901 rebuild - how do I rate syncro bands

[Cornpanzer]

I am freshening up the gearbox in my '70 and there is quite a lot of wear on the syncro bands, dog teeth and sliders.

Ques 1 - I purchased an old shop last year and I have a tub of probably 75 syncro bands. Most of them 'look' to be good, but how do I make the decision to use them or not?

Ques 2 - I have a few extra 1st/rev sliders. The one in my box is ground almost smooth. The spares I have are slightly rounded, but much better than what I am taking out. How do I determine if they are good enough to use?

Thanks to anyone who can give me pointers. I am having a blast playing with the guts in this box. Never fully understood how a gearbox worked, but now I do.

[john walker's workshop]

always taking a chance reusing them, but look for ones that still are smooth, no grooves, missing surface chunks and no worn off surfaces showing shiny base metal. i save anything that is remotely usable, for the future when they may be hard to come by or real expensive. (oh, i guess they're kind of pricey now).

[Jetjockey]

I went to a 901 clinic recently where they used worn synchros by installing them in the opposite direction. The worn side no longer makes contact and the unworn side is used to provide the friction. The worst synchro of the set was saved for 5th gear. There were a few synchros that they tossed but most were still used with a backwards orientation.

[Matt Monson]

Once you've done the visual inspection, load the synchro ring onto the dog and be sure to measure it. If it is below the minimum spec toss it. If it's too small it will not grab the slider the way it's supposed to and result in crunched dog teeth. If you don't have the spec let me know and I'll look it up for you.

And as these old 901 parts go NLA I promise you that I'll make them for you. While 80% of my customers are buying stuff for the cars made in the last 15 years, my heart belongs to my old 914 and my longnose. As long as people are driving these old cars I'll make parts to keep their gearboxes healthy...

[Cornpanzer]

HI Matt,

Thanks for the input. yes, I would be interested in those dimensions and proceedure for measuring. Or you can tell me where to find them. Thanks!

[john walker's workshop]

the measurement for the 901 syncros (same as 915 3dr/4th/5th) is 76.3mm +/- 0.18
the larger 915 1st and 2nd syncros is 86.37 +/- 0.17.
that's with the syncro installed in the dog tooth ring w/snap-ring installed. if the measurement is smaller, then suspect syncro surface wear. if it's larger, suspect inner dog tooth wear.

[911SCfanatic]

John, where is this dimension taken on the syncro, since the OD of the syncro is tapered? The largest dimension?
Also, is there a measurement to take for the inner dog surface that mates with the syncro? Is there a measurement for the slider ID? There is a groove worn in the middle of the sliders..is that normal (haven't seen what a new part looks like, I just have the used parts on my bench. I will take some pics).
Thanks for the help!

[Flieger]

The groove in the sliders is a natural result of the snychro ring wearing on that inner slider surface while doing its snychronizing thing. If the groove has much of a depth or is sharp, it is good to replace. That groove only makes the synchro clearance more critical as the synchro must be larger diameter to contact or lock into the slider.

[911SCfanatic]

So, then the inner surface of a new slider is flat?
My syncros (installed) are measuring right around 76.3 and look great. The dog teeth look good as well. I'm wondering if the teeth and syncros have been replaced but perhaps the slider hasn't. If there was some sort of slider measurement, I could know for sure. I looked in the manual and found nothing. Could someone post a pic of a new slider surface and give me some guidance on assessing slider wear?

[euro911sc]

My new sliders had a slight grooved profile, IIRC. I would be interested in knowing this inner measurement as well. Also, would like to know about the measurement on the brass shift arms and the outer groove where they sit on the slider. anyone make a billet version or would that be useless? Brass was chosen as bearing material I assume...

Matt Monson!! Start making this stuff because, as several gurus have pointed out in past posts, the current stock of parts bought new just is not up to snuff dimensionally. 901 and 915 parts please Oh and nice new website! still needs work, but its getting there!

-Michael

[911SCfanatic]

Michael, what do I need to be aware of re: new parts? Is there specific dimensions I should be checking on certain part?

