mark kibort

as some of you know, my TB jumped 3 teeth at the speed gt race. we re-timed it, tensioned it up and ran the race. i later replaced the belt, not the tensioner arm shoulder bolt ( i think thats what you guys are calling it) FORTUNATELY, it snapped off upon assembly. so, it forced me to buy a new one and install it. one thing, it didnt have the washer on the end, that the drawings show, but it did work and low and behold, my belt doesnt wonder to the edge of the passenger side pulley any more. (plus, i have a new sholder bolt. god only knows how important that is!!!!!! that thing bends or breaks, and you are , well as greg says..... toast!)

dont know what caused the jump, as it was tensioned properly, but we had been beating on the engine pretty hard that season, and the belt was 3 seasons old. also, i lost the tension sensor wire, that may have got caught in the gears and caused the mishap. i dont know, but i replaced that as well, new belt and timed it correctly. (it was suspect that i was off 1 tooth from the beginning anyway) plus, the tensioner was bone dry too.

i drilled a hole in the bottom of the passenger side timing cover to be able to push a small metal bar up and check tension. kind of crude, but i made some marks on the bar to be able to verify tension without removing the covers.

Question, does anyone have a washer on the end of that sholder bolt that attaches the tensioner arm to the water pump? it effectively tightens up the assembly to the block.

watch that bolt, its a 17mm hex head, but the bolt is really like a 12-13mm in strength and shape.

mk

deliriousga

It never ceases to amaze me to see what POs' mechanics do to these fine machines. Can't you see just see it? "Hey Joe-Bob, I just finished this timin' belt thingie and there's this washer. Wonder what it's for." "I don't know Billie-Bob, but it's just a washer, can't be that important."
Mine has a washer with a matching countersunk bolt on the end that holds the idler arm assembly on nice and tight. Replaced those too because the PO's mechanic destroyed the allen head and I destroyed the washer trying to get the darn thing off.

GregBBRD

All of these posts are a bit confusing.....only because all of the cars are different model years.

heinrich

Worse ... far worse. I almost bought a gorgeous riviera blue S4 in Georgia (IIRC) and the engine turned out to have been JB Welded shut after the crankcase had been ruptured in bottoming out. Dodged a bullet there. One of my cars had had a screwdriver banged through one cam gear "spider" to keep it from moving. One car had the heater completely bypassed head to head.

GregBBRD

Oops, sorry. Newbie pushed the wrong button. Didn't get to say what I wanted to.

In 1987, the waterpump, the idler arm, the idler bearing, and the pivot for the idler arm all changed. Prior to 1987, the pivot for the idler arm was essentially a sholdered bolt that threaded into the water pump. This has the 17mm head with the 8mm threads and frequently gets overtightened or can break off. In 1987, Porsche changed this to a rather solid pivot stud that screws into the water pump and everything goes over the top of this stud. In early 1987, Porsche used a thick countersunk washer with a countersunk screw to keep the idler arm in place. This proved to not be strong enough and they quickly changed the washer to a "strap" (using the same countersunk screw) that bolts to the water pump that adds support to the end of the "stud". You should change your washer over to the strap style. This adds significant strength to the stud. Your new water pump should already be drilled and tapped for this strap. You will probably need to modify the cover to use the new strap and will need 1 additional 6mm bolt for the strap.

Jim bailey - 928 International

Kibort's preproduction S-4 prototype has an 86 style water pump shoulder bolt arrangement among other unique bits and pieces.

deliriousga

Got all of the covers off and tracked the belt with everything exposed. It did the same thing as before moving over a little on both cams for a short time. I've checked all of the gears and waterpump pulley and none of them have play in them and wear is minimal. The oil pump and crank gears show no wear at all and the cam gears show tiny spots of where the aluminum is exposed. I'll replace those tonight just for my future comfort.

After tracking it, I pulled the belt off and put it on a flat surface to see if it actually "bumps" up in one area. Flat as can be, no deflection in the belt itself.

