mark kibort

WOW, ill have to respond to all the posts with this message.

First, i have a lot of experience with the 9201. it is hard, but once you master it, its a dead on way to measure the tension.
Now, i have nothing against the Kempf tool, but what im saying is that if you have one or two teeth closer to the pulley, its common sense to understand that the tension will read higher. In fact, one tooth can be in the gap (for those of you that know the Kempf tool) to out of the full range of the gap. SO, thats why im questioning the instruction pictures. Im sure if its calabrated for what looks like the 2 valve covers, then it needs to have a different calabration mark for the S4. it is a big deal. the one thing ALL our cars have in common when it comes to the tensioning of the belt, is that we all use the smog pump mounting bracket arm as a reference. AGREED? that said, the 9201 fits the exact tooth it needs to in setting the tension. this is one of the reasons to get to exactly TDC. at TDC , along with certain, repeatable tension of the cams acting on the belt, the position of the belt teeth in the tension checker is known.

So, my point is, the covers of my car are closer to the cam pulleys than the picture shown. MEANING that if im closer, the tension will be much greater. may as well just do the old, twist to 45degrees by hand with some effort and call it a day. Im sure that is good enough in most cases, but we all got the tool or use a tool to not have a mishap. Being one or two teeth off has a big change in the tension setting.

the good news is no matter what is the final outcome of this discussion, by using the 9201 and then calabrating (marking the tool) to the exact settings, i will be able to use the kempf tool confidently in the future.

Point is, there are differences in where my center cover ends up, which is up to 2 teeth off the instruction picture locations. and this is the main point here.

mk

mark kibort

well, understanding about chain sprocket forces allows me to sleep at night. talked to a sprocket engineer last year when all of this what happening) with 20 teeth, and one missing, the average increase of force on the adjacent teeth is about 12%. Proof in the pudding, ive raced a full season this year, and none of the teeth have broke next to the missing teeth. so, now i have 3 of 4 cams with 1 tooth missing. sure, i dont feel good about it, but my belt did slip in the last race, and why??? could have been tension, could have been belt wear, could have been that broken tension sensor strip that fell off an could have got caught in the belt and pulley. who knows. ( i wish someone did)

Mk

mark kibort

I know, the tool is fine, and a very cool idea!! the point is the position is critical for tension. one or two teeth is way out of the range of that window. (performed that test last night as you go one more tooth closer to the cam pulley, the tension goes from in the spec window to almost the full window out of it!)

Yeah, im a little sensitive about this belt stuff, knowing how many of you guys have lost motors. i havent , and dont want to be part of that club!! lack of maintenance? how is that? ive tensioned the belt every season, and the belt looks pretty good. however, it is rubber and rubber wears and gets soft, and maybe it was too old for racing 3 full seasons! (along with 10k street miles).
However, i do wish i would have tore into the center cover and changed that sensor strip when it broke. that could have caused the slip.

Im not convinced that the missing oil from the tensioner would be a cause, but if the oil is used for damping. (i would think most of the spring vibration is in the spring between the plunger and the piston that contacts the tensioner arm)
the oil i was told is mainly for heat transfer, so at worst, with no oil, the tension would be too tight under high heat ) do those metal discs absorb force like a spring too?? anyway, new oil, new boot, new sensor wire and we are off to the races!!
MK




Mark your belt slipped for one reason: lack of maintenance.

heinrich

Mark though your engine is not my business and I respectfully admit that ...... the info we recently gleaned from our 968 brethern is that the reason for cog tooth destruction, is worn timing chains. So ..... had your engineer alerted you of the **reason** for the breaking teeth, as opposed to the **physics of cogs**, then you would likely have managed to lose one tooth and never again ... all for the price of a timing chain and cam cover gasket set.

At this point I believe you may need new timing chains. All besides the point for this discussion ... on to the tensioner:

The tensioner MUST have oil. Oil lubes it. You have some very moveable parts in there, and they all work against each other. There is no guide for the plunger, except the inside teflon-covered walls of the tensioner. The absence of oil there .... not a good thing. Binding of the washers on the tensioner cylinder may (I theorise) cause slack, harder impact and shorter belt life (alongside more general strain on all components).

Now .... You recently discovered that your cam timing on one bank was off .....

This in my mind means that one cam was working against the flow in the universe of your timing drivetrain. Hence the worn cam chain, hence the broken teeth.......

And the cause? Could it be that dry tensioner?

Mark Anderson

Put down the Met wrench and step away from the car.

JE928Sx4.

I stand corrected.

Tony

The oils main purpose is to transfer the heat from the block to the tensioner washers. The transfered heat changes the over all length and thus the tension on the belt a bit. My guess is it is more crucial during start up/warm up as the heat will evantaully transfer to the washers anyway but not nearly as quick if they had oil surrounding them.

Gretch

Correct, and a slower heat transfer rate results in over-tension of the belt, not slack.......for the extended heat soak period of time...then tension should be back to the same level it was with the block cold.......

Steve J.

I know it seems like it would make a big difference but I found in actual practice that a tooth or two either way makes little difference in the reading. Like Ernie says, stay away from the ends. BTW - My Kempf tool and my 9201 do not agree. I have the calibration bar for the 9201. Which should I believe? The 9201 reads actual belt tension, but the K-tool measures the distance to the plastic TB backing piece. Is that distance really the same on everyone's car? I can change the reading by half the K tool's window by pushing harder against the plastic after the tool makes contact.

