Can you identify big valve head without removal?

[Johnfm]

So, previous owner told me the car had a big valve conversion by previous owner.

I assuem there is no way to confirm without taking off the head?

Maybe check the carb jetting (the guy would have re-jetted for a big valve head, no?

[rgh0]

No easy way to tell if a big valve conversion for the valves themselves, without pulling the head to measure. But the valve sizing was the least important part of the big valve specification

The cam, the carbs, the exhaust and the electric fan contributed more to the HP gain than the valves! and all of those can be easily checked

cheers
Rohan

[pharriso]

So, previous owner told me the car had a big valve conversion

They could have replaced the original head with a big valve head, or they could have machined your head to Big Valve Specs by shaving .040" off & putting bigger intake valves in.

Measure the thickness... BV should be 4.60" thick I seem to remember... all others 4.64"

The Sprint Cams have two rings around them, but you will have to loosen the timing chain & remove the sprocket to check that.

[rgh0]

Almost all heads these day are big valve spec or thinner after 40 plus years of skimming !!!

Getting the big valve compression ratio comes as a given normally

cheers
Rohan

[Craven]

Hi,
A simple way to get a clue is using just a pair of vernier calipers, cheap digital are adequate. Measure the cam lift, make a measurement of the smaller diameter, reset zero, then measure the larger or peak of the cam lobe. The reading will be the cam lift, obviously it won?t give any timing info but will give you a guide to type of cam. Lots of Twin cam cam?s on Net but from memory standard cam 0.350 inch, Sprint or CPL2 0.360, L1 0.410.
For what it?s worth Ron.

[Johnfm]

Good idea.

Need a dial gauge and magnetic base too now...

[GrUmPyBoDgEr]

Good idea.

Need a dial gauge and magnetic base too now...

That will provide you with a load of fun on an aluminium cyl.-head

Cheers

[Johnfm]

Heh!

Good point!!

[Craven]

Hi,
Don?t misunderstand, it?s only a measurement of the relative sizes of the cam lobe you are measuring. Base diameter, 90 deg to peak and then peak. Example small dia at 90 deg = 1.1 inch. peak 1.45. = cam lift 0.35.


Ron

[john.p.clegg]

Ron
No,he's also after setting the timing accurately...

John
You'll have to get a small piece of steel to stand the magnetic base on on the head...

John

[mbell]

If it's an original big valve head it will have a z/N stamped on the head.In front I'd spark plug one.

See the pic.

[Johnfm]

My compression numbers were 197,196,179,203.

Seem high if it was a standard set up. Or could it just indicate the head has been skimmed in a previous life.

[rgh0]

You can generally get a set of callipers on the head to measure its thickness with it still in place.
Originally they were 4.640 inches standard. 4.600 inches Sprint.

cheers
Rohan

[Jeff@Jae]

If the previous owner was only referring to the valves you won't know unless the cylinder head is off. Using the Sprint valves in place of the standard valves is common. The so called big valves are only .030" larger but they do have a much better head shape for improved flow and with a little throat porting the gain is pretty good for little effort. As others have pointed out the real Sprint conversion would be a cam change/head skim or different pistons to gain the increase in compression ratio. We do it with the pistons here to save the cylinder heads and have much better pistons with a contemporary ring design. So measure the head thickness and cams to see what was really done and don't worry about the valves for now. Jetting of the carbs should have been changed if the compression and cams were done.

[stugilmour]

I just had my compression measured prior to valve adjustment at 155 150 154 150. Federal Stromberg head; not sure valves & cam. Guys, does this give me a CR of (155 + 14.7) /14.7 = 11.5? Is my CR low to expectations as certainly lower than John's numbers? When we did the build I believe the head was skimmed a bit for cleanup and 40 over pistons installed. Dave Bean recommended & supplied a thicker head gasket to avoid detonation problems. Car runs well on premium, but will ping like crazy on regular. Noticably better on 94 octane Super Premium than 92 octane Premium, using standard NA Pump octane rating (MON + RON) / 2.

Anyway, picking up on John's numbers, is there an 'expected' or 'right' CR or compression test range? You can probably tell I only have expertise at writing engine building cheques.

Stu