I took the Idler Arm assembly off to see if I messed something up when I put it on. That's when I believe the problem showed up. When I started to take the 3mm allen screw out of the end of the Idler Arm Pivot (Shoulder) Bolt, the bolt wiggled a tiny bit and the whole bolt unscrewed from the water pump. Looks to me like it loosened a little or in one of those great moments in life (probably about 2:00am) when I forgot to tighten it down completely. Another thing I noticed was both of the new rollers on the idler arm have a tiny bit of back-and-forth play in the bearings. I'm guessing this is normal since they are both brand new, but just want to make sure.

So, do you think this (the possible loose bolt) would cause the odd tracking with the belt moving over a little here and there? It's the only thing I can find that was not normal so I'm at a loss if this isn't it.

Thanks!

Garth S

At this point, check the male and female threads of the stud and pump body. Assuming they are fine, I'd be severely tempted to torque the pivot down to 20 N.m with a drop of blue locktite ( please verify for me the thread size - M8?) - and reassemble everything: save all the other parts for the next time. Restring the same belt on its index points, retension - and verify the tracking.
As you said earlier, the belt apparently had one "bump" per belt cycle. As one belt cycle is 222 teeth, it is out of phase with any of the drive gears (multiples of 24); therefore, in the remote possibility the bump reappears with the same belt, chalk mark it - and start counting .... we will then know.
However, I suspect that all the previously installed parts are fine - and ready to go.

heinrich

absolutely

GregBBRD

Install your new factory belt. Throw the aftermarket belt in the trash can. If you have the 3mm screw in the end of your idler arm support, the threads in the water pump are larger than 8mm. I do not have the spec. book here, but I recall the torque being greater than 20nm for this style stud.

greg brown

Garth S

John,
Thanks for the confirmation of 10mm threads - I'll cross link to this thread for future archive use ... and my memory .. ).
The max . torque on generic M10 is 40N.M/29 ft.lb +15%

Quote:
Originally Posted by Garth S
"Check the thread size - it may have been bumped to 10mm, but 8mm is my recollection."

Originally posted by deliriousga
"It's 10mm at the threads. Put it back in this morning and torqued it to 20ft lbs.

Thanks for the info guys! "
___________________
response by John Schilling

deliriousga

I'll tighten it up tonight before putting everything back together. I replaced the cam gears and put the belt on last night and tensioned it. I'll pull the belt off, tighten the idler arm bolt, re-install the belt, tension it, track it several times then put everything back together. Should be complete by midnight.

mark kibort

Hey Greg,

as Jim said, i do have the 86 style boss on the waterpump. what i was talking about, and i may still be confused, but besides the thick washer that fits in the tensioner roller assembly, there is a think washer just beneath the 17mm bolt head. its on the microfishe, but there was not one there before and my belt was walking forward on the passenger cam pulley before. it looks as though that washer makes the tensioner roller, more parellel to the system. since my old bolt broke and i used a new one, maybe before it was bent a little. however, it did have a little click of movement that would have been taken up by a think washer. anyway, the belt is not walking forward now, 1000miles later and ready for its first race.

Mk

GregBBRD

Mark:

All of those parts make sense. That thick washer actually holds the small roller in place and locates the idler arm at the correct distance, so that the tensioner roller is in the correct position. I've seen a few cars that have that washer missing, too. Can't figure out what people think when they have a big special washer left over after they complete a repair! Must be an extra??? Sounds like you have yours well under control and have installed all of the needed parts. Good catch.

greg

mark kibort

thanks. some of these jobs SHOULD be left in the hands of the pros!

by the way, i think that little small washer at the base of the 17mm bolt head would make things tighter, but the original didnt have it nor, did other TBs tensioners ive done. is it like a shim, where its a judgment call?

I cant believe that story of the thick special washer off the roller being missing. wonder how that didnt just explode everything!

I left you a PM (email) when you get a chance. got a quick question for you on another subject

thanks,

Mk