Tech Breakdown of Factory Valve Relief

I don't do fake book, but early heads each have their own part number per side, a number then a or b for which side, or a different number?

Post war, like 59ab means it will fit either side.
 
Basically these were the salient points as to why he claims blocks were factory relieved:

1. Camshaft is offset from the crank centreline by 0.265”
2. Valves do not run parallel with the bores (49.5 vs 52 degrees) from centreline of the block.
3. This results in more of an “eyebrow” on one side of the block than the other.
4. These “eyebrows” (area between bore and valve seat) would get quite thin and as such were prone to cracking.
5. The heavy industrial motors got the relief milled in the block to get rid of the “eyebrow” and increase gas flow. He shows a 59A style motor in the vid.
6. These engines with the relief milled into the block have special heads to compensate for the differences in material milled out of the block on each side.
7. There are over 150 different part numbers for the heads for these early 24 stud motors.
8. Left side heads typically only have one port in the head pointing backwards for a temp sensor.
9. The right side head typically has two ports.

Some pics from the vid showing the areas he is referring to as the “eyebrows”.




1. Blocks without the factory reliefs were prone to cracking between the bore and valve seats.
 

Attachments

  • IMG_0558.jpg
    IMG_0558.jpg
    32.5 KB · Views: 269
  • IMG_0559.jpg
    IMG_0559.jpg
    36.8 KB · Views: 266
Last edited:
Earlier production Edelbrock 39 - 48 heads used to have a right and a left . . . to compensate for the different valve angles in their chambers. All the modern versions are the same (side to side) - at least as far as I know. I really don't like the modern chambers, but they are what they are! LOL
 
Ziggster said:
Basically these were the satient points as to why he claims blocks were factory relieved:

1. Camshaft is offset from the crank centreline by 0.265”
2. Valves do not run parallel with the bores (49.5 vs 52 degrees) from centreline of the block.
3. This results in more of an “eyebrow” on one side of the block than the other.
4. These “eyebrows” (area between bore and valve seat) would get quite thin and as such were prone to cracking.
5. The heavy industrial motors got the relief milled in the block to get rid of the “eyebrow” and increase gas flow. He shows a 59A style motor in the vid.
6. These engines with the relief milled into the block have special heads to compensate for the differences in material milled out of the block on each side.
7. There are over 150 different part numbers for the heads for these early 24 stud motors.
8. Left side heads typically only have one port in the head pointing backwards for a temp sensor.
9. The right side head typically has two ports.

Some pics from the vid showing the areas he is referring to as the “eyebrows”.




1. Blocks without the factory reliefs were prone to cracking between the bore and valve seats.
Click to expand...
591557


591558
 
While we're on the subject . . .

Here is a good look at a stock relief --> as used in industrial, some trucks and other heavy-duty situations. This is 42 WWII block - NOS.

BlockStock1.jpg

BlockStock2.jpg


Here is the same block after I performed an experimental stepped relief. There is a raised flow eyebrow to direct the intake charge into the cylinder - similar to how some of the Harkey KR cylinders were done.

SteppedRelief-41Merc-A.jpg

With valves -> 1.72" intake and 1.6" exhaust

SteppedRelief-WithValves.jpg

Here is a "traditional" race-style relief done on a 59L block (came out of the Kenz-Leslie streamliner):

RelievedBlock-59L-Closeup.jpg

Here is a 'street relief' that smooths the transitions from the valves, but doesn't remove a lot of material. This is a 59AB block. This helps keep the compression up (which is definitely good for the average performance street motor):

Street-PartialReliefWithRadiusBoreEdge.jpg
 
I'm a true believer of the factory relief as far as helping to reduce the valve-cylinder cracks. Years ago I was given a trailer mounted ww2 vintage welder with a 24 stud V8. I was told it had an overheating problem, and man, Did it. This was in the mid 90s and I had no V8 contacts. Just books. I tried everything and I'd think it was cured and I'd be welding on something, and another serious geyser. Never any water in oil though. I had read a lot about cracks and seen quite a few in other V8s. Well one day I got sick of the heavy old thing. I scrapped the welder and kept the engine. Then gave it to a friend who'd inherited a '40 ford. A week later I was in his shop and the heads were off and block was really cleaned up. First time I'd seen a factory relief (a thing of beauty) and with my young eyes straining, not a hint of a crack anywhere. He still hasn't put it in the 40 though, and I'll cut him some slack as he just retired a few months back. Anyhow, a tough block.
 
Bored&Stroked said:
While we're on the subject . . .

Here is a good look at a stock relief --> as used in industrial, some trucks and other heavy-duty situations. This is 42 WWII block - NOS.

View attachment 591679

View attachment 591680


Here is the same block after I performed an experimental stepped relief. There is a raised flow eyebrow to direct the intake charge into the cylinder - similar to how some of the Harkey KR cylinders were done.

View attachment 591681

With valves -> 1.72" intake and 1.6" exhaust

View attachment 591683

Here is a "traditional" race-style relief done on a 59L block (came out of the Kenz-Leslie streamliner):

View attachment 591682

Here is a 'street relief' that smooths the transitions from the valves, but doesn't remove a lot of material. This is a 59AB block. This helps keep the compression up (which is definitely good for the average performance street motor):

View attachment 591684
Click to expand...

