Intermediate gear/crank gear backlash

Jim Williams · 05:19 PM

Let me start the post by saying I spent some hours trying to locate an answer to this by doing searches on "layshaft", "countershaft" and "intermediate shaft". Lots of hits, but nothing I could find on my particular question. I did find a thread describing *no backlash* (not the problem here).

I'm into another engine rebuild .... One area I've never found to be of a lot of concern is that of the crank gear to intermidiate gear play. I've done the measurements before -- I have the spec books -- but can't recall all the outcomes, and can't recall ever being concerned enough about what I measured so that I bought a new gear or found a reason to change anything... Just put the intermediate shaft/gear back as it was.

On this rebuild, I have not proceeded as quickly as in the past due to some issues with the nature of this particular engine, and have spent more time than ususal researching some things. ( Anyone ever been into a '79 Factory Upgrade 3.1 liter engine, or ever seen one, for that matter?)

Anyway, the '78 -'79 spec book gives backlash numbers in the range of 0.016 mm to 0.049 mm. That's only 0.0006" to 0.0019". My measurements show 0.12 mm of backlash. Almost 10 times over the minimum number. This is about an 80K mile engine. Everything in the bottom end looks to be in reasonably good shape. The crank and rod bearings and journals show very little wear. The intermediate gear teeth surfaces don't *look* to have any abormal wear. The case is a "0", the layshaft gear is a "0", and I seem to find a "0" lightly etched on the crank gear. The backlash numbers, BTW, seem to be installation numbers, and not wear limit numbers.

I have several used countershaft assemblies lying around, and decided to check out the backlash of these while I had the dial indicator set up. None of these had any *less* backlash. All 6 were "0" stamped, except for one "1". The "1" had more backlash than the others (just more worn?). All of these are used gears, so I'm not sure what all this says. I thought perhaps one gear might have less backlash than the one from the engine and give me some clues.

The spec book shows a measurement technique for measuring the gear alone, using 4.5 mm steel rollers. Now, let me see, where in my meaurement tools drawer, did I mis-place those 4.5 mm steel rollers? The numbers given here are for the wear limit, so if I can ever find those pesky 4.5 mm steel rollers (doesn't everyone have a set of these?), I might get the answer.

What happens when the gears in question are out of the installed spec with regard to the backlash? Cam timing hard to set? Issues with engine performance? Issues with premature wear of something? I can't find any discussion of this "problem"(Is it?).

Replies from anyone with any insight appreciated.

ChrisBennet · ChrisBennet's Garage

Until the expert chime in...
I looked through my records and found the the numbers for a 964 motor I did. It had .04mm of backlash.
I dug a little further and discovered some notes on a 3.0L that had .14mm (!) of backlash and a note that the backlash was measured with new (uncrushed) bearings.
Now I measure the backlash before I remove the intermediate shaft from the case. I suppose assembling the case to crush the bearings would also work though I haven't tried it.
-Chris

Jim Williams · 11-15-2005, 08:35 AM

Hi Chris,

Thanks for the input. These measurements I took were on the gear just after the left side case was pulled off, with the crank, intermediate shaft, and old bearings still in place. I wouldn't think that bearing crush would be much of a factor in an "open case" measurement, as the crush should occur on the bottoms as well as the tops of the bearing shells, sort of averaging out the movement of the gear in the "towards" as well as the "away" movement of the gear shaft in the direction of the crank.

I'm hoping that there will be others to chime in, as I hate not knowing about things that are in these engines that might affect either perfomance or durability. BA's handbook addresses making the measurement, and uses the same numbers as are in the little "spec" books, but he doesn't address what needs to happen if the measurement numbers fall outside the quoted ones.

ChrisBennet · ChrisBennet's Garage

Quote:

Originally posted by Jim Williams
Hi Chris,

Thanks for the input. These measurements I took were on the gear just after the left side case was pulled off, with the crank, intermediate shaft, and old bearings still in place. I wouldn't think that bearing crush would be much of a factor in an "open case" measurement, as the crush should occur on the bottoms as well as the tops of the bearing shells, sort of averaging out the movement of the gear in the "towards" as well as the "away" movement of the gear shaft in the direction of the crank.

I'm hoping that there will be others to chime in, as I hate not knowing about things that are in these engines that might affect either perfomance or durability. BA's handbook addresses making the measurement, and uses the same numbers as are in the little "spec" books, but he doesn't address what needs to happen if the measurement numbers fall outside the quoted ones.

When I got such a large backlash with the new bearings I assumed that the bearing wasn't squished down into the case i.e. it was riding a little high from natural springiness.
-Chris

NevenM · 11-15-2005, 11:21 AM

Hi

Just a suggestion, for the 4.5mm rollers use a couple of 4.5mm drills. This may be a little more difficult for you in the US (Still imperial), Though from what you say I don't think its the Intermediate gear.

First try measuring it with a new bearing shells if you are going to replace them, this is after all an 'as built' spec

Neven

Neven

Jim Williams · 11-15-2005, 03:04 PM

Neven,

Thanks for the hints about the drill bits. I found that a size #16 drill bit shank is very close to 4.5 mm. As a matter of fact, looking up the decimal equivalent, it is 0.1770", or 4.496 mm.

