**Milling .030 off '920 - 273 heads**

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65Val

65Val

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Just spit-balling here...my '67 - 273 has the lo-po 8.8:1 2bbl pistons in it still, and I was wondering what the comp would be if I had, say, .030 taken off the stock '920's' head surface?

I assume I could still use my original, solid-lifter pushrods and iron adjustable rockers with the solid cam thats in there now, but would .030 off the head require custom pushrods if I went with stock, stamped-steel, non-adjustable hydraulic rockers ( and hyd. cam, of course)?
 
You could take .030 off the surface. I took .030 off mine but I ended up machining the intake, using different pushrods and shimming the rocker shafts to make it all work correctly. The bad thing about it all is the heads and intake should always be together as a unit. No splitting those guys up! LOL Most of my info is in my build thread tmm
 
Darwin what head gaskets are you using? you can get them in +/- .025 from the general .040's - that might help all on it's own. Or find the factory correct steel shim head gaskets, but good luck with that. Another problem is the bore size they don't make for a 273 anymore (that I have found) and the normal SBM gasket bore is around 4.100
 
I agree with Rocky, 273 Head Gaskets are a little tough to find.

When I replaced the heads on the 273 in my truck, I got them from a specialty shop in town (I've pretty much bought up all the mopar stuff they had). The old man behind the counter disappeared into the back room, and about 10 minutes later he came back with a pair of NOS 273 Head Gaskets that were covered in so much dust, they must have been on the shelf for 40 years.

I tracked down a set of 273 Gaskets, because I didn't feel comfortable running a 318/340/360 gasket. Anyone who's ever placed a 318/340/360 gasket on a 273 block knows what I'm talking about.

A massive void around the cylinder for carbon to build up (and I do mean massive), not to mention the fire ring is extremely close to the "Notch" found in the side of the 273 Deck (required to clear the steering box on the Early A, I believe)
 
You can keep the original rockers and run a hydraulic cam. Then you have some adjustment to take up that .030 with. But you'd need different pushrods, anyway.
 
Thanks for all the responses, guys!

Rocky...I'm using the .025 Mopar Performance gaskets now with the '302' heads. The 302's have a~62.5cc chamber and the old '920's have a~57cc chamber, so, I was thinking of milling .030 off the original 920's surface, and heads' intake face (so as not to have to mill an original 4 barrel Commando/Charger intake) to up the comp a bit to take advantage of the custom cam I had ground.

Mike...thanks...I'll check out your build thread...I had forgotten about that.

74Dusted....I hear you on the 273-specific gaskets being NLA. I'm using the thin MP ones as I thought that would be the best substitute.

Max...I was thinking about maybe running a hydraulic cam, but was concerned whether the stock hyd. lifters (using stamped-steel, non-adjust rockers) would adjust to the .030 less distance.Shimming the shafts would be the answer there. (I have all the hyd rocker gear for a swap)

Thanks again, guys...all good "grist for the mill", pardon the pun!
 
toolmanmike said:
You could take .030 off the surface. I took .030 off mine but I ended up machining the intake, using different pushrods and shimming the rocker shafts to make it all work correctly. The bad thing about it all is the heads and intake should always be together as a unit. No splitting those guys up! LOL Most of my info is in my build thread tmm
Click to expand...

From your build thread...
I used the Egge 10.5 domed pistons and with a thinner head gasket but still big bore size I ended up with a 9.6+ compression after taking .060 off the heads
Click to expand...

Mike...from your thread, did you end up taking .060 off each head?
 
Your 273 heads will need to be milled .040 to get down to 57 cc's. I always mill the intake side of the heads, .0095 on the intake side per .010 on the block side, so only the heads have to stay together and you can swap intakes whenever you want. Not sure you will gain anything by the head swap. I've always run the Felpro gaskets and never noticed a difference in power using the larger bore gaskets and they don't leak. I used steel shims under the rocker arms to get the right geometry after milling the heads and running a 340 cam and rocker setup. I prefer the solid cam and rocker setup though.
 