And, just thinking out loud re: my early question on slider wear...if the existing syncro measured in spec and the teeth on the slider are still sharp, this should mean that the inner slider surface is still good. An overly worn inner slider surface would evidence itself by worn slider teeth, no?

[euro911sc]

Originally Posted by 911SCfanatic

Michael, what do I need to be aware of re: new parts? Is there specific dimensions I should be checking on certain part?

And, just thinking out loud re: my early question on slider wear...if the existing syncro measured in spec and the teeth on the slider are still sharp, this should mean that the inner slider surface is still good. An overly worn inner slider surface would evidence itself by worn slider teeth, no?

I think your logic is good here on the implied internal wear given good teeth. Though I would remove the synchro and look for spot wear. You are there so you might as well be thorough in your inspection

I remember Steve Weiner mentioning that the new manufacturer of parts wasn't very good and that the new parts needed to be measured before use. It used to be just slap the new parts in and go kind of thing... Which is why I asked about the outer and innner dims on the sliders and the brass parts.

Best regards,

Michael

[Marc Bixen]

This link is to a great 915 tutorial. It will certainly help you identify worn syncro parts and show you how to measure an assembled gear set. Enjoy
Porsche Wiki

[Matt Monson]

Originally Posted by euro911sc

My new sliders had a slight grooved profile, IIRC. I would be interested in knowing this inner measurement as well. Also, would like to know about the measurement on the brass shift arms and the outer groove where they sit on the slider. anyone make a billet version or would that be useless? Brass was chosen as bearing material I assume...

Matt Monson!! Start making this stuff because, as several gurus have pointed out in past posts, the current stock of parts bought new just is not up to snuff dimensionally. 901 and 915 parts please Oh and nice new website! still needs work, but its getting there!

-Michael

The new website goes live in about two weeks. What you see there just didn't work out with Flash being a problem for many people. We're going a different direction with the new one.

I'm starting to hoard some NOS stuff that I've managed to buy in the last couple of years because of the problems people are finding with the replacement parts. I've got 901,915,and 930 sliders sitting in a stash that is not for sale. These are things I hope to be able to provide in the next year or so. There's just a lot of new products on the stove right now and there's got to be a priority list. For now, 901 gears are the top priority along with Cayman gearsets and mainshafts.

One of the single biggest problems has been the dogteeth. We get ours from the OEM source and they tend to be pretty good. However there's 3 or 4 other manufacturers out there and what we've been seeing the last few years is that the dog teeth are not relieved enough on the backside where the synchro band rests against it. What happens then is that a brand new synchro when installed into the gear is smaller than the proper spec. and until the synchro wears in and gets a groove in it the box doesn't shift cleanly. This is worse than the typical break in for a 901 or 915 box. I know of a handful of professional builders who take a diamond wheel on a dremel to the backside of the dogteeth to relieve the teeth a bit until the synchro is the right dimension when stacked.

As mentioned above, the slider should have a little groove in it when it's new. The point of this groove is that it mates to the concave outer edge of the synchro when you engage the gear and provides a little lock. It's that reassuring click of engagement you feel when you shift into a gear.

While I don't have an actual spec of what's good versus worn out on them, one test we use is to put the stack together and just drop the slider down under the force of gravity onto the gear/synchro stack. If it shifts into gear like that or with the bare minimum of force from your hand it's worn out. It should not be too easy to get it to shift into gear.
You can also go that to a certain extent on the bench with a loaded gear and just see what it takes to push it over the synchro. If it's too easy it's a bad thing. You want to have to use a little force.

[Flieger]

I stand corrected. I thought the shallow V groove in the middle of my 2nd gear slider was just from wear. I did not think a dip so shalllow and smooth could provide a "locking" mechanism. (My mechanic gave me the old parts from my rebuild and said the groove was too big= slider worn out.)

[Matt Monson]

Just to be clear, what your tech told you was correct. It's when the groove gets too big that it won't catch and it's worn out. It's not so much the groove that actually wears as much as the shoulder of it where the synchro goes across it over and over every time you select a gear.