I used a couple of these drill bits on either side of the intermediate gear, and measured 137.22 mm across them. It looks like this gear passes the test, as the wear limit is 136.5 for a "0" gear (135.55 for a "1"), with lower numbers indicating what would be more wear. So it would seem the backlash numbers are only installation numbers, and as such are useful only when installing a new gear. I guess I could go out on a limb here and say that backlash numbers don't appear to be of any use when checking the intermediate gear to see if it is OK to reuse in a rebuild situation. One needs to measure the intermediate gear using the method in the little spec book to get anything meaningful.

NevenM · 11-15-2005, 03:09 PM

Jim

When I rebuilt my 2.4 the backlash was within spec, but that was installed in new bearing shells, Are you renewing these?. This backlash measurement as it is may indicate your bearings need replacing

Neven

Jim Williams · 11-15-2005, 05:26 PM

Neven,

You say your backlash was in spec. I guess you are saying that it was within installation spec range. That was fortunate, I think if mine had been within the installation spec, I probably would not have raised the question in my intial post, which was: what are the implications of the backlash being outside the installed spec range of values?

I do plan to use new bearings. I don't think I have ever considered opening up a case for a rebuild without replacing all the bearings. The intermediate shaft bearings are showing copper, so they will come out for sure. The crank bearings look perfect, but I will be replacing these as well as a matter of course.

But to fast forward to installing the new bearings (they are on the way but not here yet), let's suppose that when the bearings are replaced, the backlash doesn't change significantly enough to get it withing the installation limits (which I'm guessing it won't).

It seems like there should be a wear limit on the backlash. I don't think that's what the numbers in the spec book represent. If my intermediate gear itself checks within the wear limit, and I replace the intermediate shaft bearings, I can't see what else to do. I don't expect that the steel crank gear is worn. But there are no specs on this gear that I'm aware of, so there's apparently no way to check it.

If the new bearings bring the measurement in specs, concern goes away. But if not, do I just assume at that point that there will be no engine problems because of this out of spec condition? Again, this is the first time I have dwelled on this issue in several engine rebuilds, but now that I have a question, I am looking for answers.

NevenM · 11-15-2005, 09:23 PM

Jim

Quote:

I don't expect that the steel crank gear is worn

1/ This is a dangerous assumption

2/ If you look at the tech spec book its obvious that the 0 was the undersize and 1 was the oversize in the machining tolerance (which is why they permitted 1 Item of the other spec)

3/ I think your 0.12 is to high (and therefore out of spec) but until you put in new bearings and test it you won't know as measuring it now will bear no relationship to what it will be when you disturb it as all the wear in the journal/bearings and teeth will be reflected in this backlash measurement

4/ I'd measure it during the install and if still out of spec change the crank gear, if still this far out then get a machine shop to check the crankcase axis distance because maybe someone has altered it by a bad line bore? or you have a faulty factory case that a sloppy porsche employee let slip thru

HTH Neven

Jim Williams · 11-16-2005, 09:24 AM

Neven,

Quote:

3/ I think your 0.12 is to high (and therefore out of spec) but until you put in new bearings and test it you won't know as measuring it now will bear no relationship to what it will be when you disturb it as all the wear in the journal/bearings and teeth will be reflected in this backlash measurement

Why do you think it is too high and therefore out of spec? The largest of these numbers in the book is 0.049 mm, or 0.0019". I find it hard to believe that just under 0.002" would be a reasonable number as a wear limit for backlash for these gears after 80K miles or so. As I've said, I don't think the installation numbers in the spec book table are vaild as a wear limit.

The thread seems to have wandered off from my original questions. I understand the differences in the tolerances of the various components at play here. 1) the crank gear, 2) the intermediate gear, 3) the case,and last (and least) the bearings. In fact several different combinations of these different components were allowed (save the bearings), and the installation tolerances for these combinations are listed in the spec book.

I don't know which of us is correct at this point, but it is apparent that we are seeing the tolerance numbers in a different light. I am seeing the numbers as installation tolerances, and you are seeing them as wear limits. As the book is not clear on this, I still waiting for someone to step up to the plate who has more experience than you or I, and set me straight.

I see the numbers in the light of the way pistons and cylinders are treated in the spec book. Install tolerances are given for the pistons and also for the cylinders. Comparing these numbers against each other gives another set of numbers, the installation running clearances, also found in the spec book. But the book goes further and gives the wear limits, being a comparison of actual measured piston dimensions against actual cylinder dimensions. So it is pretty clear that there are two kinds of limits for P+C, the installation limits and the wear limits. The book is clear on this issue.

Another point of logic I neglected to mention: a wear limit is stated as one number, not a range of numbers. (It has no meaning to say "I'm going shopping, but my expense limit will be no more than $20 to $40.") Another point: why would the book give a wear limit on the intermediate gear wear and also the backlash? Which takes precedence over the other? (Since both are measurable, a wear limit for the backlash would seem to me to be more meaningful, but the book doesn't do this.)

The case of this engine has never been split, leaving out the possibility of an aftermarket line bore figuring into the mix.

