Distributor shaft gear

[Jeff G 78]

Last year during an 8 hour endurance race, my 260Z started missing and running like crap before it dies completely.  We tried to diagnose the issue at the track and finally had to throw in the towel that race.  We could not get the car to start no matter what we tried.  Everything looked normal with the timing, valve springs, plugs, etc, but it wouldn't start even on starter fluid.  We parked the car in the garage and until today, we had no idea what was wrong or how much damage was done to the engine.

Today, we pulled the plugs and looked in the cylinders with a borescope.  Everything looked great.  We then looked down the front cover and all looked normal.  The chain, tensioner, and crank gear looked good, but I couldn't quite see the drive gear on the distributor shaft.  We then pulled the oil pump and all became clear.  The drive gear on the shaft was a mess.  The teeth were stripped.  We then drained the engine oil into a paper towel and found no debris.  Next, the oil pump came apart and looked okay, but there were signs of small debris going through it.  Hopefully, when we pull the engine and tear it down, we won't find any damage to the journals.  I think we'll be okay, but it's a shame to pull it apart again.

What causes the shaft gear to strip?  I have never had this failure before.  The shaft gear looked great when we built the engine which now has about 20 race hours on it.  The oil pressure was always good right up to when the engine quit, so it was clearly still spinning, it just couldn't keep timing correct.

 

[Zed Head]

I think that the gear is just a press fit on the shaft.  Yours look like it might have moved, there's a colored spot on the shaft.  Also looks like it might be bronze or some other non-iron based metal alloy.  I don't know what the stock gears are made from but maybe yours is aftermarket?  And not as durable as Nissan stock.

I always wonder when people worry about debris making it to the bearings.  That oil is supposed to be filtered, so only material smaller than the filter limit would make it, or if the filter clogged and the bypass opened up.  Just saying.  Cut open the filter and you might find the rest of the gear, in oily dust form.

[Diseazd]

Jeff....I remember back in the 70's there was a problem with the worm gear ( the gear that slides onto the crank nose and meshes with the teeth on the distributor sprocket) under harsh racing conditions. The two it seems were incompatible and caused wear under race conditions. The solution was a bronze worm gear that you could get from Nissan (NLA). I bought them for most of my engines before they became NLA. Seems to me that someone was making them again.....try googling them or maybe eBay. If not, I would replace all 3 items .....oil pump, worm gear and sprocket and go at it again.

[siteunseen]

I assume that's where the "endurance race" gets the name.  That spindle gear endured all it could.  

[Jeff G 78]

Zed Head, the shaft came from the donor ZX engine which appeared to be all original with unknown mileage.  The cylinders were in perfect shape as were all of the journals.  I'm sure you are right about the filter collecting any gear debris, but I'd hate to blow up in my next race because of stray shavings.

Thanks Guy, I will look around.  The engine is all 1982 ZX stuff and since I rebuilt it, has about 20 race hours on it.  Previously, my L26 engine had about 60 race hours on it and the gears looked like new.  Oddly, the gear on the crank has no damage at all, at least when viewed via borescope.  I will replace the pump, shaft an crank gear.

[Chickenman]

I blew a spindle gear like this once ( and the worm gear ). Turned out to be a slightly bent crank snout. 

Check the radial runout on the crank snout. Check at very end of snout.  Will destroy the spindle gear in short order. FSM should have specs or a good machine shop. 

Endurance racing will really amplify any vibration/harmonics issues. Is this a stroker engine by any chance? What size is the engine, has it been balanced and what RPM range are you using?

What are you using for a front damper?  Front damper design can really affect spindle and worm gear life. Secondary vibrations in a inline 6 can get very high. A good vibration damper is critical to crank and component life on an Inline 6. Never use an aluminium or solid billet steel pulley on the front of the crank. 

