Don Terrill’s Speed-Talk

For roller camshafts, 8620 seems to be fairly common. Several manufacturers now seem to offer "Tool Steel" cores. A little digging I found that S7 was used, just curious as to what else was used.

Also, what are some instances where you've seen a core break?

For a project I may have to go down the road of making some new cores, just wanting to get some insight in to why certain materials are used over others.

Thanks in advance

8620
8660
9310
S7
M3

In 99% of roller applications, 8620 is the best way to go.

steel cores breaking, is very rare.
When it happens, it's usually an induction hardened cam core, where they mistakenly hardened it between the lobes.
Carborized 8620 cores that have been masked between the lobes, to block the heat treat, are extremely reliable. The only time we see issues is when the barrel of the core, or base circle, gets below .900" diameter.

Is 8620 the best for flat tappet too?

CamKing wrote:steel cores breaking, is very rare.
When it happens, it's usually an induction hardened cam core, where they mistakenly hardened it between the lobes.
Carborized 8620 cores that have been masked between the lobes, to block the heat treat, are extremely reliable. The only time we see issues is when the barrel of the core, or base circle, gets below .900" diameter.

I examined a billet Isky SBC solid roller on Sunday. It broke just ahead of the second-last lobe in the fillet between the lobe and the base circle after less than an hour in a 7500 RPM road racer. I thought I could see a tiny imperfection at the crack initiation site, but the surfaces were a bit beat up and I might have been seeing what I wanted to see. It was a uniform medium grey everywhere except the lobe and journal surfaces.

It's a dry sump engine so the only load to speak of is from the lifters, making it strange that it failed at the rear unless there was indeed a flaw. At least this meant that 14 valves dodged a bullet!

Speaking to Mike's highlighted sentence, I didn't measure but it had a very noticeably small barrel...

MadBill wrote:

CamKing wrote:steel cores breaking, is very rare.
When it happens, it's usually an induction hardened cam core, where they mistakenly hardened it between the lobes.
Carborized 8620 cores that have been masked between the lobes, to block the heat treat, are extremely reliable. The only time we see issues is when the barrel of the core, or base circle, gets below .900" diameter.

I examined a billet Isky SBC solid roller on Sunday. It broke just ahead of the second-last lobe in the fillet between the lobe and the base circle after less than an hour in a 7500 RPM road racer. I thought I could see a tiny imperfection at the crack initiation site, but the surfaces were a bit beat up and I might have been seeing what I wanted to see. It was a uniform medium grey everywhere except the lobe and journal surfaces.

It's a dry sump engine so the only load to speak of is from the lifters, making it strange that it failed at the rear unless there was indeed a flaw. At least this meant that 14 valves dodged a bullet!

Speaking to Mike's highlighted sentence, I didn't measure but it had a very noticeably small barrel...

The last section of the cam having the widest journal spacing may be a factor.

A little off my original topic, how much material is typically left on the lobes prior to grinding? I'm going to reverse engineer a camshaft I have to produce a few cores.

Agree with Schmidt, last span is longest & more likely to flex.

In 30 years of this business I have had about 8 or 10 SBC cams break in front of #8 ex lobe. Mostly small barrel stuff. All but 1 was advertised as having been made of 8620. The other being induction hardened.

Question here for Cam King. Does it hurt to have a larger cam barrel than the base circle as long as proper heat treating has been attained? Other than cosmetics and generous radiuses are employed.

jdperform wrote:In 30 years of this business I have had about 8 or 10 SBC cams break in front of #8 ex lobe. Mostly small barrel stuff. All but 1 was advertised as having been made of 8620. The other being induction hardened.

Question here for Cam King. Does it hurt to have a larger cam barrel than the base circle as long as proper heat treating has been attained? Other than cosmetics and generous radiuses are employed.

The reason the barrel is cut down smaller than the base circle is to keep the camshaft straight because it distributes the residual stresses evenly.

CamKing wrote:8620
8660
9310
S7
M3

In 99% of roller applications, 8620 is the best way to go.

what are the pros and cons of these materials in this application

Another question. Is flat tappet hyd&solid cams made from same material and is it possible to convert hyd to solid with lifter change.

DaveMcLain wrote:

jdperform wrote:In 30 years of this business I have had about 8 or 10 SBC cams break in front of #8 ex lobe. Mostly small barrel stuff. All but 1 was advertised as having been made of 8620. The other being induction hardened.

Question here for Cam King. Does it hurt to have a larger cam barrel than the base circle as long as proper heat treating has been attained? Other than cosmetics and generous radiuses are employed.

The reason the barrel is cut down smaller than the base circle is to keep the camshaft straight because it distributes the residual stresses evenly.

What was the name of that core design that was in vogue ~ 30 years back that had the lobes 'webbed' together and a maxed-out-everywhere barrel diameter, to increase strength? Was it 'lobe-locked'?

MadBill wrote:

DaveMcLain wrote:

jdperform wrote:In 30 years of this business I have had about 8 or 10 SBC cams break in front of #8 ex lobe. Mostly small barrel stuff. All but 1 was advertised as having been made of 8620. The other being induction hardened.

Question here for Cam King. Does it hurt to have a larger cam barrel than the base circle as long as proper heat treating has been attained? Other than cosmetics and generous radiuses are employed.

The reason the barrel is cut down smaller than the base circle is to keep the camshaft straight because it distributes the residual stresses evenly.

What was the name of that core design that was in vogue ~ 30 years back that had the lobes 'webbed' together and a maxed-out-everywhere barrel diameter, to increase strength? Was it 'lobe-locked'?

Yes, I think General Kinetics did some of those but I've been told by several people that they were a real pain in the butt to produce because they are very difficult to keep straight.

Any time the wheel grinds below the existing base circle and into the barrel of the core the cam instantly bends.

Runit wrote:Is 8620 the best for flat tappet too?

No