Z

POS tool = Porsche Official Service tool?

mark kibort

EXCELLENT POINT Steve!

never thoughtof that. so, if your belt is riding on the edge of the cam pulley, it will have to twist farter for the KP tool to meet the plastic sheet against the block. this brings me to my next concern, i dont dont have a plastic backing plate either, is that normal for the S4 or 1985-86???? i know my old S '84 did.

those two components sure do give a lot of slop for the reading.

the fact that some have used the 9201 and had them disagree, worries me too.
there is a calabration tool for the 9201?? now, im even more worried. I know this tool im borrowing from CT automotive is over 15 years old. (probably was used to adjust my very first timing belt back in 1987.

Mk

mark kibort

Hey, i respect your and the rest of the groups opinion. I have a thick skin, so dont worry about it. I also apreciate the time you spend responding back on this and other issus!!

Now, as far as causes. you may be on to something. hard to believe the chains are worn, but im not a chain expert and your right, the engineer didnt really address causes. we were all under the assumption that maybe these cam sprockets had some metalurgy flaw and now the strongest survived. ??? wishful thinking ?? probably !

isnt the plunger different on the 85s vs the 2 valve tensioner. as i look at the parts, there is nothing that really moves unless the plates expand or contract to change the tension. looks like the little spring between the tensioner and the final plunger that fits in to the roller arm, are not in the oil mix. so, i wonder where the issue is with a dry tensioner? anyway, thats fixed now and hope that solves some of the issues.

what about that tensioner roller arm sensor strip? could that have got caught up in the cam pulley and forced a cam belt 3 teeth jump?

Thanks,

MK

Garth S

Mark,
Understand what you are trying to do - interested to see your correlations between the two tools! On that note, try the archives - there was a very extensive correlation reported by, IIRC, the Scandinavian contingent.
Re. the pic from my earlier post and Heinrichs comment

" The tensioner MUST have oil. Oil lubes it. You have some very moveable parts in there, and they all work against each other. There is no guide for the plunger, except the inside teflon-covered walls of the tensioner. The absence of oil there .... not a good thing. Binding of the washers on the tensioner cylinder may (I theorise) cause slack, harder impact and shorter belt life (alongside more general strain on all components)."

Close, but not entirely correct - the 35 belleville washers are 'along for the ride', being captive on the rod from the top piston which is centered in the well of the lower piston - see the copper coloured stack between the two pistons in the center image. The 4mm deep well in the lower piston ( with the O-ring and check valve) allows enough slack under compression, or belt tension, to prevent the centering rod from bottoming out. This leaves the 7 opposed groups of 5 washers each captured by the rod to act as a powerful spring that absorbs and dissipates the harmonic energy of the TB into the oil, tensioner walls - and so on ...

As far as your speculation on
"what about that tensioner roller arm sensor strip? could that have got caught up in the cam pulley and forced a cam belt 3 teeth jump?"
I believe that to be entirely possible in light of the following , but true story: Several months ago, a field mouse decided to stock up acorns, etc., and nest under the manifold of the V6 of my Toyota T100. One fine day, I started the truck - and the engine began 'pinging' madly, as if it was way out of time: it was!
The mouse made its escape into the cam cover, and took a ride around with the TB - on the wrong side!!! The right cam jumped 4-5 teeth and the engine ran like crap - a piece of metal could likely the same if it were long enough to cover ~1/4 of the gear teeth..... a mouse gets pretty long - for an instant

mark kibort

Funny!! thats what i think happened, but i dont think mine was as messy as below!

by the way, comparing the two tensioners, it looks like the S4 verson has that extra spring between the tensioner and the part that engages the roller arm. the earlier version has no spring, but its washers are spaced and maybe that acts as the spring force. dont know, but what i do think is the oil has little function in the force compressing the tensioner roller arm even under harmonic vibrations. if that was the purpose, they wouldnt have had that spring on the S4 model . its a pretty light spring and allows for a certain amount of floating of the tensioner roller arm.

by the way, i also notices that the tensioner arm has a bolt and a big thick washer for attaching to the water pump rise boss. however, how much play should there be there? im thinking that there needs to be a washer on that bolt, but im sure it needs to be thin as the tensioner arm needs to float a bit too. On the PET, it shows a washer, but on most 928s ive worked on , i havent seen it. this may be one of the reasons my belt is walking to the outside of the cam pulley! I know the arm has removeable plastic bushings, but ive been told that mine are fine. (in othewords, when bolted together, the roller can be moved side to side slightly, almost like a loose wheel bearing click)

mk

Vilhuer

Mark, interesting that you get different reading from two separate teeths. I would expect this to happen when teeth are close to sprockets but not in middle. Our findinds support fact that it doesn't make significant difference as long as measurement is done about middle point between cam sprocket and tensioner roller.

https://rennlist.com/forums/showthre...=127171&page=3

I believe at least S4 and newer engines have cast aluminium (not plastic) piece between belt and head/block. This should leave out all uncertainty about how hard Kempf tool is pushed. In 16V engines we have done we did twist tool only until it barely touched plastic.

If you have used uncalibrated factory tool I wouldn't trust those results one bit. Maybe this is just me being overly worried about engine health but I would check belt tension, tensioner oil level, cam chain condition etc many times over racing season, not just once a year or less. You've been very lucky so far. Change the cam chains please.