Check the TC bushing to make sure it hasn't elongated. Can cause spindle shaft whip. Are you running a Crank fire ignition by any chance and have eliminated the Dizzy? Dizzy lower bushing gives support to top of spindle shaft and helps to limit shaft whip. If Dizzy eliminated, top of spindle shaft should be machined for an upper bearing. It should not be left unsupported. That will cause spindle shaft whip for sure and destroy the gears. 

Other possible causes are excessive oil pressure. . What oil and weight are you running? Low levels of Zddp in oil could also cause this issue, but I'm leaning more towards a vibration or bent crank issue.  

[Chickenman]

FSM calls for crank run out limit of less than .0020" ... but that's measured in middle of crank. The runout on the snout can be more critical, because of the length that is unsupported and the  leverage involved. I'd prefer to see it around .0010" on a race motor. Measured at end of snout. A good crank shop should be able to straighten a bent snout fairly easily. Just make sure they know it's a Race engine and that " Good Enough " is NOT acceptable. 

[zKars]

The only problem I've ever had with these was on an L18 in a 610, a REAL long time ago. The oil pump pressure relief valve jambed up and the oil pressure followed RPM like a tach. Then it got quite high, then It went to 0 and stayed there. It had sheared the tang off the end of the shaft.

Point being, oil pump malfunctions can also cause spindle failure.

Edited April 18, 2017 by zKars

[Chickenman]

^ Very true. 

[Chickenman]

One thing to remember is that factory oil filter systems are a bypass system. It does not filter all of the oil in a single pass. If you have removed and plugged the bypass valve in the oil filter mounting pad, then yes, it is a full pass filter. But if bypass valve is still in the OF mounting pad, then some unfiltered oil is always bypassing the filter.  . Normal contaminants eventually get filtered and are small enough ( and soft enough ) not to damage the  bearings or anything else.

But the factory bypass system will  not save the bearings from mechanical engine damage and the resulting metal chips/shavings,... FWIW.  .

 

Edit:  Filter does NOT have to be plugged for the bypass valve to open. It will always be open somewhat under racing conditions. Thta's why engine builders plug them.

Edited April 19, 2017 by Chickenman

[Jeff G 78]

Thanks Chickenman, I will check the crank runout for sure.  

To answer some of your questions, the engine is an '82 L28.  The original head, block, crank, rods, and pistons were used.  I had the head milled 0.050" and adjusted the timing using the SBC eccentric cam dowels per the "How to Modify..." book.  The guides have been slotted and moved in to get the proper chain tension and position.

I run a turbo oil pump and stock ZX distributor.  Oil pressure held very steady throughout the race and never got too high or low.  The crankshaft damper is a used ZX unit that Guy (Diseazd) gave me.  I always paint a line across the edges of each pulley so I can tell if it ever starts to slip on the rubber bond.  

One other thing that happened, though I cannot say if it was a cause or effect, was that the crank damper bolt came loose as well.  The car ran great for the first four hours of the race (engine had 14 hours on it from a previous race).  The driver got into the car at the four hour mark and after about 90 minutes, he reported that the car was missing at high RPM.  We had seen this before due to fuel boiling, but it was a cool and very rainy race, so that seemed odd.  We left him out on the track and he reported that the miss was getting worse and that the car now was starting to buck.  A lap or two later, it died just before pit-in.  He entered the pits and coasted to our pit stall.  We tried the usual diagnosis and couldn't get it to fire.  That's when we found the loose crank bolt. Yesterday when we found the stripped gear, we figured that the missing and likely backfiring due to bad timing caused the harmonics to "kick" the damper and loosen the bolt.  If the crank is bent, it still won't tell me if the crank bent first or was a result of some other issue.  The crank was checked when I built the engine and it was within spec per the machine shop.

[Chickenman]

That's how I bent my crank. Crank bolt came loose. Didn't fall out. Just became loose. Engine builder ( not me ) did not check how far the bolt went in. Front damper was a Tilton torsional damper and was supposed to use a longer bolt because the center hub was thicker. He used a stock bolt and it worked loose. 

Edited April 19, 2017 by Chickenman