compressed.

Cut figure = deck clearance - (desired quench - gasket thickness).

For example the piston is .030" in the hole, you want your quench at .046", and the gasket thickness is .043".

Cut = .030-(.046-.043)
cut = .027"

[Cheromaniac]

here are the numbers that I used...
6 cyl
4.6 liter
53.6 CC volume average
.005 deck clearance
.043 gasket thickness
4" gasket bore
15.5cc piston dish
5,500 feet elevation
=9.58:1 static, 8.12: dynamic (with the comp 235 cam; 254 intake duration, 111 LSA, 4 degrees advance)

Your SCR and DCR numbers are applicable at a 5500ft elevation but at sea level I'm getting an SCR of 10.68:1 and a DCR of 9.22:1.
Your engine should be fine running premium at 5500ft but at sea level you're gonna have a problem (unless you convert the engine to run on propane).

[FlyinRyan]

here are the numbers that I used...
6 cyl
4.6 liter
53.6 CC volume average
.005 deck clearance
.043 gasket thickness
4" gasket bore
15.5cc piston dish
5,500 feet elevation
=9.58:1 static, 8.12: dynamic (with the comp 235 cam; 254 intake duration, 111 LSA, 4 degrees advance)

Your SCR and DCR numbers are applicable at a 5500ft elevation but at sea level I'm getting an SCR of 10.68:1 and a DCR of 9.22:1.
Your engine should be fine running premium at 5500ft but at sea level you're gonna have a problem (unless you convert the engine to run on propane).

I'm sorry Dino, but this is not correct at all. The math is fine, but the assertion that a 10+:1 SCR/ 9+:1 DCR combo cannot work on pump gas is absolutely not true.
Proper plugs, proper fueling, proper timing, proper quench....It WILL work, and it will work great.

[Cheromaniac]

I'm sorry Dino, but this is not correct at all. The math is fine, but the assertion that a 10+:1 SCR/ 9+:1 DCR combo cannot work on pump gas is absolutely not true.
Proper plugs, proper fueling, proper timing, proper quench....It WILL work, and it will work great.

I wasn't saying it's impossible; just very challenging, and that propane would be a good alternative fuel.
Glad that you're around to give expert advice.

[akadeutsch]

I live in Denver and I really just don't care what it will run like at sea level. It is more likely to see 13,000 feet than it is to see 500 feet.

[Cheromaniac]

It is more likely to see 13,000 feet

Wow. Looks like you and your Jeep will need to acclimatize.