For the folks who are rebuilding their own transmissions, what are you using for a backlash measurement? The manual says around .007, but that seems a little tight. We're trying to get some temperature out of the transmission and were thinking about running a little looser. Anyone tried that?

I wouldn't try that, maybe it's time for a pump and cooler set-up

Have a cooler and pump. The 901 is set very tight compared to most other transmissions, so we thought it's worth a try. What do you set yours at?

I don't race so mine is just stock.
But, I'd would worry that the loading/unloading would cause excessive wear. I'd keep a close eye on the wear pattern, if it gets to close to the root or edge of the tooth it will begin to break things.

The backlash and contact pattern go work together to make the (sliding) contact area large enough and in the right place. It doesn't have to be dead center but that's ideal.

Years ago I broke a pinion tooth by setting the proper backlash and the wrong contact pattern. Bad juju that.

You can have good settings and contact and still break stuff.

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I use the factory tool, or at least a clone of it. The factory states that .012-.018 (edit:McMark 0.12-0.18mm or 0.005-0.007") is tolerance. The way that the back lash is set at the factory is that the pinion is mated to the ring at a set design measurement "N" and then the unit is run on a machine and slowly adjusted out from the center line of the ring gear until the quietest operation is attained. This number is then scribed onto the pinion head, i.e. n=18 would mean that you need to set the pinion back from N +.018.

To do this correctly, you need the special tool. Varying in or out will not give you much in the form of performance and I do not think it will effect cooling all that much either. One can surmise that the quieter running factory setting is the least amount of friction one can get from the R/P.

These dimensions are incorrect.

New to forum, reviewing old topics and came across this. Numbers above are noted as 1/1000 of something, but incorrect whether mm or inch. Factory dimensions should be noted as 1/100mm:

Backlash range: 0.12mm to 0.18mm
Pinion dimension: N18 = +0.18mm

In inches, equivalent backlash range would be 0.005" to 0.007".
Hope the good doctor wasn't using those dimensions himself, nor anyone else.

Jon B.
Vista, CA

Not using inches, why would you assume so? Decimal was out of place, sure.

I didn't assume numbers were in inches, I said "1/1000 of something" because it wasn't noted, just .012-.018 of something. But in US when people see .018 they do assume inches. Matt began thread with .007, an inch dimension although also not noted as such.

Just felt it should be corrected in case future forum members search "backlash".

No?

Jon B.

Sure, whatever

Jon - Good call.

As an update, we're going to pull the transmission this winter and still plan on loosening the R+P some.

We also have a clone of the factory tool that we made.

I'd like to see a video or write up when you do that, Matt.

Some might not know Jon B. But as an FYI, he's one of the premiere 901/915/G50 trans experts on the west coast. Jon, good catch on the decimal thing. Only an expert would have caught that.

Matt, what kind of differential do you use?

Rich

If thats Jon Bunin, then add 356 trans guru as well.

Matt,

It's best to stay within factory specs when adjusting a ring & pinion, even for racing. Porsche manuals note "...the pinion shaft and ring gear is matched as a pair during production. They are checked on testing machines for correct contact pattern and low noise level in both directions of rotation". Gear noises are the result of excessive vibrations. These vibrations can be annoying but also destructive, as they contribute to material fatigue. It's best to take advantage of their tests and stay within factory specs.

Jon B.
Vista, CA

Photo below of factory tool, P258, with all accessories.
This tool will only measure pinion depth; other tools are needed to determine ring gear backlash and differential bearing preloads.

I'm curious to see the clones being made.

Click to view attachment

Jon B.
Vista, CA

Just have an open diff in it right now.

Mine was made by a machinist and has the same dimensions as the factory tool. I only have to use it when replacing R/P for failure of inspection or traumatic failure. Most of the time, and I have covered this extensively here and in my DVD, I instruct people to measure gaskets and maintain factory set specs. The vib issues is not much of a concern in the small increments we deal with. It is mainly a nuisance. Many other manufacturers do not do the finite test in leu of just matching patters with dykem or other indicator to a visual pattern.

In a race box, though, I would keep as much friction and vibe out of your setup as fatigue will be highly magnified. Bearings wear and tolerances are quickly lost.

Any pictures of these various copies of the P258??

Ya, in my free time Sorry, I am at my 9a-9p job and then I will be gutting a few more boxes for goodies.

Search on Pelican as the guy who made them had a thread over there. Just search P258.

see post #43

http://www.914world.com/bbs2/index.php?sho...=91491&st=0

Um, no. That is not it.

Post 21 shows one though.

http://forums.pelicanparts.com/porsche-911...tml#post2402521

Ding ding ding

Matt,

Have you considered replacing the diff housing plates on the outside with thicker ones?

I wonder if you may have experienced flexing, thus heat....dunno

I use the VW tools. Got them to do 915's, but work equally well on 901's.Click to view attachment

Click to view attachment

We have a billet intermediate plate and side plate.

