Oh-oh !

[917guy]

I have been putting 1966-82 911 engines together with great success for 30 years and 356 engines since the 60s. I just replaced seals and crank bearings on 1972 S. As I tried to time the first cam, with a single rocker arm in the engine, the engine would not rotate more than 320 degrees, with the problem centered on the #1 at TDC. The cams rotate freely. I pulled both cam housings, the chain housings and the #1 and #4 heads. I had been worried that excess cam chain or whatever had jammed in the counter-shaft sprocket area. No sign of anything wrong there, but now the range of travel is 1.4 teeth of the timing gear, from TDC to a few degrees. I put finger tension on a chain, and poked it with an 1/8" aluminum welding rod, but it is tight even on the back side. The fact that the range changed from symmetrical to all one side of TDC, and became so much less, makes me think there is something flopping around in there. The engine has been sitting half built for weeks, but in a quiet basement shop area, so I doubt anything dropped into it.

I cannot think of anything to do short of taking off all the heads, cylinders, and pistons. I cannot see anything through the open sump hole, the chain holes, etc. I could flip it over, and move the crank a bit in case a 'lucky coin' falls out ;-)

[917guy]

Do any of you have any suggestions what could be stopping this crank?

Nothing in the pump gears or hitting the rods. If it were the rods/ pistons, it would have hit before the engine came apart, and as I was assembling the lower end, and its range of travel would not have changed so drastically. I did check for clearance, as it went together.

[Walt Fricke]

I suspect it will take a while for responses. The initial 320 motion could have been from a variety of things, but the fact that this got progressively worse rules out some things (like one of two pistons hitting its head, or a valve which is bent and not completely closed, or a bolt sticking through the flywheel.

So you have no cams, no cam housings, no chain housings, and the 1#4 heads are removed. So those aren't the problem.

But when built it to the point of timing the cams, did the chains all appear to be the right length? The left fit over the cam gear, and the idler was in about the right position? That would seem to rule out the left chain being folded up inside at the gear.

What about the right? Are both chains in that nice slack position where they are when installing the Ps and Cs and cranking the engine over a number of times? I do this with the engine on edge, so to speak, so one hangs down nicely, while the other tends to hang up, causing me to have to pull it back out from time to time. But this has never been an issue, And if things got a bit stuck, rotating the other way did the trick. Sometimes with a pretty firm yank

So are both chains free or not? If I recall, with one cam opening "up":, you can let the slack chain drop in some, even to the point of getting it to move by a link at a time on its IS gear. I'd think you could determine if it was a chain issue (seems the most likely one to be progressive) or not.

For $100 you can purchase a borescope from Harbor Freight. With the 90 degree mirror attached, you might be able to do a little inspection through the oil return tubes. And with your detachable oil strainer, you ought to be able to poke it into a lot more places. Maybe even snake it along the floor of the case into the area of the left IS gear.

I can't think of how a foreign object in the oil pump would have led to the initial 320, though maybe something could bind more and more.

I had a clearance issue using a 66mm crank in a '76 930 case: the crank counterweights interefered with the main bearing webs when the case was bolted together. Turns out this is a known issue (wasn't to me, of course), and some knife edging of the crank will solve it. Or, in my case, some hand clearancing of the throws did the trick - it was only light rubbing, but very distinct. But I doubt this has much to do with your issue, and why would it get worse? You are just putting parts back where they came from with new bearings.

Not getting the locating pin in its hole in the #8 main (nose) bearing, but not by accident in the oil relief, will lock the crank as soon as the case is torqued together, so not that.

I'd for sure rotate the engine a number of times, turning this way and that and jostling it.

[Flat6pac]

Sounds like your cam is not right on the dot and ahead so the valve is open too much to allow the piston to come all the way up.
You didnt mix the cams up to the wrong side?
Bruce

[Walt Fricke]

Bruce - having made that mistake - timing the #4 with the cam upside down -, that was my first thought. But then he says it not only continues after he has removed both cams (and their carriers, etc, plus #1 and 4 heads), but got a lot worse. No longer a 320 degree and stop issue.

So he is wondering what might be loose inside, etc. I'm drawing a blank, especially since he is not a novice, and these motors are not all that complicated.

[Flat6pac]

Walt, I should have reread for understanding.
Can you turn it backwards 360 degrees or 720?
Bruce

[kwikt 911]

Almost sounds like the cam chains are getting bound up on the crankshaft timing gear. Or even a piston skirt bottoming out on bottom dead center but wouldn't happen unless you changed pistons.

[irobertson]

I had a similar symptom. Mine turned out to be one of the bolts I had screwed into the flywheel to help facilitate turning the engine over had threaded itself in a bit too far and was interfering with a chunk of the case. My heart was in my throat four a while until I figured it out.

[917guy]

It looks like the engine will be coming apart soon. Thanks for your thoughts.

The chains were the same ones which were in it before, and once they are on both sprockets and have a tensioner on, they cannot bunch up on the far side of the drive sprocket. Chain bunching was my first guess, as it could explain the asymmetric later travel range, but I can see that it is not an issue.

In the past, I have sometimes had issues with the chain falling off its drive sprocket when installing a chain housing, and pulling the upper end of the chain away from the flywheel to clear the housing's internal projections. The chain lower end moves an inch or so toward the flywheel, pops off the sprocket, and is difficult to get back in place.

There is no flywheel on it, and I have peered carefully at the countershaft gear, where it runs in a groove in the case, and the pressure and scavenge pump gears, and see nothing amiss. When it was at 320 degrees of total crank travel from left 'stop' to right stop, it seemed like a piston or rod interference. The interference was symmetrical within a few degrees of the travel of the #1 & 4 pistons, so I went there. That it got so much worse on its own, and now is not symmetrical (it runs from about TDC until #1 has moved about 3mm after TDC in running direction.) makes me believe that it could not be a piston or rod thing. I suppose it could be a rod, other than #1 or 4, catching something like the wire screen, which it is not.

