Date: Sun, 12 Jan 1997 23:45:01 -0800 From: Mike West Subject: VTG and the Camshaft connection I need to shorten this yarn on Valve Train Geometry up some so I guess it gets put into several parts, maybe three. Yes, I could talk less. :-) Some guy says he needs more. There is considerable "monkey motion" going on in the rocker box and it's all predicated on the cam doing it's thing. VW, or at least the "Bentley" doesn't say what the stock lift or duration is so I had to turn to Bill Fishers book. Duration got covered in an earlier post and isn't something we can readily check anyway so pass on that. Duration by the way, is the length of time the valves are open to some degree. If it could be instantaneous, that would be exactly 180 degrees but since we have no "perfect", it's 224 intake, 228 exhaust. Now the lift: according to Fisher again, it's .290"(7.25mm) intake and .280"(7mm) exhaust. This is at the cam. Plus/minus tolerances. So before you slip that "bump-stick" in there you might want to check those numbers and even write them down. I have fetched one in the house so you may follow along as I check it. It's a pretty thing, brand new gear on it but I suspect it's a reground cam shaft. So we go to #1 exhaust bump. That's the one farthest from the gear. Have you noticed there are only four bumps? Ah yes, :-) the opposing cylinder thing! The #1 exhaust bump is also the #3 exhaust bump. Isn't that clever? :-) So we hold the cam shaft in the left hand and turn the bump down. The bump being on one side leaves three sides to measure the "bottom" of the lift on, so flare your calipers and slide it on there. Rock it around a little and make sure you have a tight and square fit. Get it up to the light and it reads 1.217"(30.9mm) write that down. Now turn the bump towards you so you can get the calipers over it and across the back. By the way, make sure you take this reading out in the middle of the lobe, where the follower will ride. Get it square and rock it over the bump till you're sure you have the high reading. It says 1.450"(36.82mm) write that down and subtract the other number from it. So 1.45 minus 1.217 is .233"(5.82mm). This being a long damn way from the .280 we were looking for, yes? That's why I think it's a "regrind". The one in my case out in the shop does say .280, in fact it says .283. We're talking .050"(1.25mm). Do you want that in your "small wonder"? Now we go on to the #1/#3 intake just behind it. Reads 1.225"(31.11mm) Over the bump: it reads 1.507" (38.27mm). Second number minus first number is .282" (.7mm). Not exactly the .290 we should have but close enough. The new one out in my case reads .295 average. These engines, when you boil it all down are "self powered air compressors". If you can't get it out, you can't take it in. Or vice versa. This cam shaft would be referred to as a "mother-in-law". Something you'd slip in your mother-in-laws car. It's going to run alright but no matter what you do, it's not goiing to have any top end. I don't know if it would run hot or not. I think maybe so. Getting back to the real numbers, .290 and .280, you're only started with those so put them in your safe for the time being. You're putting the case together right now. Those two numbers go up thru the followers(lifters), solid or hydraulic, and push on one end of the rocker arm. The stock rocker is 1.1 times as long on the valve stem side as it is on the pushrod side so you have a 1.1:1 increase in movement. So, if you have .290/.280 lift on the cam, you have that times 1.1 or .319/.308 lift on the valves. If you have after market "ratio rockers" you multiply by that number. Now as you've heard recently in another post the rocker is traveling in an arc as it pushes down on the valve stem. The center of the adjusting screw slides from the side away from the rocker shaft on the valve stem head to the side closest to the rocker shaft. To visualize this just reach out, arm fully extended in front of you, palm down. Now let your hand drop down towards the ground. See how your hand gets closer to your body? Samo samo on the rocker. Now the reason we need those numbers is that there is more monkey motion going on than just equal wear on the valve stem head. The adjuster is supposed to be in the center of the valve stem when the valve stem and rocker are halfway thru their travel. Example: using an even number of .320 for lift on the valve, you set the valve at halfway down, or .160 and at that point, the adjuster screw should rest right in the center of the valve stem in the up and down direction. That's the direction the stem travels as it slides across the stem. Always more to the story: As valves wear in or seats are reground, the stem rises higher in the rocker box. Even if VW did give you a perfect setup at the factory, it isn't there any more and you have to check it and "Shim" the Rocker Assembly, "under" the hold-down posts to get it to where it does meet that center again. When I speak of the "hold-down posts", I'm talking about the post that the stud in the rocker box goes thru to hold the rocker assy in place. We'll get into the rocker box and the rocker shaft setup in another post. I'm still looking for some soft springs to do the real setup in the box with. Even tho I've given you some numbers on lift that will allow you to set up the rocker assy, I think you see from the one I just measured, that there might be other reasons to "dab a calip" on your bump stick. west yeah, I guess we're still pushing back the dark. :-)