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How shaving a head has an effect on cam timing

OVERDRIVE

OVERDRIVE

Well-known member
Joined
Jul 31, 2017
Location
Brooklyn, Neu York
I'm putting my motor back together today, .062" is removed from the distance from the crankshaft to camshaft from a mixture of decking the head and an MLS head gasket. I've heard 1° every .010" thrown around but I wanted a clear answer and I haven't seen the calculations documented.

I whipped up this low-res diagram. The cam gear has 38 teeth and a new Gates belt measures 14.25" across the circumference of the gear. This is not a tensioned measurement, the cam gear circumference when using 4.5" inches as the diameter is 14.137" so I'll just call it 14.2". 14.2" / 720 (remember we're calculating degrees at the crank) is .0197" for every degree.

The equation to get crankshaft degrees retarded at the camshaft where D is the distance removed from the deck surface in inches is
(12.25 - (√((11.863-D)² + 9.339))) / .0197

This is just a simple Pythagorean theorem equation to find the change in the distance from the auxiliary shaft to the camshaft, then that distance is divided by our constant, how long is the distance on the belt for every degree.
Mathematically this isn't a linear equation but when calculating for thousandths of inches removed on such a large triangle it's moot.

.040" removed calculates to 1.98°. Remember that this is crank degrees, so the camshaft centerline will need to rotate clockwise only 1° to correct this. This is pretty close to every .020" being 1° which is half as much as I've heard mentioned many times over.
timing1.jpg
Infrequently asked questions:

Q: What about a Pinto belt?
A: If you need to run a shorter belt, that slack will be removed from the idler/tensioner side, this does not impact the distance from the crank to cam gear on the auxiliary shaft side of the belt so it doesn't impact the camshaft angle.
Q: I have an STS gear and shaved my head .020", so I advance it one tick mark for 1 degree?
A: No. Each tick mark on an STS gear is 2 crank degrees.
Q: But I degreed a cam when installed and got a different result.
A: These are quite small measurements and there's too many variables for valve events like valve lash, how worn/stretched the belt is, if the roll pin in the cam and gear are a little sloppy, if the roll pin in the cam isn't 100% at the centerline that you see it advertised at etc.
 
You are correct.

As a very vocal 0.010"/1 proponent in the past, constant measurement of this stuff has proven that this isn't true. While it isn't quite 0.020"/1, that is a much closer round number to use (it's really like 0.017ish or something). However, as you mentioned in the last "A", there's worn belts, roll pins, cam gear on cam nose fitment, etc. When I had the head milled this Spring to a total of 0.060", I checked it out in this way: using the spec for the V cam out of the green book (0.7mm valve lift is 11 BTDC or something. Don't quote me) and a BRAND NEW belt, I measured where it was actually taking place. For the 0.060", it was a little more than 4* retarded. Definitely not 5*.

I see where the folly in my thinking was previously, that the gear rotates back not only for the trailing edge of the gear, but the leading edge also. Wrong. The tensioner takes that up. Any previously executed measurements I did used a used belt, which I can't trust, and neither should you. Ultimately, if you're building an engine where you feel like it matters, you should be degreeing it in. No excuses. If you're not, and you've just whacked 0.040" for the bump in compression, you can advance it 2*, you can advance it 4*, you can leave it retarded and just run the stock gear if you don't want to spend the $100 on a pretty nice product that supports the community. I would probably still advance it 4* to accommodate for belt stretch over time, and a hair advanced is probably better than a hair retarded on a car that gets driven on the street.
 
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No need to worry about trig. It’s simple, pi times d divided by 360 gives you how far a degree is. When i decked blocks 2-3mm to build cheater engines with stock smog cams i left the timing retarded to move the power band up.
And as stated, always degree it.
I could dig out notes from a build but it’s irrelevant, really. As long as it makes power and makes you happy and doesn’t bend valves it’s all good.
 
mikep said:
No need to worry about trig. It’s simple, pi times d divided by 360 gives you how far a degree is. When i decked blocks 2-3mm to build cheater engines with stock smog cams i left the timing retarded to move the power band up.
And as stated, always degree it.
I could dig out notes from a build but it’s irrelevant, really. As long as it makes power and makes you happy and doesn’t bend valves it’s all good.
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You can also use the pitch of the belt teeth, which is 3/8”. Makes the math pretty easy, if you assume the sin error is minimal (which it is) on the belt angle from bore centerline.
 
