Differences in 906 heads....

[BSB67]

We did cc the heads and all chambers were within 1 cc of one another. They have been planned .060" and to our surprise still came out to around 84cc. I thought there would be less than this with having so much material removed....

Your machine shop? What's the invoice say?

Measure the thickness of the outside row for the head bolts. An uncut head will be about 1.00"

 

[mopar_chuck]

Your machine shop? What's the invoice say?

Measure the thickness of the outside row for the head bolts. An uncut head will be about 1.00"

Do not have an invoice from the machine shop as this whole project was started MANY years ago by my father and that is when the heads were planned. We have just recently picked back up on the restoration and engine build.

Anyways, the 1” measurement is exactly what I’m looking for in regards to a reference measurement. Thank you. I will check it tomorrow and report back.

 

[66Satellite47]

We did cc the heads and all chambers were within 1 cc of one another. They have been planned .060" and to our surprise still came out to around 84cc. I thought there would be less than this with having so much material removed....

Just looked at a couple of my sets of 906's. A .060 mill should get pretty close to 80 to 81 cc. The .030 cut gets you 84 cc +/-. Just a rule of thumb, castings are all different.

 

[mopar_chuck]

Well mystery solved.... measuring from the deck surface to the head bolts shows roughly 0.975” which would indicate that only about 0.025” was taken off and would be consistent with the 84-85 cc chamber volume.

Will need to go back to the shop to have the other .035” removed because we know the the intake face had 0.072” taken off to match the 0.060” that we thought was removed from the deck surface. Lots of fun!

 

[PRHeads]

Unless you guys cut the intake side yourself, or had it done........ I think I’d be mocking up the intake fit before I cut the heads more.

 

[mopar_chuck]

Unless you guys cut the intake side yourself, or had it done........ I think I’d be mocking up the intake fit before I cut the heads more.

You are absolutely right but my father actually milled the intake faces himself. However to be cautious, we will do it anyway. I ordered a Holly street dominator intake off eBay last week and should be here tomorrow.... There is also a concern of piston to valve clearance because the pistons are flat tops with no valve reliefs, but shouldn’t be an issue since our cam is not that big (.484”).

 

[66Satellite47]

I always check piston to valve. A .484 type cam should not be a problem, but if you check stuff you know. BTW the minimum piston to valve is not max cam lift. The piston is chasing the valve or the valve is chasing the piston on intake or exhaust. You need to check both. Long duration cams will be tricky.

 

[BSB67]

You are absolutely right but my father actually milled the intake faces himself. However to be cautious, we will do it anyway. I ordered a Holly street dominator intake off eBay last week and should be here tomorrow.... There is also a concern of piston to valve clearance because the pistons are flat tops with no valve reliefs, but shouldn’t be an issue since our cam is not that big (.484”).

Might want to check your piston height too before you do anymore milling.

 

[PRHeads]

Might want to check your piston height too before you do anymore milling.

Good call.
Now is the time to figure out what you really have.

 

[mopar_chuck]

I spent a good part of today cleaning the crank and main bearings to get it ready to install in the block for test fitting. Everything was caked with assembly lube that had been put on from about 20 years ago. What a job to remove!!! However, it did preserve it and all looks good with the crank so I’m happy about that. I used a lot of varsol and elbow grease to clean everything up. When my kids got home from school, one of them said, “daddy you smell like the road runner really bad.” Lol.... I took it as a compliment.

 

[PurpleBeeper]

Just a fyi on Isky's… I run a set of old Isky's and I did have to clearance the sides of my rocker pedestals + hold down tabs. I'm running the 71/72 iron heads & none of the stands were narrow from the factory. It's interesting that there are 906's both ways. I always "thought" the pre-'71 heads were all narrow at the top?...guess not.

 

[66Satellite47]

The Isky rocker arms were a very good product. I abused them with extreme RPM. The pedestals on all of my 906 & 915 heads fit fine. I did shave a few of the stock hold downs before I switched to custom solid spacers. Wish I had a good set of Isky rockers now.

 

[mopar_chuck]

Just got done test fitting the piston deck height..... the pistons are too far down in the bore to work with so a new set will be in order. Our block was supposed to have been decked and looks like it was not. So instead of shaving more off the block and the heads, we will just do it the easy and more practical way of buying new slugs. A bit disappointing but at least we can get exactly what we want this way.

 

[POMONAMISSEL]

yes , different casting dates , but there are an alfa letter as well , different letters have better porting ramps than others , and all 906 casting .

 

[mopar_chuck]

One of the heads is on the bench and the other one is in the seat and guide machine getting the valve job and porting done so I cannot check the cast date on that one. But when it gets finished up (next week hopefully) I will check the casting dates on both heads and then determine which date had narrow and which date had the wide pedestals.

 

[PRHeads]

Just curious.......just how far down is “too far” down?

 

[mopar_chuck]

Just curious.......just how far down is “too far” down?

0.062”. With 84cc heads and the steel shim gasket, it only figures to about 8.7:1 according to my calculations....

 

[BSB67]

0.062”. With 84cc heads and the steel shim gasket, it only figures to about 8.7:1 according to my calculations....

At 0.062", my guess that the deck has been cut, probably 0.020" or so.. The piston probably has a 1.991" CH.

I get 9.7:1

 

[Garys1969RR]

Try to get a piston with a 2.067 C Height, or better. This will put it about .015 below deck. Then with a closed chamber head, and a .020 steel shim gasket, you can get some good quench at 35 thousandths. That goes a long way in improving throttle response, as well as a quick burn in the chamber, and resistance to detonation. Then you can run 10 or 10.5 to 1 compression ratio, as the Roadrunner motors were originally designed to have. Chrysler engineers designed a very good motor, but the heads were usually larger chambered than specs called for. 906 were supposed to be 79 cc from the factory. 915s were 73 cc.

 

[66Satellite47]

FYI, un-milled 906 heads are about 88 CC. If the block has been milled straight, get the correct piston for the CR you want. As Gary said 2.067 gets you in the ballpark with about .010 down in the hole. That's where I ran most of my 440's. The 915 head gets you about .75 + CR increase. Both 906 & 915 really like pocket porting. The area under the valve seat is very poor in stock shape. The new valve job machine equipment can correct for this. A little work with the grinder makes big power improvements. I have done several dozen sets of 906 & 915 heads, not so much variability in them. I grind out the differences in the ports. Mill the deck to your specs. If you measure everything, you know what you got. Later head castings have bigger CC chambers.