John,

Those are excellent tools, and Porsche-recommended alternative to P258 for 901/911/914 transmissions. I use my set often. Lower photo from 915 manual, showing tool in use with diff cover removed...

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Jon B.
Vista, CA

Matt,

Were billet IP and diff cover already installed, or new for transmission?
Do you race this car?
What type of engine are you running?

Jon B.

The billet IP and diff cover were from a previous transmission that killed itself. It is a race car with a stock 3 liter six in it. 2050 pounds. It gets a lot of time - we'll do almost 3,000 track miles and around 60 hours this year.

Matt,

You realize that's alot of engine and torque for a 901/914 transmission. I'm not surprised it's running hot. It will be worse with a TBD or LSD installed. I'm curious to see your cooling system.

Compare 901 to early 915 pinion below. The 7-tooth 915 pinion is from a 2.4 liter road car; Porsche went to an 8-tooth pinion in 2.7 liter series. And you're racing with a 3.0 liter engine...

Click to view attachment

Jon B.
Vista, CA

It is a lot and around the limit of what you can put through a 901. We pull oil from the drain and put it back on the gears and R+P. The cooler is about 11x8 and has a blower forcing air through it.

Okay the magic mystery tool is really not needed. Okay thats is a miss statement, but if you want to make one, why not measure your own diff, get the exact center. Mark / Machine the position on the diff so you can measure from the same spot each time. Grab another side cover, drill a nice hole in it above the pinion and then use a bore gauge to measure your depth , a little math and you just replaced the VW and Porsche super secret tools.

You don't even need a fancy bore gauge a simple T gauge and a set of calipers will do the trick.

I used this method after talking to Groot about it. And I do believe it worked. I ran that trans for 6 years now, no signs of bad things happening. I pulled the side cover last month and pulled the LSD out. Ring and Pinion where in perfect shape. It might have been luck, I tend to think it was just taking time and doing the math to get it all right. YMMV


Now Pinion Backlash, I hate doing that, shims, pulling in and out. Subaru got that one right on the 5MT. Two bolts and a spanner wrench. Done.

Agreed, I hate setting preload.

An inverted 915/930 is ok too. The R&P is on the "right" side so the gear-stack doesn't have to come out to change spacers/shims.

curious question.
Can you replace the ring and pinion with the 8 tooth setup yet still fit inside the 914/901 box and have everything work properly?

Not with a 915 ring & pinion, it will not fit earlier transmissions.

Porsche made an 8:35 ring & pinion for the 901 transmission, but they're fairly rare and expensive, even in used condition. They're still smaller than a 915 R&P, and only a marginal improvement to the standard 901 set...

http://www.ebay.com/itm/300572148895
http://www.ebay.com/itm/300510781903

Jon B.
Vista, CA

Going to more teeth on the pinion of a 901 will actually net you a weaker pinion as the teeth will be smaller/thinner. The 915 is thicker overall in all parts.

More teeth increase the gear's load bearing surface and help decrease material fatigue. Gear tooth design can be adjusted if the pinion isn't enlarged, for example, the root circle can be increased, shortening the tooth. It's unlikely that Porsche would've offered a weaker design with no specific advantage, such as significant change in ratio.

The 915 8-tooth pinion was increased in size from previous 7-tooth pinion, with ring gear offset 2mm on differential.

Jon B.

I disagree. When it comes to the 901 pinion, more teeth takes away from the overall girth of the pinion base with smaller/thinner teeth. They would definitely put this in a box that was not intended to handle crazy tq. These weaknesses become evident when high tq is passed through a box not built for it. When keeping in with parameters speced for the boxes, the number of teeth is not an issue.

Your mention of the change made to gain an extra tooth on the 915 supports what I am saying. Increased size was mandatory and that box handles more tq/hp than a 901. All parts are thicker in that one.

More teeth increase the gear's load bearing surface and help decrease material fatigue. That is a fact. Each tooth surface bears less sustained load per cycle, as time spent in contact is briefer. And having a slightly taller ratio, the 8:35 pinion has fewer cycles per mile as well.

We can disagree.

In any case it's really not a viable solution for anyone, as sets are both scarce and expensive.

Jon B.

True that.

Late, but none the less relevant.

It only began in April, so not quite a resurrection of Biblical comparison.
Hopefully prevented some potential grief, contributed new information.
Matt still hasn't done anything since beginning the thread, so still relevant as well.

Jon B.

Yup, relevant, addressed two misplaced decimals, I have no idea what I was doing and in what state of mine I was in when I posted that; work, drunk, sick with bad cold?

I just saw this meme and it made me think of this.

Your set is certainly more complete than mine. I found that some of these tools including the storage box are still available new. I see that you have several fixed depth setting gauges as well as your master gauge. If you have duplicates that you wouldn't mind parting with, I may be interested.

Thanks,

John