I kept my mouth shut at first because i don’t want to discourage thinking, and because i could always be wrong. The way i look at it, no matter what, we are still making these things work better than the 117 firebreathing hp monsters of old, and that’s a good thing.
 
mikep said:
I kept my mouth shut at first because i don’t want to discourage thinking, and because i could always be wrong. The way i look at it, no matter what, we are still making these things work better than the 117 firebreathing hp monsters of old, and that’s a good thing.
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I distinctly remember degreeing in a cam on one of these for the first time in my little tiny garage in Philly, and it was off 4 degrees after having cut the head 0.040". I probably just took it as gospel, without knowing where the head deck had been before I started, or if the belt was bagged out, or whatever. I feel like I should have said something after I knew better, but at least someone did.
 
Is this reviving a zombie thread if it hasn't been six months yet? Stumbled across this and got me curious. I shaved my K-Jet head .040 and ran the stock belt for ages - putting on a new belt, should I be running a T013?
 
Bricktothefuture said:
Is this reviving a zombie thread if it hasn't been six months yet? Stumbled across this and got me curious. I shaved my K-Jet head .040 and ran the stock belt for ages - putting on a new belt, should I be running a T013?
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Keep the stock belt.

The shorter belts are when you’ve taken off ~0.090” or more.
 
The shorter belt will not go on until you've cut that much.

Who wants to help me figure out what to do when you have 1.5mm off? We need a bigger tensioner pulley. Something I could press on, keep in pleace with some Loctite...
 
shoestring said:
The shorter belt will not go on until you've cut that much.

Who wants to help me figure out what to do when you have 1.5mm off? We need a bigger tensioner pulley. Something I could press on, keep in pleace with some Loctite...
Click to expand...
1.5-2mm is straight to timing belt jail.

A slip fit sleeve with sleeve retaining compound would be real handy, I was just thinking about this the other day.
 
1.5mm (more realistically .062") on my engine. I use a pinto belt with the tensioner maxed out. I have to remove the cam gear to get the belt off. I tried a stock belt and I felt like it was at risk of being too loose.

Back to the original topic, my timing marks line up with the aforementioned math, and the cams I've tried do seem to like where I've put them, I've yet to put a degree wheel on and actually verify.
 
OVERDRIVE said:
1.5mm (more realistically .062") on my engine. I use a pinto belt with the tensioner maxed out. I have to remove the cam gear to get the belt off. I tried a stock belt and I felt like it was at risk of being too loose.

Back to the original topic, my timing marks line up with the aforementioned math, and the cams I've tried do seem to like where I've put them, I've yet to put a degree wheel on and actually verify.
Click to expand...
That's impressive. I did a 2L ford belt at 0.080" off and it was tiiiiiight with the tensioner backed off. I can see how removing the cam gear would buy you a little wiggle room.
 
shoestring said:
What's the interference fit for something like this? Do we need something more durable than say a seamless mild steel?
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Loctite sleeve and bearing retaining compound is good for a big ol gap, so you could do a nice 0.005 or so slip fit for the bond gap.

You could use DOM tubing, but in my experience starting from a tube usually means you're going to have some real hard sections and a weld seam to deal with. Going from bar-stock allows for thin walls to be made easier as well.

Random thought, is there a speedi-sleeve that would work?
 
culberro said:
Loctite sleeve and bearing retaining compound is good for a big ol gap, so you could do a nice 0.005 or so slip fit for the bond gap.

You could use DOM tubing, but in my experience starting from a tube usually means you're going to have some real hard sections and a weld seam to deal with. Going from bar-stock allows for thin walls to be made easier as well.

Random thought, is there a speedi-sleeve that would work?
Click to expand...
I can't say I've ever seen one as thick as what I think we need for this.

I'm gonna get on this when I get home.

On a side note, man you need a lot of fiber as you get older, just for maintenance. Maybe move this to TBOT.
 
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