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Old 02-19-2010, 03:38 PM
JB Welder
 
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milling heads

I am playing with an amc 360, I know, not a great platform, and I don't have any experience with amc's, but because of the mildness of my mods and not wanting to go back into the short block, I want to mill the heads a little when I get a valve job done (for an exhaust valve not seating right) to bump the compression up from the stock 8.4 to about 9.5 max. These heads have the larger combustion chamber and I have heard milling about .042 will net 1 pt in compression. I feel that's about as far as I can go without changing pistons and rebuilding the short block again. (Extremely tight budget) Please correct me if I am wrong about anything.

My question is, does anyone know how much I can safely mill before quench factor issues and all with the stock dish pistons. And how much can I go before I need to start shaving the intake manifold port flanges? I was hoping to mill .050. Is there a ratio for shaving the intake, or is it just shave and check, shave and check?

Thanks for any input, this is a great site.

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Old 02-19-2010, 04:45 PM
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Not sure on a amc head but on sbc heads the limit is the edge of the intake valve seat and or the spark plug boss. Wether the heads will be durable after that much machining is another story. Flat milling will get you so much chamber reduction, angle milling will get your more, but costs more.
You don't mill the intake to correct, you mill the intake joining face on the head. You may need to enlargen the intake bolt holes.

Is there another amc cylinder head that has a smaller chamber volume you can start with?

Shaving the heads by .042" will get you about 1/2 a ratio gain assuming the same head gasket.
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Old 02-19-2010, 05:09 PM
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There are the dogleg heads that flow better with smaller combustion chambers, but then I will start getting into serious money that I don't have.

Do you have any idea about how much the ratio would be for shaving the head intake port face for a given amount of milling?
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Old 02-19-2010, 05:14 PM
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Quote:
Originally Posted by Raufus
There are the dogleg heads that flow better with smaller combustion chambers, but then I will start getting into serious money that I don't have.

Do you have any idea about how much the ratio would be for shaving the head intake port face for a given amount of milling?

the intake face milling correction ratio is based on the cylinder bank angle and the angle of the intake face.. I have the chart for this buried away somewhere. A qualified machinist will help you with this.

I would look into angle milling your amc heads to get a significant net chamber volume reduction 11-12cc to get a whole compression ratio increase on your motor. A thinner head gasket helps too.

Last edited by F-BIRD'88; 02-19-2010 at 05:20 PM.
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Old 02-19-2010, 05:39 PM
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Quote:
Originally Posted by F-BIRD'88
I would look into angle milling your amc heads to get a significant net chamber volume reduction 11-12cc to get a whole compression ratio increase on your motor. A thinner head gasket helps too.
These stock heads are in the neighborhood of 78cc and I was hoping to get more than that by flat milling. The older dogleg port heads have a ~56cc chamber but will end up costing close to edelbrock head money fully dressed. hmmm May not be a cheap way out of this.
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Old 02-19-2010, 06:22 PM
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If you were to look under the hood of 50 AMC street machines or race cars, 99% of them will have a sbc or bbc under the hood.
There is a good reason for that. $$$$$
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Old 02-19-2010, 06:33 PM
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Its in the Knowledge Base on the bottom of the page,
http://www.hotrodders.com/kb/general-engine
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Old 02-19-2010, 06:54 PM
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That's it. Thank you both.
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Old 02-22-2010, 10:19 AM
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Yeah, the main reason is people just don't know that an AMC 360 is hands down a better built engine than a run-of-the-mill 350 Chevy. The heads are closer to LS-1 heads, not the standard 350 heads, though they are run-of-the-mill AMC heads.

Seriously, the 360 has some faults the same way the SBC 350 and all other engines do, but it's as good as any in it's size, with the heads being some of the best standard production heads ever made, equal to the big three performance heads in most cases. There aren't as many parts available for AMCs, but the ones that are still available are the ones that work well. Decide on the rpm range you want and you don't have to wonder which of five or more will be best, you'll only have one or two choices -- but known to work well choices.

