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Looking to increase the compression on my 5.3 gen 3 (lm7) engine that has 862 heads on it. Hoping to find the right amount of head milling and head gasket to get up to a compression ratio that will run on 91 octane. Also doing a small cam upgrade 212/218 but I’m sure the PTV will be fine since It’s minor modifications (pistons are dished).
For that small of a cam I wouldn't bother with milling the heads. The 862 are the smallest chamber factory LS heads made. The heads you have will work with 87, 89, 91 or 93 octane.
I see OP's point. The LM7 only has 9.5:1 compression. A .040 gasket might get him up to 9.72:1.
About .020 off the head besides the gasket mentioned will get him to just over 10:1
1FastBrick found my source! LOL!! Fun to play with..
Which number do you use for milling 862/706's? I thought .006 per 1cc reduction, coming up with about .020" for 3cc, but it looks like you use about .008/1cc to get .025" for a 3cc reduction.
I guess that could be different depending on the model of head.
1FastBrick found my source! LOL!! Fun to play with..
Which number do you use for milling 862/706's? I thought .006 per 1cc reduction, coming up with about .020" for 3cc, but it looks like you use about .008/1cc to get .025" for a 3cc reduction.
I guess that could be different depending on the model of head.
Its not a true number as each shape is different.
I just had some 706 heads done and they came out to 58cc with a .025 mill. I measured the stock castings around 61cc
1FastBrick found my source! LOL!! Fun to play with..
Which number do you use for milling 862/706's? I thought .006 per 1cc reduction, coming up with about .020" for 3cc, but it looks like you use about .008/1cc to get .025" for a 3cc reduction.
I guess that could be different depending on the model of head.
As I mentioned size and shape of the chamber plays a role.
These are 706 stage 2 heads that where done by Richard at WCCH earlier this year.
As I mentioned size and shape of the chamber plays a role.
These are 706 stage 2 heads that where done by Richard at WCCH earlier this year.
Thank you! Richard knows his stuff, especially lately with the 706/862 truck heads! Then I shall consider your numbers golden...
It appears .025 will get 706 heads down to 58cc instead of the .020 I mentioned. The smaller the chamber, the more cut per cc...
Is it possible to run a stock head gasket and mill the heads for .035 instead of .025 and still gain the 10:1 (ish) compression? (Thinner head gasket is very expensive) Also hoping to run Skip to the beginning of the images gallery
Stage 2 Thumpr 226/237 Hydraulic Roller Cam for GEN III LS 5.3 ls
Is it possible to run a stock head gasket and mill the heads for .035 instead of .025 and still gain the 10:1 (ish) compression? (Thinner head gasket is very expensive) Also hoping to run Skip to the beginning of the images gallery
Stage 2 Thumpr 226/237 Hydraulic Roller Cam for GEN III LS 5.3 ls
will that still have safe PVC?
Thinner head gasket increases the quench which allows you to run higher compression with out detonation.
Measure the chamber before and after you mill it, not all 862s are 61cc, (61.5) they can be as much as 63cc. I have run 11.6:1 static on 91 with 26* of total timing with less than .040” quench without detonation or timing retard.
Measure the chamber before and after you mill it, not all 862s are 61cc, (61.5) they can be as much as 63cc. I have run 11.6:1 static on 91 with 26* of total timing with less than .040” quench without detonation or timing retard.
That's better than I thought! How much compression could a guy get away with running 87 or 88 with a small (200-210 degrees intake at .050) cam?
Measure the chamber before and after you mill it, not all 862s are 61cc, (61.5) they can be as much as 63cc. I have run 11.6:1 static on 91 with 26* of total timing with less than .040” quench without detonation or timing retard.
12-28-2019, 07:20 AM
862 Head milling for compression. 