You can't go by a printed spec. CC each head and you will know exactly what you have.
 
mderoy340 said:
You can't go by a printed spec. CC each head and you will know exactly what you have.
Click to expand...

Agreed. Also when I shaved my x heads .035 instead of plaining down the head on intake side. I plained the intake. Makes it easier if you have to take more off to get a perfect match.
 
57 cc is NHRA minimum allowable. I've done quite a few 273 heads and it takes .040 to get to 57 cc on the largest chamber. Then .038 on the intake side of the head. I can use any intake and not have to worry about if it will fit and seal. Or try to remember or figure out what I or someone else did. It also takes .040 off J heads to get to min NHRA specs. As for the rest I do not have any personal experience.
 
Thanks 66....I'll do the intake side of the heads, for precisely that reason....also don't want to mess up a stock Commando intake.

If I did do .040, with my 2bbl. 8.8:1 pistons, what would be your best guess as to CR after the milling, using the .025 thin Mopar head gaskets?
 
Is the 8.8:1 a printed spec? If so you need to measure how far below the deck the piston actually is and calculate the CCs.
It's best to know the block CC first to calculate what head CC chamber you will need to reach the desired comp ratio. If street car stick with 9 to 1 for pump gas.
Once you know the CC required for the head have the machinist mill the head to meet your spec.
The other thing you need to look at is Piston to Valve clearance when shaving heads and running a thin headgasket. Most of the early SB motors had flat tops and no valve reliefs. .040 milled, .025 or 30 less headgasket thickness will place the valves closer to the piston.
Build the motor to run 8.2 Dynamic Compression and it will make real power and run on pump gas.
 
Okay I'll be the one to ask -Why ? You just got back on the road , adding 1/2 a point of compression isn't going to give squat performance wise , if it were me I would spend on porting the heads and a mild cam upgrade . It will never be a rocket ship so why not enjoy it as the sweet *** vert that it iz ?
 
cannucky said:
Okay I'll be the one to ask -Why ? You just got back on the road , adding 1/2 a point of compression isn't going to give squat performance wise , if it were me I would spend on porting the heads and a mild cam upgrade . It will never be a rocket ship so why not enjoy it as the sweet *** vert that it iz ?
Click to expand...

This is where I was trying to go. Both the "302" casting and the "920" casting are closed chambers, small valve, and probably flow close to the same. If you have already milled .030 off your 302 castings, you have already decreased the chamber volume by about 5.7 cc. 273 pistons are generally .02 in the hole, so there is good quench and good compression. If you have bored your block that tends to increase compression slightly. I would say that if you would like more power, to tune what you have. It sounds like a great little motor. What are the specs on the cam? What distributor and timing are you running? What Carb?
 
I agree, compression is one of the most expensive with little pay back per HP with in itself. Most street engines are some where between 8:1-10:1 and going from the 8:1 to 10:1 will only give you 8-10% power increase. Cam and head flow are where the majority of the power is. I'd only worry about getting enough or close enough CR that your cam requires, so unless your going for a larger cam and or porting your heads I wouldn't worry about trying to bump you CR through milling your heads.
 
65Val said:
From your build thread...

Mike...from your thread, did you end up taking .060 off each head?
Click to expand...

I believe it was .030 and not .060. I looked the other day and I can't seem to find my build specs. I don't know where they went. tmm
 
66fs said:
57 cc is NHRA minimum allowable. I've done quite a few 273 heads and it takes .040 to get to 57 cc on the largest chamber. Then .038 on the intake side of the head. I can use any intake and not have to worry about if it will fit and seal. Or try to remember or figure out what I or someone else did. It also takes .040 off J heads to get to min NHRA specs. As for the rest I do not have any personal experience.
Click to expand...

I have taken up to .070 off of late low compression 340 heads and had the intake side planed as well so a stock intake will still bolt rite up. Seems crazy to me to plane an intake so it is now only good for that particular set of heads. JMO I have some nice vintage intakes I am not ruining.
 
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