Is cam regrinding purely subtractive?

David in San Antonio

David in San Antonio

Well-known member
I searched past threads but couldn’t find the answer:
I was told that when a Model A cam is reground it is purely a subtractive process. That is, the lobes are not first built up with additional metal prior to grinding. This sounds like the reground cam could have a different shape but at no point could its diameter exceed that of the original.

Q1. Is that true that regrinding an A cam is purely subtractive?
If so, is this true for other engines?

Q2. If they are not built up, isn’t there potential for the lift height to be diminished? I presume the lift height generally isn’t increased or the valve could strike the head, especially if it’s a higher compression design. But don’t we want to at least maintain the original lift?

Q3. If not built up wouldn’t this limit the duration the valve is open? I’m picturing a pointy lobe with steep ramps vs a more rounded, wider nose, holding the valve open longer.

Thanks for helping me understand.
 
You can counteract the decreased height of the cam lobes by lengthening the valve+tappet linkage. The cam grinder will tell you the correct lash to set.
 
Another way to explain this is, let’s say you had a stock cam, and you wanted to keep the valve open longer. You take out the cam, grind the top of each lobe to be less pointy, and put it back in. This doesn’t get what you want, because the valve and tappet now sit lower overall. Aha, you think, I’ll adjust my tappet to be longer. Now you get the behavior you want - the valve rises earlier and lowers later - but you’ve altered the valve lash, so now there’s not enough clearance when the valve is at the bottom of its travel. So now you have to take the cam out and grind material off the round portion of each lobe to bring the lash back to the correct amount.

So you see, regrinding a cam involves reshaping each lobe to be slightly smaller all the way around. You then re-adjust the length of each tappet to get the correct clearance again - which might be different than the stock cam clearance.
 
They also grind what is known as the base circle that gives you the increased lift but you can only grind down so much before you would have to build it up!!!
 
To regrind any cam, it is mounted in the grinder "on centers". Look at each end of all cams and you will find shallow holes, these are exactly in the center of the bearings diameter-wise. The grinder is similar to a crankshaft grinder, only a 'master' lobe is mounted on one end with a follower so the grinding wheel moves in and out as the cam rotates. The grinding wheel can be moved along the cam so it can reach each lobe. There is a special chuck that the cam is mounted in, it is rotated to properly align with the lobe. That chuck can be set for 4,6, or 8 cylinder cars, it can also be rotated by degrees to properly align with a lobe, this is done on the first lobe to be ground, then the chuck is rotated for the number of lobes (cylinders of the car). Metal is then ground to match the master. For a performance grind more metal is ground off the heel of the cam but very little off the nose. If that cam is now inserted in the engine you will have to adjust (lengthen) the lifter to the proper gap. So the lifter is now closer to the center of the cam, but the nose is farther away, so the lift is greater. The master lobe is 4" or 5" in diameter, depending on what company made the machine, this insures the cam lobe will be exactly as you want it to be. I hope this is understandable?
 
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This is a Winfield 1R grind. You can see how the heel has been cut down to increase lift.

Duration 266 Lift .350 Winfield 1R grind - fast action, decent idle
 

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So, to the OP questions,
Yes, grinding is a subtractive process. The typical cam regrind will leave the lobe smaller.
As Jim has mentioned, if you remove from the base circle it will increase the lift of the lobe.
Welding is an option to build up the lobe surface to increase the duration of the cam and grinding/blending the lobe to final shape/profile.
I made cam blanks for racing as shown in the picture to allow for changing the lobes layout for different crank firing order and also reverse porting the motor.
These also allowed for the block to be bored out and use a larger cam diameter, this gave .500 lift and did not back cut the base circle.

John
 

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When the legendary Ed Iskenderian made the famous 404 radius lifter camshaft with.404 lift for the flathead V8. That was about the most lift you could get without making the base circle so small that the cam would flex and break and still fit the current bearing size of the flathead V8. Then later he made a 505 radius lifter cam with .505 lift. The way he was able to fit this larger lobe in the block was to run it without the cam bearings just like a model A cam runs!!!
I run the 404 radius lifter cam in my 296 Flathead V8 Nostalgia dragster in my avatar.
Ed “Isky” Iskenderian is I believe the last of the living Hot Rod Pioneers and Innovators and is currently 103 years old!!!
Yes I know I strayed off Model A’s.
 
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