The "dog-leg" heads were made from 1970-91. The early model heads have smaller chambers but were only made 70- mid 71. Those have a 51cc chamber and 9:1 compression on 2V engines, 10:1 on 4V. The 4V engine had a piston with a smaller dish, heads are the same. Mid-71-91 heads have a 58cc chamber and 8.5:1 compression with the same pistons as the 2V early engine.

The head gasket compresses to about 0.042" (0.048" before compressing -- stock head gasket). With the stock 4.08" bore that's 9cc. So cutting the head 0.040" will reduce the chamber by 9cc, 0.020" by 4.5cc. Do the math and figure out what your compression ratio will be. Using the calculator at http://www.rbracing-rsr.com/compstaticcalc.html, I come up with a 0.32 increase in compression with a 0.020" cut (4cc chamber reduction), a 0.66 increase with a 0.040" cut (8cc reduction). My AMC books don't say how far you can cut the heads. 0.020" is safe, I don't know if 0.040" is or not -- talk to your machinist. With the 0.020" cut you come out with 8.78:1 compression, 0.040" = 9.12:1.

The "291" heads (70-mid 71) are pricey because they increase compression by 0.5 points simply by swapping the heads. Other than chamber size they are identical to the later heads. They are generally referred to as "291" heads because the last three digits of the casting number are 291. Some call them "319" heads for the first three digits of the casting number. Valve sizes are the same for all 360 and 401 heads (heads are the same) -- 1.680 exhaust", 2.025" intake.

The only real difference is the chamber size, but the price of the 291 heads has sky rocketed because some idiot in one of the rodding magazines stated that they were the "best" heads. They aren't. If you're building a mid 71+ AMC engine just buy the appropriate pistons to get the compression you want, don't waste money buying those heads! They are times when they would come in handy -- like upping compression on an already built engine, but otherwise they aren't worth the inflated prices.

Even the performance book AMC printed in the early 70s states that the dog-leg head exhaust port flows "50% better" than the 66-69 343/390 rectangular port head, but someone was smoking something that day! The port is no more than 15% bigger. I know size isn't everything, but other than the "dog leg" the port shape is about the same as the earlier head. I seriously doubt flow increase is more than 20% at best. For flow numbers on the newer heads see http://theamcforum.com/forum/cylinde..._topic159.html.
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Old 02-22-2010, 11:42 AM
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Quote:
These stock heads are in the neighborhood of 78cc and I was hoping to get more than that by flat milling.
I don't see 78 cc heads for the AMC 360. Largest I see are 60 cc nominal. You will lose cc's by milling- I'm sure you meant CR when you said "get more than that".

As it is, with a 58cc chamber, you'll need a 20cc dished piston to get a 9.45:1 CR.

This is with a .042 head gasket thickness, zero deck, stock B&S, 4.125 head gasket bore diameter.

Some info on OEM and aftermarket AMC heads: CLICK.
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Old 02-22-2010, 12:08 PM
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Quote:
Originally Posted by cobalt327
I don't see 78 cc heads for the AMC 360. Largest I see are 60 cc nominal. You will lose cc's by milling- I'm sure you meant CR when you said "get more than that".
I don't remember exactly what cc the head chamber is, but I thought it was something like that. (After some checking I believe they are 64cc.) That plus the dish pistons add up to a stock 8.4:1 compression ratio in my year range. Yes, I mean lose more cc's or gain more compression by milling. Sorry I wasn't very clear. I'm bad about that. Thanks for the link.

Last edited by Raufus; 02-26-2010 at 05:50 AM.
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Old 02-22-2010, 12:17 PM
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Quote:
Originally Posted by Raufus
I don't remember exactly what cc the head chamber is, but I'm pretty sure it was something like that. That plus the dish pistons add up to a stock 8.4:1 compression ratio in my year range. Yes, I mean lose more cc's or gain more compression by milling. Sorry I wasn't very clear. I'm bad about that. Thanks for the link.
No problem.

