XTREMEGSXR

I have seen quite a few comments about number 5 & 7 heads. How are the number 6 heads for a mild build? Also have read that 403 rods are best.

2blu442

Yes, the #6 heads are similar in performance with the 5, 7, 7a. Its the #8 that have a significantly larger combustion chamber. Someone else will have to speak up about the 403 rods. John

Octania

403 "XC" rods are indeed beastier, slightly.

shakeyhands73442

I am going from 8 to 6. I've been told it will be a pretty noticable difference.

olds 307 and 403

You will have 10+cc smaller chamber plus better exhaust side flow. Make sure to use the .028" Corteco composite head gaskets from Rocket Racing or the .011" shim head gaskets from MJ Proformance unless at 0 deck. All Olds rods are so so strength wise plus heavy. I am sure the Scat sbc rods I am using, will much stronger and lighter.

XTREMEGSXR

What cars had the 403xc rods? Does anyone have a set they want to sell?

olds 307 and 403

I may have a set soon. They will need the big end resized, pretty sure a rod bearing is spun.

MDchanic

Just for the sake of thoroughness, as I have seen vague references to "Smitty at MJ Performance" before, with no clear contact info, please allow me to state:

MJ Proformance
Mark "Smitty" Smith
4444 Meese Road, NE
Louisville, OH, 44641-9119
(330) 875-8096
(330) 495-4582
MJProformance.com ("Under Construction" placeholder page)
smitho455 at att dot net

By the way, what does he get for those shim gaskets?

- Eric

Sampson

Are the shim gaskets used in conjunction with head gaskets to adjust compression?

MDchanic

No (and yes).

The original head gaskets used by the factory were just 0.016" thick steel stampings. That's what we're talking about here.
They can be hard to get nowadays, and the most common FelPro gaskets are 0.030" thicker, so will drop compression about a point (if I recall my calculations from a while ago).

On the other hand, back in the '70s, people sometimes used shim gaskets (not the same as the OEM gaskets we're talking about) to reduce the compression of high-compression engines, in order to be able to use the gas that was available at the time. It is possible to run across cars with these gaskets on occasion.

- Eric

Sampson

Thanks for the info Eric. I may be using bad etiquette here and do not mean to be stepping on this thread. I am installing Speed Pro flat tops in my 350 that are 30 over in conjunction w/#6 heads. Compression will be a little above 10:1 which has me worried about pump gas. Are the shims still available to bring CR down a touch. I would like to be between 9.5:1 & 10:1

MDchanic

I have no idea, but if they are available, they are not a "best practice" sort of a modification.

Did you cc the heads to calculate that CR, or did you use the nominal 64cc volume? The nominal volume is seldom accurate, and the actual volume tends to be higher.
Also, did you calculate the CR with the thicker FelPro gaskets or other gaskets?

- Eric

J-(Chicago)

You can order cometics gaskets in any thickness you like, they just aren't cheap.

Sampson

Thanks for the info. I also talked w/Smitty from MJ Performance today who told me that he could also make gaskets to order.

Sampson

I was using .039 from Fel Pro. Talked w/ the builder today and he said the heads. C/C 'd. 66cc before cleaning and valve job so it only gets better from there. Also talked w/Smitty from MJ Performance today who said he will build up to 10:5 cr with iron heads without issue. I will sleep better tonight. Thanks for the help

MDchanic

You're welcome.