What do I do about this (idler arm)?

safe · safe's Garage

I am currently looking over my cam chains and tensioners, (including chaninging my chains, different post.) and have now found this.

Is the idler arm were too much?

And what about the shaft its mounted on?? I'm guess the idler shoud not where on the shaft so it must be from corrosion, or what?

john walker's workshop · 03-20-2005, 12:57 PM

binding there causes most tensioner failures. the old, no bushing arm is the most prone to binding. the late style wider and bushed arm is recommended. i would have to actually see and feel the shaft to decide if the housing should be replaced. possibly with the later arm, you could get away with it unless there's a big notch.

Jeff Alton · 03-20-2005, 02:52 PM

Careful of your word choice there, someone might get the wrong idea!!

Jeff

safe · safe's Garage

John: I cleaned it up abit with a fine sand paper. I took away most discoloration (spelling?) but you definetly feel it with yor fingers. Im no expert but to me it feels bad...

Can you replace the shaft and not the whole housing? I think I remember a tread about that, cant find it.... Anyone remember?

safe · safe's Garage

Found the thread!
boogered chain box shafts

john walker's workshop · 03-21-2005, 07:49 AM

you could , but the housings are fairly common. a later aluminum one will work also.

safe · safe's Garage

Im replacing them I think, cheeper and faster.
How much play should there be between the shaft and idler arm? Mine where a little loose. Should it be like that?

hcoles · 03-29-2005, 05:11 AM

My 2 cents. When I saw the wear on my arm bushings that are the updated type, it made me feel like for sure you don't want the old "short coupled" type. It seems there is quite a bit of torque with the chain pushing on one side and the tensioner pushing on the other. Not a great design in my opinion.
-h

safe · safe's Garage

My thought was the other way around, with that amount of force from the tensioner, no way the arm could bind. But I'm probably wrong... Dont't mind me

Tim Hancock · 03-31-2005, 04:09 AM

Safe, if you have machining equipment available you can install a bronze bushing in your old arm then ream to size.

If you install your screwed up arms on new shafts, You will probably eventually gall up your shafts. Best bet would be to install new style idler arms (which have bronze inserts).

When I redid mine (see my earlier referenced post), I had no discernable play in the shafts and bronze bushed arms. Just a perfect slip fit.

Obviously you need to have decent machining equipment to maintain squareness when boring/reaming idler arms. I am all for DIY type work, but if you do not have the proper equipment, it is not worth the risk to a relatively high value 911 engine.

Good luck!

hcoles · 03-31-2005, 06:58 AM

Tim,
Wayne's book lists 16-45 microns as the clearance range. I assume this is a diameter difference. At this point after I opened one of the bores up to 15.00 I did get a nice feeling slip fit with no noticable play.

I think I'm still not up to 16-45. Is this the clearance you measure?

-Henry

Tim Hancock · 03-31-2005, 07:15 AM

Henry, if I remember right, .001" = 25 microns. Therefore the clearance that was mentioned = .0006"-.0018". I ran a 15.0 mm reamer thru my bushings which ended up fitting nicely over my 15mm ground tool steel shafting. The reamer probably cut .0005-.0008" over size. I did not measure after reaming, but it was a nice fit. I also used a die grinder to make a slight oil groove in the id of the bushing to help ensure lubrication.

I have about 2000 miles on the rebuild so far and all is good.

hcoles · 03-31-2005, 08:22 AM

Tim,
Yes 25 microns is 0.001" I like very much your idea of cutting an additional oil path. The stock design just has one hole in the middle of the two bushings in each arm. Can you please describe a bit more where you put the grooves? I want to consider this I'll see what I can do with hand tools or talk to my machinist Ted Robinson and see what he thinks. Again very good idea.
-Henry

Tim Hancock · 03-31-2005, 09:20 AM

I just ground a slight groove/channel starting at the edge of the hole (on the inside of the bushing surface obviously) and went along about 90 degrees along the inner surface. I did this in both directions. My goal was to ensure decent pentration of oil aroiund the shaft. I do not know if it will help (or is even needed) now that I have new shafts and bronze bushings. It cant hurt.

