993 Cam timing variances...what is the preload?

[Mike Juzenas]

Well my rebuild is up to the cam timing now...everything has gone together nice and neat so far. But now I have a dilemma. If I had the factory tools it would not be a problem...its how much to preload the chain ramps to get the timing right?

I have made up a tool out of an old brake piston tool to preload the chain/ramp...works like a champ but I am not sure how much pressure to put on the ramps. I notice a significant movement on my dial guage if I adjust the tool...I can get the swing to be 0.3mm and more. Since I am after a +-0.1 mm accuracy it makes it really difficult to say how much pressure is correct.

I know the amount of chain tension is a common question and I have read all the posts from a search but this 964/993 ramp system seems to have made it much more sensitive.

Can anyone give advice on how much pressure or how long the distance should be between the ramp and the top of the tensioner holdown? If anyone has the factory tools then that can be measured....

Since the right and left sides are different lengths it also makes it difficult to judge the pressure.

The index mark on the pully is about a mile wide as well...I have been using the middle as the true spot but since the timing is on a steep part of the cam the difference between the beginning and the end of the timing mark is over 0.1mm!

I am trying to set the cams at 1.1mm (taking the average between jerry woods at 1.0mm and camgrinder at 1.2mm). ANy comments?

Oh, here is a pict of my mickeymouse chain tensioner:



Thanks in advance!

Cheers,

Mike

[ChrisBennet]

Note: I'm not an expert of anything, this is just what I do.

Through experimentation I've determined that if I increase the tension on the chain I get more consistent readings - to a point.

If I tighten the tension on the chain to a certain point I'll get pretty consistent readings. If I increase the tension past that point the readings move but they don't get any more consistent. Sooo, I increase the tension just to the point where I get readings that are consistant within a range. Then I repeat the process for the other cam using the same range.

For something like a 964 cam I adjust the tension to the point where I get consistent reading within say a .02mm range on a side.
If you have old sprockets and chains or a more agressive cam you might pick a different range. You can determine the range for your particular motor through experimentation.

That is, I rotate the motor over say 8 times, (taking 4 readings) and if the highest and the lowest are within say .02mm of each other I stop increasing tension. Example:

First try:
Sample reading= 1.35, 1.37, 1.34, 1.36 = 0.03mm spread, not tight enough

Increase tension and try again:
Sample reading: 1.38, 1.37, 1.38, 1.39 = 0.02mm spread, tight enough

Notice that the 2nd set or readings "moved" (bigger #'s) but they got more consistent.

Sorry for the rambling, I'm tired.
-Chris

[Mike Juzenas]

Thanks Chris!

I evantually used not-too-tight and not-too-loose pressure (how is that for accuracy?) and did the reading multiple times and got the same readings bang on. I then decreased the pressure and did the same...with a little variance in the readings. So I guess I followed your method.....I guess I will see when its fired up! ;-) I checked the valve/piston clearances and they are fine so its off to putting the rest together...

Thanks!

Mike

[Jeff Alton]

Fire that sucker up Mike!!!