Ok i am swapping some modded LT1 aluminum heads on my stock bore 305. It turns out that i have an LG4 with dished pistons. I measured the depth of the dish from the block deck and got 0.15" I calculated that there is 22.3cc's from the deck. I also cc'ed my heads and got 53cc's. I will be using the Felpro 1904 .015" shim head gaskets that have 3.2cc volume.

I am shooting for 10:1 compression so thats 69cc combustion volume. So i need to reduce my head volume be 10cc's. I am thinking that .006" milled = 1cc so i need .06-.07" milled.

Will this cause any problems? Will i need to get my intake milled to fit? Anything else? Thoughts, comments, suggestions?

Thanks

Leigh

 

If you in fact do have 22.3 cc of dish volume on a 305, that's a lot. By my rough calculations, it would require about 0.062" of deck material removal from the heads, and that would definitely require machining the intake to match. That would net a static CR of 10.01:1 with your given specs.

EDIT: You'll need to closely watch exhaust valve interference with that much milling, provided the aluminum heads have enough deck thickness to allow that much material removal safely.

 

ME Leigh, how is that shim gasket working for you? I was always under the impression that you couldn't use a shim gasket with aluminum heads. That's why I am asking.

 

Hmm i don't know about the shim head gasket. I haven't run it yet on the aluminum heads. But i did have a shim gasket with my old 416 heads on there.


The 22.3cc's is total combustion volume below the blocks deck. So its piston dish, and deck height.

How much do you think i could mill?

Will i have problems with the head bolts?

 

Yeah, it seems like 22+cc of dish vol is a lot.
About double what I would have expected.
It seems like it would be good to double check that if possible, before making the decision to mill .060 off your heads.
I'd use modeling clay, and bring a piston up to TDC then just pack it til it's level and measure the vol of the clay. I'm sure you're familiar with that procedure.
That would rule out mathematical error.
KB 145s are the OEM replacement piston for that engine and they're 12cc.

 

Ok well i took about 20 more measurements. The dish is .105" deep and 3.0" in diameter= 12cc's. The piston is down in the hole .03" .

Now if i measure from the deck to the dish of the piston is is .13" The dish is 3.0" wide and there is a .2" wide lip then about .15" angle on the edge of the piston and the conical edge of the cylinder wall. So i figure about 3.4" of .13" deep dish. Which equals 19cc's.

Confused yet?

So 19+53+3=75. So now i only need 6ccs taken off or .036".

I'm gonna measure some more from different locations on the deck and more cylinders to get some better data.

 

When I switched to my aluminum L98 heads, I was told that aluminum heads require a different type of head gasket than iron heads. Even though I was going down to a 58cc head (from my stock 64cc head), the typical gasket that the aluminum heads needed (.039") negated all the advantages of the smaller combustion chamber. (My L98 had flat-tops with small valve reliefs btw).

I did some searching and ended up going with a head gasket from TPIS (.029") which was suppose to be the thinnest head gasket designed for aluminum heads.

Before the aluminum L98 heads, I had never had aluminum heads on any SBC that I've owned, so I was relying on what I was told and read. So you may wish to double check this to ensure that you are not setting yourself up for a head gasket failure.

 

I took a look at the KB/Silvolite site and located a cast piston (#1449) that's supposedly a more direct replacement than the hyper one (#145) listed above, it's pretty close to what you described.
I did a CR calc also and it looks like what you're saying is true, regarding how much would need to be milled from the heads to raise the CR.
If I were in your position, I'd give some consideration to buying a set of flattop pistons that have a taller compression ht, and a weight that matches your factory ones, and then having the deck milled to zero.
I think your overall satisfaction will be greater, and in addition to that, you won't have to screw up your nice heads by milling so much off them that they need to have the intake surface milled to make them work. For which the cost savings for machining would be nearly equal to the cost of a piston set anyhow.
Then when you're done your heads would be of more value if you want to sell them, and you could use a thicker gasket, and you might be able to use stock length pushrods.
And I'm not sure, but is there enough deck thickness to begin with, on the LT1 heads, to accomodate a -.060" or .080" cut?
It just seems to me like flattops with 1.56" compression ht, and a deck milling job to match would be the solution to every one of the problems you've listed.
And you'd be more stoked on the performance too.

 

Well i don't want to do all that, this is really just a play motor. I don't want to spend alot on it, its only a 305 and i don't really care about it.

I really just wanted aluminum heads for the weight savings. And LT1's fit the bill in several ways the best so...

I'm gonna do some more measuring tomorrow. But right now i'm thinking that i might just mill them 0.03" for about 5cc's That should bring me to around 10.11:1 according to the KB compression ratio calculator.

Thanks for the help.

 

Ok i measure the #1 piston today and got .025" deck height and .128 to the piston dish. So off of those i think i'm just gonna mill .020" off and live with it. I took the heads to the welder today so hopefully they will be done soon.

 

Good luck.

Are you also going to try using the shim gasket too (just to see what happens)? If it's just a "play 305" and pulling the heads off again (in case the shim doesn't work) doesn't bother you; go for it.

I have never been given a GOOD REASON why aluminum heads can't use a shim. Maybe brinneling.

 

Yes i'm gonna try a shim head gasket, i believe Summit said it was fine. That is according to there website. It said the gaskets were for irona and aluminum. But then again i could be stupid!