My old shafts were severly galled and one tensioner was actually siezed to the shaft. The car I bought had sat for long periods in it's life. My guess is that over long periods of time,the oil runs off the shaft and condensation corrodes the shaft/arm. At startup the corrosion abrades the shaft and arm. After enough of these cycles the shaft galls and tensioner fails.

hcoles · 03-31-2005, 04:29 PM

Tim,
Thanks...not sure I completely invision the cuts in the bushings. You cut in and around in a spiral fashion making a total of 4 cuts per arm? I assume we don't want to cut where the tension is on the shaft but just near that area so oil can migrate hopefully. Maybe you a send a jpg or ?
I like this idea because I see my bushings had fretting, they were too tight and jwworkshop says this is the cause of a lot of failures which is starting to make sense considering no one is putting forth other tensioner failure modes.
-Henry

Wayne 962 · Wayne 962's Garage

This is absolutely one of the precursors to tensioner failure. The idler arm is very, very worn, and so is the shaft. I would definitely upgrade to the later-style arms without even thinking one more minute about it. In fact, even if there were no wear marks, I would recommend the upgrade...

-Wayne

Tim Hancock · 04-01-2005, 06:42 AM

I do not have pictures of my oil grooves, but just think of providing a couple of small grooves eminating from the oil hole to help oil get farther around the shaft.

I do not think this is neccesary, it is just something I decided would be a possible improvement. The bronze bearing alone will probably preclude any galling issues in the future.

Just for fun, your choices in my OPINION are:

Best choice: Buy new car
2nd best choice: Buy new engine
3rd best choice: buy new chaincase and new style arms
4th best choice: replace shafts and install new style arms (my favorite)
5th best choice: replace shafts and install bushings in old style arms
6th best choice: re-assemble and sell car on E-bay

I probably missed a few in between #5 and #6 but anything in between would probably not fall under the heading of "best choice".

Life is full of compromise based on skill level, finances, equipment, etc.
Good luck

Wayne 962 · Wayne 962's Garage

Installing bushings in the old arms isn't going to help the core problem, which is that the bearing surface is not wide enough. The Porsche engineers figured this out, hence the longer arms. I wouldn't doubt them...

-Wayne

hcoles · 04-08-2005, 05:54 AM

Tim,
I'm now doing a closer inspection of the shafts and arms. If I replace the arm bushings and then have a machine shop bore/hone to size exactly what type of bushing material should I use. I'm not confident that the Porsche bushing is the best considering it was specified maybe 17 years ago or more. The shaft feels smooth but the bushing in the arm has a definite wear spot and if this is buffed out I may be getting close to the upper end of the clearance spec. While I'm at it and can figure out what the good bushing material is I'll probably rebush both sides. Thanks, in advance, for your help. BTW - my arms are the "new" type wide with bushings.
-Henry

Tim Hancock · 04-08-2005, 10:16 AM

Henry,

I do not know what type of bronze is stock, I therefore cannot recommend a "better type".

I used 841 as I had a piece in hand.

841 bronze is the type many off the shelf bronze bearings are made of (sintered, oil impregnated). 932 & 936 all also considered useful for bearing applications (about $10.00 for a foot long chunk). Others may be suitable also. Mcmaster Carr sells these types in solid and hollow rounds.

If you have to pay someone to do the machining, you might want to just buy new ones. I luckily have a lathe at home and access to a machine shop at my day job.

Wayne, I do not doubt that the newer wider design is better than the old narrow design without bushing and I understand the offset load that the wider arm addresses. However, in my OPINION, installing bushings in old narrow arms on GOOD shafts SHOULD give ACCEPTABLE life. As a mechanical engineer with a not so large income, yet lot's of expensive hobbies, I often find myself looking for ways to save cash in exchange for my shop time. If this is a sin then I am guilty as charged.

(By the way, I also admit to re-ringing my Alusils instead of buying new P&C's. 2500 miles thus far w/ no smoking!)