Learned something interesting today... has anybody out there CC'd their D heads?

The machine shop measured mine today after the valve job, and they measure 82cc. So any information out there about D heads having a smaller combustion chamber than other BBO heads is likely wrong.

They are often advertised at their NHRA minimum of 69.75cc, but I had heard other information they were in the low-70's. I don't have an old Mondello engine manual to see what that says.

Yes, they are real 400370, D heads, 110 date code... were pretty much untouched virgin heads. Curious to see if anybody else has checked this before.

 

This is what 442.com says... these heads have one side of the crossover blocked, I'm not sure anybody has actually seen a D head with both exhaust ports feeding the crossover.

D 400370 69.75cc 2.072" 1.625" 68-69 400/455's, with W-30 or H/O [except A/C '68 H/O's- see C].

Large valves, 2.072 and 1.625". Shallow spring seats. No hard exhaust seats. Smaller combustion chambers than other BB heads, possibly due to the small bore of the G-block 400 motors. Rumored that the '68 versions had both center exhaust ports connected to the crossover, whereas an improved later version as used in 1969 had one port blocked off from the intake heat crossover port for better power.

 

People always think that the published numbers are as-cast. They are not. The published numbers are the NHRA and manufacturer's BLUEPRINT numbers. There's a reason why you blueprint an engine for max performance. The as-cast chambers are ALWAYS larger than the blueprint number. This allows the heads to be milled more than once without exceeding the design number. It also ensures that production cars, built to mass-production tolerances, won't have too much compression and thus be prone to detonation (and warranty work).

The published numbers are what NHRA and Oldsmobile agreed were the DESIGN numbers that a pure stock race car could be machined to for max performance. They have no bearing on the real world.

 

I get the difference there and we see that all the time. Even the Mondello tech manual published D heads at 72cc, smaller than the rest of the BBO heads, which isn't the NHRA minimum number.

 

That said, I have noticed that NONE of the "ordinary" heads I have ever seen were factory surface down to where the casting ID characters were touched. However, ALL of the D heads I have seen are cut until these casting ID characters are affected.

A bit more cutting.

 

Here's the only pic I have of these on the combustion chamber side.

 

Here is mine