What You’ve Always Wanted to Know About Shaved Heads,
Cam Timing & Adjustable Cam Gears, But Were Afraid to Ask
I decided to write this article in collaboration with Rod (Zuffen) to help enlighten fellow hotrodders on this subject, which is probably as critical to making power out of your DOHC engine as the black art of head porting. At least this subject is fairly straightforward and not too difficult to understand, whereas to really understand head porting you probably need a PhD in fluid dynamics and have access to a supercomputer.
So what do shaved heads, camshaft timing and adjustable cam gears have in common? Plenty, but first some background.
When the OEM’s design and build an OHC engine, they have to connect the cams on the top of the motor with the crankshaft at the bottom of the motor so the crankshaft can spin the cams at exactly half the speed of the engine, and the cams will open & close the valves at very precisely determined intervals.
The method of connecting the cams and crankshaft is the timing belt or timing chain, and once the distance is established and fixed between the centerline of the crankshaft and the cams, the position of the timing marks on the cam covers can also be fixed. (This is part of the design bit that we hotrodders don’t get to see)
Once this geometrical relationship is fixed, and the distance doesn’t change between the cams and crank, the timing marks will always be aligned and we can be sure that the valve events will always occur when they’re supposed to. For sake of illustration, let’s refer to this distance in terms of timing belt teeth, and say there’s 100 teeth between the timing mark on the cam and the timing mark on the crank of our engine.
And let’s mark the belt tooth 0 on the crank, when the #1 piston is at TDC. Let’s also mark the position of belt tooth 100 on the cam when the #1 piston is at TDC.
Now, let’s assume the distance between the crank and cams changes. What do you think will happen to the engine timing when that distance is shortened, and now there are only 99 1/2 teeth between the cams and the crank? No problem you say, the timing belt tensioner will take up the slack. And this is what most people would think also, but it’s wrong. Now, with this shortened distance, all the events that the crankshaft expects to happen with the valves when it’s on the 0 tooth of the belt, will not have happened yet, because the cam side is on the 99 1/2 tooth, not the 100th. In other words, the cam events have become retarded (or late) by half a tooth.
So what can change this cam to crank distance? Head shaving or resurfacing, as well as block decking will achieve this result.
As a part of every high performance rebuild any good engine builder is going to “true up” or resurface the heads and block of the engine, in order to ensure “his” build has the very best flat sealing surfaces possible. Let’s take another hypothetical example to see what happens when he does this. Let’s say the builder takes off 0.005” from each head, and from each side of the block.
That’s 0.010” from each bank, or 0.020” total, right? Now does that mean the timing belt has become 0.020” too long? No, it has become TWICE that, or 0.040” too long. Why? Refer to the diagram below, and notice what happens if a cam timing pulley is moved inward toward the crank by 1mm. Is the length of the belt affected by only 1mm? No, the effect is double, because the belt passes on both sides of the pulley, and has 180 degrees of wrap, or more.
Now that we’ve seen how taking off 0.020” worth of material has translated into a timing belt that is 0.040” too long, let’s translate this into timing error. We know that the upper pulleys have 48 teeth and the pitch of these teeth is 8mm. Therefore the “pitch” circumference of the pulley is 48 x 8 = 384mm. Now if we have an error of 0.040”, this is 1.02mm, or 1.02 x 360/384 = 0.96 degrees at the camshafts, or twice that at the crankshaft.
So simply by skimming 0.005” off each head & block surface in order to get a flat and near perfect sealing surface, we have just introduced 2 degrees worth of valve timing error into the engine. This is why some OEM’s do not recommend resurfacing their DOHC heads or blocks at all.
OK, so now all our timing events are 2 degrees off; what can we do about it? There are multiple solutions. First, the material that was taken off the heads or block can be restored, in the form of a thicker head gasket. This could also be a good way of dropping, or restoring the original compression, if the motor is being built for turbo or supercharged duty. However, it could also upset the delicate squish relationship, if one isn’t careful….. Another way is to re-establish the original timing sequence with adjustable timing gears on the cams.
This has the effect of allowing you to realign the timing marks and timing events based on the new, shorter geometry of the engine. And yet another way is to go to the cam manufacturer, and ask them to grind the cams with more advance, to compensate for the retardation introduced by the resurfacing. However, neither of these last two methods will account for the shortened distance between the cam sprockets and the crank, and the resulting timing belt looseness. Hopefully the timing belt tensioner can take up this slack, but if not, then the tensioner must be shimmed, or a shorter belt considered.
Now the Really Smart Guys amongst us will say “Hang on a minute. If I align my cam sprocket marks with the static marks on top, and my crank timing marks on bottom and I fit the timing belt on, and it’s good & tight, then the timing can’t be off.” And you’d be right, but only if the loss of material has resulted in an error of exactly 1 full tooth or multiples of that. If the loss of material results in a fraction of a tooth, then somethinghas to shift ½ a tooth for the belt to go on properly. And a ½ tooth shift is 0.5/48*360 = 3.75 degrees error, at the cams, or 7.5 degrees at the crank. Scary.
So there you have it, the relationships between head shaving, block decking and cam timing. The key concepts that should be remembered are:
Resurfacing takes away material, shortens the distance between the cams and the crankshaft, and retards the cam timing. Retarded cam timing actually “retards” or delays the peak power of the engine and shifts it to a higher RPM, which can actually be a good thing, but at a loss to low RPM power.
Adding material, and restoring the distance between the cams and the crank will advance the cam timing, and will “advance” the peak power back into a lower RPM area.
In conclusion, after you’ve plunked down good money for a high performance rebuild, and a new set of cams, don’t be surprised if those cams don’t “degree in” as expected.
Secondly, after a high performance rebuild, if you find that you have an unusually loose timing belt, you need to pin the builder down on exactly how much material he had to take off the heads and block, as you’re likely in for a difficult time with your cam timing.
Bottom line, a good set of adjustable timing gears are a wise investment after an engine rebuild, and may be the only solution to get the top & bottom of the motor working together again.
Authors’ Note: Most of the information you’ve just read is not original. All this is available in bits and pieces from engine building manuals and people who are genuine engine gurus. However, no sources were able to provide a clear, concise explanation about why resurfacing heads and blocks retards the cam timing. So after a lot of thought, and a lot of pictures and diagrams, it’s now presented in, what we believe to be, a clear & concise manner. We hope you “get” it from our explanation, and are now able to use it for your own benefit.
John Cribb (Cribbj) and Rod Garnett (Zuffen)