I guess what I was getting at, is no milling is needed, just a smaller dish than the engine came with should get you all the CR you need.

This won't help if you wanted to keep and use the original pistons, though.

Good luck.
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Old 02-22-2010, 12:34 PM
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Quote:
Originally Posted by farna
The "dog-leg" heads were made from 1970-91. The early model heads have smaller chambers but were only made 70- mid 71. Those have a 51cc chamber and 9:1 compression on 2V engines, 10:1 on 4V. The 4V engine had a piston with a smaller dish, heads are the same. Mid-71-91 heads have a 58cc chamber and 8.5:1 compression with the same pistons as the 2V early engine.
It's great to see another AMC'er.

Quote:
Originally Posted by farna
The "291" heads (70-mid 71) are pricey because they increase compression by 0.5 points simply by swapping the heads. Other than chamber size they are identical to the later heads. They are generally referred to as "291" heads because the last three digits of the casting number are 291. Some call them "319" heads for the first three digits of the casting number. Valve sizes are the same for all 360 and 401 heads (heads are the same) -- 1.680 exhaust", 2.025" intake. The only real difference is the chamber size, but the price of the 291 heads has sky rocketed because some idiot in one of the rodding magazines stated that they were the "best" heads. They aren't. If you're building a mid 71+ AMC engine just buy the appropriate pistons to get the compression you want, don't waste money buying those heads! They are times when they would come in handy -- like upping compression on an already built engine, but otherwise they aren't worth the inflated prices.
Yeah, I agree, they aren't the best. The 291 heads have the smaller chamber but supposedly don't flow as well as the later model stock heads.

Quote:
Originally Posted by farna
Even the performance book AMC printed in the early 70s states that the dog-leg head exhaust port flows "50% better" than the 66-69 343/390 rectangular port head, but someone was smoking something that day! The port is no more than 15% bigger. I know size isn't everything, but other than the "dog leg" the port shape is about the same as the earlier head. I seriously doubt flow increase is more than 20% at best. For flow numbers on the newer heads see http://theamcforum.com/forum/cylinde..._topic159.html.
I don't believe 50% either. Maybe in lab conditions, or in the theoretical, but not in real world circumstances. Thanks for the link.

If I had big bucks to spend, it would be easy just to get a pair of Edelbrock heads, but counting shipping that would be close to 2k I'm sure they are good heads, but money is tight.

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Old 02-22-2010, 12:41 PM
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Quote:
Originally Posted by cobalt327
No problem.

I guess what I was getting at, is no milling is needed, just a smaller dish than the engine came with should get you all the CR you need.

This won't help if you wanted to keep and use the original pistons, though.

Good luck.
Yeah, that's one of my issues. Even if I did want to rebuild the short block, there aren't many pistons other than the dish type out there. I would have to get custom forgings, or bore to a chevy size and turn the crank to a small journal chevy size or something like that to use chevy pistons and rods. Then it turns into big bucks. I'm sure that's not the only way to do it, but there aren't a lot of options with a thin wallet.
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Old 02-22-2010, 02:07 PM
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Quote:
Originally Posted by Raufus
Yeah, that's one of my issues. Even if I did want to rebuild the short block, there aren't many pistons other than the dish type out there. I would have to get custom forgings, or bore to a chevy size and turn the crank to a small journal chevy size or something like that to use chevy pistons and rods. Then it turns into big bucks. I'm sure that's not the only way to do it, but there aren't a lot of options with a thin wallet.
The specs for compression height and pin diameters, etc. are tantalizingly close to SBC sizes, it makes you start to scheme how different combos could come together.

A standard bore 400 piston on the AMC rods bushed to fit SBC pins, block bored 0.045" and the deck cut 0.020" would work (piston down the hole ~ 0.020(?)- or offset grind the crank to make up the difference, etc.)- but at what cost? It all adds up!

Anyway, keep us posted.
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