The bottom line for me is that I am seeing only one wear limit for the crank/intermediate gear check, that of the intermediate gear, when measured per the spec instructions.

I will definitely re-measure the backlash with new bearings in place when I receive them, but regardless of how this new measurement turns out, it's not going to answer the questions in my original post.

Anyone else? Henry Schmidt? Steve Weiner?

NevenM · 11-16-2005, 10:00 AM

Jim

I looked at the Service manuals and the int gear dims are wear limits, so as yours are not under the wear limit something else is going on (and you should be within the backlash limits), I await Henrys opinion on this as he must have done a few

Neven

Jim Williams · 11-20-2005, 06:23 AM

Anyone? How much backlash is too much? Reasons?

Eagledriver · 11-20-2005, 07:48 AM

I've seen this discussion before, a search may yield more info. I think the concensus was that the backlash is a non critical spec. (not a fact just speculation). I believe this to be true from my own experience. I took apart my 2.7 RS replica engine after 80k miles and the aluminum gear showed obvious wear. The teeth were asymetrical and had worn about .005" or more. The back lash of course was very large. I put in a new 0 gear (that's all that's available) and the backlash went to just about zero. I think the most important thing to look at is the wear on the aluminum gear. If the gear looks ok then reuse it. If not then replace it. It wasn't that expensive. By the way I was supposed to use a 1 gear due to having a 1 case and 0 steel gear. I found that the 0 gear was big enough and I think a 1 gear would have been binding.

-Andy

Jim Williams · 11-20-2005, 11:26 AM

Andy,

Thanks for weighing in. As I said, I did several searches for info; I thought I read through everything that popped up. Maybe I just missed this discussion you reference. The aluminum gear is in spec according to the wear limit figure. I used two different measurementt methods of check the gear (including a visual for signs of obvious wear) just to be sure I had it right.

So I feel pretty comforatble with the aluminum gear. If you remember anything about the thread location, title or time frame, please let me know.

Jim Williams · 11-27-2005, 05:21 PM

Hello out there ----

Can anyone speak to the question of what should be the allowable limit on intermediate gear backlash and what numbers out of the little spec book or their own experience they base the limit on?

Inquiring minds want to know.

Jim Williams · 12-03-2005, 12:32 PM

Well, I guess this question doesn't hold much interest for the engine gurus. So I'm gonna take a hack at answering my own question. My take is, since there is no wear limit spec on the backlash (my thought is that the numbers in the table in the spec book are installation tolerances and not wear limits), that it's not really not all that important. I measured 0.12 mm backlash, and the intermediate gear measured 137.22 mm - within spec. So what do I compare the 0.12 mm to?

If I am getting 0.12 mm of backlash, my math tells me this is equivqlent to about 0.2 degree at the crankshaft if you could hold the intermediate gear immobile ( crank gear has roughly a 76.2 mm diameter, circumference = pi x D = 249.4 mm. (0.12/249.4)x360 deg = 0.173 deg). Or if you hold the crank immobile, half that, or 0.1 degree at the cam.

Now, how much would this affect the cam timing? Since trying to move the crank by 0.2 degree is dam near impossible, even using a degree wheel, I'm thinking the answer to this one is, so little you can't measure the effect on cam timing either.

So gear noise, if any, would seem to be the only effect on engine operation, unless I'm missing something else.

If anyone else has anything new to add, please post.

89turbocabmike · 12-03-2005, 09:00 PM

Jim I've been subscribed to this thread hoping to learn as well. I'm surprised that none of the specialists have chimed in. I'm certainly no guru but I have installed and timed my own SC cams as well as rebuilt 6 non-porsche engine so I understand your query. Your noise assumption makes sense. As long as proper procedure is used timing the cams,turning the crank always clockwise directly to TDC, the clearance still would seem to have minimal effect on timing.

dtw · 12-04-2005, 12:39 AM

Jim,
I can't see as well as I'd like in the pics you've posted, but it looks like those steel sprockets are pretty worn. Are you planning on replacing them?

Jim Williams · 12-04-2005, 10:22 AM

Mike,

It seems like as long as the cam chain is pulling from the side it is supposed to pull from, and not pushing, the effect should be minimal....... ;^]

dtw,

You're right, the sprockets have some side wear from the chain, just enough to make the sides shiny. Although the engine showed no signs of ever being "apart", there were carrera tensioners added, and the chains had the removeable links. So someone in the past saw a need to replace the chains. I don't know if the wear occurred with the first set of chains, or the second. Anyway the sprockets will get my attention before I decide to use them (or not). I have several good used intermediate shaft assemblies, and may decide to borrow some sprockets from these.

I checked out your website with your 2.4 rebuild from a couple of years ago. Nice storybook, very methodical, I trust all turned out well.

Jim Williams · 04-24-2006, 05:08 PM

Hi guys,

Long time no input to this thread.... Due to unforseen circumstances, I am just now able to start on the engine assembly

I never received any opinions from some of the more experienced 911 engine builders on the backlash limits condsidered acceptable for reassembly. Anone want to step up at the last minute and provide a number? The case is going back together in a couple of days.