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emoore924 · Joined: 13 Apr 2004 Posts: 2822

Ok, I was setting up a cam today and I had something weird happen.

We were checking valve clearances and found an intake would fit a .006 feeler gauge but not a .007 (target: .004). Upon looking at the adjusting screw, we found that the one-notch screw was at the end of the adjustment, so we opted to move up to a two-notch.

(for the uninitiated, more notches = .005 thicker per notch).

So we put in a two-notch, put back together, checked clearances..., .011 fits, .012 doesn't...Whut th- That ain't right.

Um, so mebbe I measured wrong. So we need to move about .0075 now, so let's put in a 4-notch and we'll end up at the beginning of the adjustment. Change screws, recheck clearances..., .018 fits, .019 doesn't. HEY!!

So, I start checking the screws with the caliper.

Bottom line: THEY WERE WAAAAAYYYY OFF. The 4-notch was 5.90. A 2-notch was 6.15. Found a 3-notch that was 6.17.

So, IDK what was up, but given the age of these cars, you don't know what you're going to get. My advice would be to measure your screws before you use them. They may not be what they seem to me.

Rich H · Posted: Mon Nov 23, 2009 6:47 pm Post subject:

???

The more notches are thinner/smaller screws - the valves recess into the head as they wear so the screws need to be thinner as the gaps close up. You need a blank/zero notch screw.
_________________
1994 Lotus Esprit S4 - Work in progress...
1980 Porsche 924 S2 DITC Turbo - Original spec
1978 Homo-Sapiens - Tired spec
1953 Landrover S1 - Pensioner Spec

RC · Joined: 25 Mar 2007 Posts: 2637 Location: Australia

There is quite a variation between screws in the same group, no notch, 1 notch, and so on. The number of notches places them within a defined upper and lower tolerance group. Occasionally 2 screws in adjacent groups will measure equal, eg. a large 1 notch and a small 2 notch.

Recall there is a similar thread by Mike924 who gave measurements in a chart.
http://www.924board.org/viewtopic.php?p=197655#197655

It is difficult to measure these screws accurately, especially at the small end of the taper. As there is no standard here there will be much variation. Personally I measure the width at the thicker end with the flat face level on one jaw of the calipers.

emoore924 · Joined: 13 Apr 2004 Posts: 2822

Just to clarify, I'm measuring the screws in the middle, give or take a thread or two. I've gotten pretty accurate actually. I know about tolerance groups and all that.

Additionally, I was only changing the screw, not the bucket or the cam or anything else so the variation (increasing gap) was wholly attributable to the change in screw, despite increasing the number of notches -- hence my initial confusion.

My point is that sometime during their long an troubled life, someone has *ground* the screw and made it much thinner than the marking would indicate -- no a wholly outlandish scenario, especially if you don't have the right screw. But definately NOT what the number of marks indicates.

I'm sharing this so that those of you who might have less experience with such things might take advantage of my misfortune and not make the same mistake.

So the bottom line is no matter how many marks, measure the screw if you don't want to install a cam 4 times, unless you like doing that kind of thing of course.