I am voting for an object stuck in the gears of the countershaft or the oil pump. Those are 'pinch-points' that are not symmetrical with any piston's TDC/BDC. As Walt says, these are not all that complicated. Since both were clear and free when installed, this makes me suspect an object that fell into place. In that case, I would expect to be able to see it.

This is like a parlor game. An average person cracks a rare, old plate, feels sad and gets rid of it. A tinkerer looks at this as a puzzle, and seeks a repair technique. It is my opinion that the early/ air-cooled 911 community is heavily represented with those who enjoy puzzles and are high-achievers. That is why I enjoyed Porsche engine work when I first got into it in the 60s. That and I loved the cars and had no money for dealer repair. When one fully understands what the Porsche engineers had in mind, occasionally one can find ways to improve. That is exactly what a puzzle solver seeks.

David

[Walt Fricke]

You are right - a lot of us love a puzzle. And I love nothing better than to guess correctly as to what caused something. Especially if it is a system I don't understand all that well and have to make a bunch of assumptions.

So please remember to tell us all what caused this progressive problem. The IS shaft does not turn at 1/2 the speed of the crank, so its teeth keep changing which crank teeth they mesh with. Same, I think, with the chain, but why would it act up? And what would cause the progression?

You can tap the crank this way or that with a plastic or brass hammer. And get a brass drift in on the end of the IS shaft. Probably even pry it this way or that with a long screw driver. Might tell you something. But if they move, what next? Or even if they don't. Oh - teardown. And we have failed to solve the puzzle.

[917guy]

It is raining, so I am back to that engine. I pulled the 456 cam housing and the 456 heads. I did not try rotation. I pried the #4 cylinder off. It seemed to take more force than normal. With #4 cyl off, the crank was free to rotate more. The blockage runs from 5mm of #1 piston travel ATDC, backwards 300+ degrees, to 15.5mm ATDC on #1. It is independent of chain laxness. These limits do not change with 5 attempts to rotate through the full range available. I cannot find anything unusual in the #4 cylinder, rings or piston. #4 has its offset arrow pointed to the flywheel end. Pistons are Mahle 94mm, and were used happily for a summer until low oil pressure doomed a middle rod bearing. I straightened the crank and drilled its #3 and #5 journals for rod-oil supply. That story was explained in gruesome detail in previous issues of my RED RIVER SPORTS CAR ENTHUSIAST. Avoid chewing up main journals. It saves a lot of work.

I am tempted to re-install the #4 cyl and head to see if the very short travel regime returns. Do we have multiple stopping mechanisms? The engine stand has not rotated at all today, so no foreign object should have 'fallen out of place.'

#456 cylinders and heads are now installed and heads torqued. Crank still turns from 16mm atdc to 5mm atdc. It is disappointing that the (4mm travel) symptom could not be repeated. Signs of a bad experiment when repeats have such variable outcomes.

I have now spent too much time gathering data. It is time to just tear it apart until the crank turns.

[917guy]

I took off all of the heads, cyl and pistons. Of course this led to rods hitting stuff as I rotated the crank. Each time, I would touch small ends, and free that rod. Now the crank could rotate 700+ degrees. This was a good sign. I might not have to split the case. I initially thought a rod cap might be hitting a cylinder base, case edge or piston. No sign of that.

I soon realized that when the #4 is near TDC, the big end of the #3 rod moves downward, close to the top of the oil pump. Sure enough, there was a tiny gouge on the top of the pump. I was using the same crank, rods, cylinders, heads, chains and so on, but had installed a bigger oil pump. The tiny travel regime was when the corner of the rod cap had cut a mini furrow in the pump, and was only moving back and forth in that furrow. With the crank falling toward the pump when the engine was in normal orientation, and locking into this furrow, the rod was clear of the pump when the engine was pointed sump-up. The regime, with 320 degrees of travel, was with one set of heads pointed downward.

We need to keep in mind that the crank is heavy and sitting loose on its bearings. Adding up all the clearances on even fresh bearings, there is more than a few thou of difference in reach, for grabbing at nearby parts like the pump, case, etc. It is obvious that anything within about 0.011" of the crank&rods can be touched. When a plain bearing is running happily, with 0.0025" of difference in OD/ID, the clearance will be within about 0.001 of centered. When it is at rest, the journal will sag toward the earth. Remember that the piston cannot come within 40 thou of the head when at rest. Obviously the pump has to be more than 0.020 from the rods.

Path 1: split the case, and file the pump. Clean it and reinstall.

Path 2: Remove #3 rod, and shave the pump with a greasy file, onto a bed of chip catching towel... If some filings escape, at least the case can still be split.

[917guy]

I chose to take the pump out and apart. I milled the offending edge, and tested it with the offending rod at various angles. Lots of clearance now.

The engine is back together now, and ready for timing the cams again. Happily, it turns full revolutions now.

[jmh61]

Congrats on getting it sorted out. Sometimes it seems like two steps forward, one step back with these cars.

[porterdog]

Wow, that was a long hike uphill. Hope the view is worth it and more!

[Walt Fricke]

Wow - gravity caused engine angle clearance issues. Who'da thunk it?

I don't recall if I checked when installing a 930 pump in a 2.7. The clearances I was interested in were pump to web. Same with the GT3 pump I now have in an engine. Glad you shared this.

Almost makes one wonder if the bigger hammer theory would have worked - more force might have self-machined the pump, with just some harmless aluminum bits for a filter to catch. Naw, too many unknowns like funky bearing wear I suppose.

At least it wasn't an open end wrench or something, like the surgeon's proverbial sponge.