What to mill 360 heads.

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evilred73

evilred73

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Question guys, I got a low mileage 72 360 that I’m changing top end and cam. Was thinking of going somewhat to the early 340 combination. I got the cam,Springs heads done with big valves but before my head guy assembled them just thought if I should mill the heads to reach the 340 spec 10.1 compression or leave as is.
Original to that engine was 8.8 compression.
What are your guys thoughts on it. Worth doing? I’m aware of milling the intake aswell.
 
Oh Yes, I would go to 9.5:1 just to be safe. And b-4 you get them together do some port work with your Dremel or air grinder. I'd go somewhere in the 240 duration @ .050 area, 750 cfm carb. A nice thermo carb with the small primaries my give a little milage.
 
Seems like it would take an awful lot of milling to pick up 1.5 - 2 points in compression. I put the range up to 2 points because engines typically had less compression than advertised. Deck clearance & chamber sizes were more than spec. Do the measurements on the original engine and you'll probably be closer to 8.0-1.
 
Yes! Know exactly the compression ratio. It puts you ahead of the curve. But, as seen before, others have milled .060 for a mild raise in the ratio. I don’t keep anything in my head anymore but the MP engines book has milling specs. How much milled for how many cc’s etc...
 
on a typical 340/360 head..........cut .0048 inches for 1cc reduction in chamber volume. For each .010 cut from chamber side, cut .0095 off intake face. Info source, "How to Hot Rod Small Block Mopar Engines" by Larry Shepard
 
A .060" cut (about 12 cc decrease in chamber size) is gonna get you pretty close to 1.0 point increase in SCR. That'll get you to the low 9's range.
 
Make sure when you do the math for the heads deck milling that you also do the math and physical measurement of how much of the intake is milled, if you mill the intake or the heads intake mating surface to see how much of a valve cover gasket surface you have left.
 
Head gasket plays a big part also, If you cut the heads .050, and put a .050 head gasket in there, you are only gaining a net cut of about .022, the thickness of the original head gaskets
 
PO of our car had put 360 heads on the 318 and didn't mill them. Sooooo when we changed cams we had the heads re-worked our machinist did the math and recommended .040 and the thin Cometic gasket to get about 9.0 to1. Our guy said to check the intake before we had it milled because we may not need to. We didn't. And also that because of the forgiving nature of hydraulic valve trains not to mess with shims either. At .040 we would be fine. So that's what we did. And it seems to run strong.
 
None of the 360s were as high as 8.8. Don't know where that number came from. They were rated at 8.2 or 8.5. Chrysler's machining tolerances sucked, and as a result, the deck heights were high, combustion chambers were big. It was not uncommon for them to actually be in the 7.5-8.1 range.

That said, you'd have to mill into the water jacket to get the chamber small enough to go from 8.1 to 10.1. Just not enough meat to do it. If you want 9.5 or better, different pistons and or heads are in order.
 
Yep. Search the forums too. Lots of information about milling 360 heads. For a mild Cruiser engine .040 to .050 is common. And you ain't going to get 10 to 1.
 
I wouldn't mill them that far. All I would do is mill to make sure the surface is flat. Milling to gain compression is futile, as a LOT has to be removed. You can blueprint the deck height, as I am sure it is both too tall and crooked.

A 360 engine can make over 350 HP having 8.5 compression with good parts choices. It's not hard to do and will run all day long on the cheapest pump gas money will buy.
 
Yes sir, my bad. I should have restated I was talking about 360 heads on a 318 and not a 360. Just making the best of what we had. Kind of just went with what the local Mopar guy recommended. Seems to run pretty good for what it is.
 
RustyRatRod said:
I wouldn't mill them that far. All I would do is mill to make sure the surface is flat. Milling to gain compression is futile, as a LOT has to be removed. You can blueprint the deck height, as I am sure it is both too tall and crooked.

A 360 engine can make over 350 HP having 8.5 compression with good parts choices. It's not hard to do and will run all day long on the cheapest pump gas money will buy.
Click to expand...
Ultimately went with this route, as this was a temporary motor for my stroker. Great info as mentioned. But definitely pulling all stops for the stroker build. Thanks again!
 
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