Thank you for that excellent information! Those are the kinds of things I need to know.
My purpose of the engine is daily driver and off-roader, so I don't need every ounce of power I can get. Reliability is much more important. However, since this is my daily driver, and I use it for everything, I'd like to get as much reliable punch out of it that I can, as well as an increase in economy.

I have a block in my garage, so I had planned on bringing it down to the machinist anyway. I'll have him zero deck the block and go from there, and I'll probably leave the head alone. It sounds like 0.040" is a good place to leave it at.

I wouldn't mind using mid-grade fuel, but it's always nice to just stick at '87, so I probably wouldn't want to go much beyond 9.0.

My next question is with those online calculators.
I must be entering something wrong, because no matter what I put in, I get a result of 8.13:1 compression.

On summitracing:

Bore: 4.0
Stroke: 3.98
Cylinder head cc: 76
Effective dome volume: 22
Deck clearance: 0.043
Compressed gasket thickness: 0.039
# cylinders: 6

However, I've usually seen numbers around 8.4 - 8.5 compression (Rockauto.com states 8.5CR with stock 22cc pistons). Am I missing something?

 

Do not zero deck until you are 100% sure on what pistons you will be using.

Yes. I used the 68cc 240 head in my calculations. You were trying the same with the 76cc 300 head. Other than that, you are doing nothing wrong and the figures the calculators are giving you are correct.

 

No worries, I'll have everything fully planned before I have anything done.


So the 300 has 8.1 CR stock?

 

Depends on the year but 8.1-8.3, around there. I've never seen a definitive source anywhere.

I have however seen a definitive source of the 240 compression going down a bit every couple years and I would imagine that the 300 was the same. I have also pulled a few 300 heads and noticed that the earlier ones have flat valves and the later ones have dished valves. I know the head never changed and I don't think the pistons ever did either, except for the piston pin diameter change in 1968.

 

oversized valves, a little bowl work, and cleaning up the runners really wakes these puppies up! not sure if SI is still producing the stock stem big valves anymore. might have to contemplate the SBC valves perhaps.
fyi...i'm still up in the flathead valley, MT 59901...got quite a few parts and odds 'n' ends. if you end up looking for a thing or two, drop me a line before you go spending anything at the parts counter or stealership. sounds like you've got a good ground plan set up though man. keep the pics and posts coming!

 

SI is not producing oversized valves anymore. Find yourself a set of SBC valves and make them work.