Need some expert cam terminology to explain OEM regrinding

[PackardV8]

In a conversation on OEM cam regrinding, I was given the following:

Because the cam will be a smaller diameter than original, it's impossible to duplicate the original cam profile. either the lift or duration has to be different. . . . . As the cam diameter is decreased by grinding, the speed of the cam running across the lifter is decreased, so if you want the same rate of lift, the profile needs to be changed slightly. I know these are only very small amounts, but we are using the term "exact" in this discussion, so "close" doesn't count.

Since he doesn't specify the "small amounts" and "exact", we're not going to answer this assertion, but just to clarify my understanding, if the same OEM master is used to regrind a typical worn OEM cam, how will the resulting lobes differ from the slightly larger OEM lobe?

[xenginebuilder]

Based on my experience, if your regrind is using the same master as the original, the differences will be small if the amount ground off is small, like .010" or so. There should be no lift difference, just a small change to the duration. I know that the same roller master ground on a SBF cam will shrink 2 degrees ground on a SBC. If you want the "exact" profile ground on a different base circle dia, then the master lift table has to be changed, and/or a different master made.
If the regrind looses lift, then the master is a copy made from an existing cam.

[statsystems]

The problem I've found with regrinding is you are making a master off of a lobe. Then you use that master to make a lobe. It's never exactly the same as the original.

The only way I will regrinding a cam is if the guy who is doing it has a properly made master, not a copy of a lobe.


I'm sure Mike will correct me if I'm wrong, so thanks in advance Mike.

[CamKing]

In a conversation on OEM cam regrinding, I was given the following:

Because the cam will be a smaller diameter than original, it's impossible to duplicate the original cam profile. either the lift or duration has to be different. . . . . As the cam diameter is decreased by grinding, the speed of the cam running across the lifter is decreased, so if you want the same rate of lift, the profile needs to be changed slightly. I know these are only very small amounts, but we are using the term "exact" in this discussion, so "close" doesn't count.

Since he doesn't specify the "small amounts" and "exact", we're not going to answer this assertion, but just to clarify my understanding, if the same OEM master is used to regrind a typical worn OEM cam, how will the resulting lobes differ from the slightly larger OEM lobe?

If, you're talking about taking .010" off, the lift will be the same, and the duration will be within 1/10 of a degree.

[statsystems]

In a conversation on OEM cam regrinding, I was given the following:

Because the cam will be a smaller diameter than original, it's impossible to duplicate the original cam profile. either the lift or duration has to be different. . . . . As the cam diameter is decreased by grinding, the speed of the cam running across the lifter is decreased, so if you want the same rate of lift, the profile needs to be changed slightly. I know these are only very small amounts, but we are using the term "exact" in this discussion, so "close" doesn't count.

Since he doesn't specify the "small amounts" and "exact", we're not going to answer this assertion, but just to clarify my understanding, if the same OEM master is used to regrind a typical worn OEM cam, how will the resulting lobes differ from the slightly larger OEM lobe?

If, you're talking about taking .010" off, the lift will be the same, and the duration will be within 1/10 of a degree.

What about making a master from an existing lobe? Is it possible to even copy a lobe like that and make it exactly the same?

[PackardV8]

Thanks, Mike, et al. I knew we couldn't claim exact, but 1/10th of a degree is close enough for the obsolete guys I go with.

The problem I've found with regrinding is you are making a master off of a lobe. Then you use that master to make a lobe. It's never exactly the same as the original. The only way I will regrinding a cam is if the guy who is doing it has a properly made master, not a copy of a lobe. . . . What about making a master from an existing lobe? Is it possible to even copy a lobe like that and make it exactly the same?

For true; one would never use a copy master if a custom master were a possibility. But for many projects, the perfect is the enemy of the good enough. In the real world of obsolete engineering, usually the only way you're going to get a 50-100-year-old one-off cam reground is to make a master from the best used lobe available. Agree, the result is not what a current max-effort-no-budget-EMC-build would accept, but making a master from a lobe always works and is always better than nothing.

Another FWIW; close enough is usually good enough for most restorations. Many times, there is no cam core available and/or no good-enough lobe available to copy. Slightly off topic, but a local shop was doing a $250,000 restoration of a very rare classic. When the owner was told there was no cam master available and the cost of a custom generated master, he said, "Would the judges ever know the difference when I am driving across the lawn at Pebble Beach?" We work with a cam grinder who specializes in obsolete, looked at all the scarce OEM specs available for that engine, lifter diameter, head and valve train design, compression ratio, RPM range and chose what seemed to be the most compatible match master from the hundreds he has on the rack. The engine seems to like that cam better than the original.

[CamKing]

What about making a master from an existing lobe? Is it possible to even copy a lobe like that and make it exactly the same?

It's never a good idea. All you'll do is compound all the tolerances, and end up with a poor quality copy.

[CamKing]

For true; one would never use a copy master if a custom master were a possibility. But for many projects, the perfect is the enemy of the good enough. In the real world of obsolete engineering, usually the only way you're going to get a 50-100-year-old one-off cam reground is to make a master from the best used lobe available.

The correct thing to do, is have the used cam measured, and have a new master designed to the specs of the old cam. For an extra $400, you'll be getting a design that you know is correct.

[modok]

Making a copy of a copy there is some degradation. It can work, if what you are using to make the copy has 10x better resolution than the original then you could be good as new, but how good WAS new?
Not sure what kind of OEM cam we are talking here but I am sure there is some variance. With today tech maybe you could digitize several lobes and pick the best one to copy. OR, at that point once you've analyzed the original, find a more modern grind that does the same job better that would fit within the original.