Thanks for sharing! Some real interesting stuff!
 
I have two C59A engines. Both were industrial engines, and both came with the heavy duty truck 4-speed. Both had the truck oil pan with the large cleanout. However, only one had the factory relief. I find that very odd. The one with the factory relief is in my speedster. I will rebuild the second engine soon, but I’m on the fence as to having it relieved. I want to bore and stroke it out for maximum displacement.
 
Ziggster said:
I have two C59A engines. Both were industrial engines, and both came with the heavy duty truck 4-speed. Both had the truck oil pan with the large cleanout. However, only one had the factory relief. I find that very odd. The one with the factory relief is in my speedster. I will rebuild the second engine soon, but I’m on the fence as to having it relieved. I want to bore and stroke it out for maximum displacement.
Click to expand...

The factory relief will decrease the compression ratio so plan accordingly for your heads and squish.

Factory-relieved 59A and 59AB blocks were primarily truck units designed to lower compression for better performance with low-octane fuel and heavy loads. The traditional The constant-depth relief for a factory relief, removed approximately 0.100 to 0.125 inches of material from the combustion chamber between the valves and the cylinder bore.

Note that the testing done in the table above indicates these values For Offenhauser 400 heads:

For 0.100 relief, 3 cc of volume removed, CR is reduced by 0.39 (9.72-9.33 = 0.39)
For 0.125 relief, 4 cc of volume removed, CR is reduced by 0.52 (9.72-9.20 = 0.52)

In the table linked in my post #14 below, some testing was done on other after market heads with a range of about 0.26-0.37 for a 0.100 depth of relief and the upper end 0.52 for a 0.125 relief.
 
Last edited:
glennpm said:
The factory relief will decrease the compression ratio by about 0.42 so plan accordingly for your heads and squish.
Click to expand...

There was a massive spreadsheet somewhere on the interweebs that had all kinds of CRs for various bores and head combinations. Don’t recall if it mentioned anything about the blocks with the reliefs, but IIRC, with my combo (0.060” over and C7RA-A cast iron heads) it was around 7.4:1. So, with the factory reliefs, I’m guessing it’s really around 7.0:1.
 
Ziggster said:
There was a massive spreadsheet somewhere on the interweebs that had all kinds of CRs for various bores and head combinations. Don’t recall if it mentioned anything about the blocks with the reliefs, but IIRC, with my combo (0.060” over and C7RA-A cast iron heads) it was around 7.4:1. So, with the factory reliefs, I’m guessing it’s really around 7.0:1.
Click to expand...

Yes, see page #216 of my Techno pdf in my signature.

"Many thanks for David Fort, Bruce Lancaster, Joe Abbin, Armin Brown, JWL, Scott Miller and others for their input & data in this effort. Hopefully this data will be beneficial for use in our flathead Fords & Mercurys in future years.

Jim (blown49 on the BARN)"
 
glennpm said:
Yes, see page #216 of my Techno pdf in my signature.

"Many thanks for David Fort, Bruce Lancaster, Joe Abbin, Armin Brown, JWL, Scott Miller and others for their input & data in this effort. Hopefully this data will be beneficial for use in our flathead Fords & Mercurys in future years.

Jim (blown49 on the BARN)"
Click to expand...

When I click on the Techno link, I get this. Is it still a valid link?
 

Attachments

  • IMG_0611.jpg
    IMG_0611.jpg
    25.4 KB · Views: 15
Bored&Stroked said:
While we're on the subject . . .

Here is a good look at a stock relief --> as used in industrial, some trucks and other heavy-duty situations. This is 42 WWII block - NOS.

View attachment 591679

View attachment 591680


Here is the same block after I performed an experimental stepped relief. There is a raised flow eyebrow to direct the intake charge into the cylinder - similar to how some of the Harkey KR cylinders were done.

View attachment 591681

With valves -> 1.72" intake and 1.6" exhaust

View attachment 591683

Here is a "traditional" race-style relief done on a 59L block (came out of the Kenz-Leslie streamliner):

View attachment 591682

Here is a 'street relief' that smooths the transitions from the valves, but doesn't remove a lot of material. This is a 59AB block. This helps keep the compression up (which is definitely good for the average performance street motor):

View attachment 591684
Click to expand...

Dale, how deep are those relief cuts? Hard to tell in the pictures. Thanks
 
Hey Tim - it depends on the style. On a traditional full-race style relief, they are between .150 and .187 deep. One thing to note, when running a deeper relief and also a supercharger with some real boost, sometimes I will move the ring pack down a bit - to keep some heat out of the upper ring. Too much heat into it can cause it to "relax" a bit and then cylinder sealing will suffer. Now this is for all-out race style stuff, but I thought I'd mention it!

Note: Folks like Ross will take instructions as noted above ---> I like to specify the rings (always metric) and sometimes the ring pack location from the top of the piston edge.
 
Hope he chime in then. I’m more interested in the claim if I understand him correctly that there is a difference between the left and right side heads on a 59A type engine.
 
You must log in or register to reply here.
Back
Top