How much the cam is worn is easy to measure, and so is the base circle.
If they didn't have a reason to keep the base circles in a tight tolerance then they probably didn't, unless it's German, in which case they may have just out of pride.

[statsystems]

Thanks, Mike, et al. I knew we couldn't claim exact, but 1/10th of a degree is close enough for the obsolete guys I go with.

The problem I've found with regrinding is you are making a master off of a lobe. Then you use that master to make a lobe. It's never exactly the same as the original. The only way I will regrinding a cam is if the guy who is doing it has a properly made master, not a copy of a lobe. . . . What about making a master from an existing lobe? Is it possible to even copy a lobe like that and make it exactly the same?

For true; one would never use a copy master if a custom master were a possibility. But for many projects, the perfect is the enemy of the good enough. In the real world of obsolete engineering, usually the only way you're going to get a 50-100-year-old one-off cam reground is to make a master from the best used lobe available. Agree, the result is not what a current max-effort-no-budget-EMC-build would accept, but making a master from a lobe always works and is always better than nothing.

Another FWIW; close enough is usually good enough for most restorations. Many times, there is no cam core available and/or no good-enough lobe available to copy. Slightly off topic, but a local shop was doing a $250,000 restoration of a very rare classic. When the owner was told there was no cam master available and the cost of a custom generated master, he said, "Would the judges ever know the difference when I am driving across the lawn at Pebble Beach?" We work with a cam grinder who specializes in obsolete, looked at all the scarce OEM specs available for that engine, lifter diameter, head and valve train design, compression ratio, RPM range and chose what seemed to be the most compatible match master from the hundreds he has on the rack. The engine seems to like that cam better than the original.

I agree with you in the entirety.

I guess my point was there are many people I know of who are regrinding cams from copied masters for the most generic, common stuff like the Chevy and Ford and Chrysler stuff. For what you do...I agree. Sometimes you have no option. And what you can get is plenty good enough.

[statsystems]

What about making a master from an existing lobe? Is it possible to even copy a lobe like that and make it exactly the same?

It's never a good idea. All you'll do is compound all the tolerances, and end up with a poor quality copy.

Ok, that's what I though.

Thank you.

[Krooser]

Thanks, Mike, et al. I knew we couldn't claim exact, but 1/10th of a degree is close enough for the obsolete guys I go with.

The problem I've found with regrinding is you are making a master off of a lobe. Then you use that master to make a lobe. It's never exactly the same as the original. The only way I will regrinding a cam is if the guy who is doing it has a properly made master, not a copy of a lobe. . . . What about making a master from an existing lobe? Is it possible to even copy a lobe like that and make it exactly the same?

For true; one would never use a copy master if a custom master were a possibility. But for many projects, the perfect is the enemy of the good enough. In the real world of obsolete engineering, usually the only way you're going to get a 50-100-year-old one-off cam reground is to make a master from the best used lobe available. Agree, the result is not what a current max-effort-no-budget-EMC-build would accept, but making a master from a lobe always works and is always better than nothing.

Another FWIW; close enough is usually good enough for most restorations. Many times, there is no cam core available and/or no good-enough lobe available to copy. Slightly off topic, but a local shop was doing a $250,000 restoration of a very rare classic. When the owner was told there was no cam master available and the cost of a custom generated master, he said, "Would the judges ever know the difference when I am driving across the lawn at Pebble Beach?" We work with a cam grinder who specializes in obsolete, looked at all the scarce OEM specs available for that engine, lifter diameter, head and valve train design, compression ratio, RPM range and chose what seemed to be the most compatible match master from the hundreds he has on the rack. The engine seems to like that cam better than the original.

Not only did that car win at Pebble Beach but it does an 8 second 1/4 mile!!

[CamKing]

Look at my Cheater 604 cam design.
I measured the stock GM cam, and designed my own lobe, that's the same duration at .006", .020", .050", .100", .150", .200", .250", .300", and the same exact max lift.
Even though it looks the same, it's a more modern design, it's easier on the valvetrain, and makes an extra 8-hp.

That's the correct way to "Copy" a cam.

[DaveMcLain]

In a conversation on OEM cam regrinding, I was given the following:

Because the cam will be a smaller diameter than original, it's impossible to duplicate the original cam profile. either the lift or duration has to be different. . . . . As the cam diameter is decreased by grinding, the speed of the cam running across the lifter is decreased, so if you want the same rate of lift, the profile needs to be changed slightly. I know these are only very small amounts, but we are using the term "exact" in this discussion, so "close" doesn't count.

Since he doesn't specify the "small amounts" and "exact", we're not going to answer this assertion, but just to clarify my understanding, if the same OEM master is used to regrind a typical worn OEM cam, how will the resulting lobes differ from the slightly larger OEM lobe?

If the cam is a flat tappet why would the lift duration etc change at all when regrinding the cam?

[statsystems]

Look at my Cheater 604 cam design.
I measured the stock GM cam, and designed my own lobe, that's the same duration at .006", .020", .050", .100", .150", .200", .250", .300", and the same exact max lift.
Even though it looks the same, it's a more modern design, it's easier on the valvetrain, and makes an extra 8-hp.

That's the correct way to "Copy" a cam.

That was my point. You can regrind a cam with a master you made rather than a copy from a lobe and get the same or better.