Things got cozy

[DieselSJ]

[kwmech]

I'll say. What happened?

[DieselSJ]

We think the gear drive wasn't machined correctly and it advanced the cam. I'm going to do some measurements on the gear drive this week to see. Luckily nothing broke...just a nice indentation in the top of the piston.

[mongocanfly]

looks like you need to buy a lottry ticket....you got lucky you didn't have a explosion

[PGSigns]

Thats a good reason to check cam timing and piston to valve clearance during the build. You did get very lucky. Was it just this one hole? If so look at the valve depths on the heads while you have it apart. Either cut the seats all to the same depth or use this hole for the checks as it is the one with the most chance of a strike. Degree the cam, check the pushrod lengths and then look at piston to valve clearance with a set of checking springs. Shoot for .060 on the intake and .100 on the exhaust for a good safe setup.
Jimmy

[DieselSJ]

Originally Posted by PGSigns

Thats a good reason to check cam timing and piston to valve clearance during the build. You did get very lucky. Was it just this one hole? If so look at the valve depths on the heads while you have it apart. Either cut the seats all to the same depth or use this hole for the checks as it is the one with the most chance of a strike. Degree the cam, check the pushrod lengths and then look at piston to valve clearance with a set of checking springs. Shoot for .060 on the intake and .100 on the exhaust for a good safe setup.
Jimmy

Thanks, I do know all of that.

This was a 100% stock engine from the factory, not a rebuild. The only change made was a gear driven timing set. The cam timing on these is not adjustable, nor is the valvetrain. Gears were aligned correctly at install (verified during tear down). This wasn't a screw up during a rebuild or during machining of block or heads. A user on another forum had a similar issue after installing one of these gear drives. He did some measuring and the gears are machined out of round on the crank, cam and idler gears. In his particular case, cylinders went from as much as 6 degrees advanced to 2 degrees retarded, and they also varied on intake timing and exhaust timing.

[70K10Hunter]

that doesn't sound right if the gears were machined out of round, the engine would lockup because the gears are binding.... what do you mean when you say the valve train is NOT adjustable? 6 degrees advanced or 2 retarded shouldn't do that. if the cam advanced timing that would mean the gears were built the wrong size and the ratio is off, was this a running motor?

[DieselSJ]

Originally Posted by 70K10Hunter

that doesn't sound right if the gears were machined out of round, the engine would lockup because the gears are binding.... what do you mean when you say the valve train is NOT adjustable? 6 degrees advanced or 2 retarded shouldn't do that. if the cam advanced timing that would mean the gears were built the wrong size and the ratio is off, was this a running motor?

Why would things bind? Have you seen a gear drive with a dual idler? The idlers float so they would simply move around to compensate for the out of round.

What part of "the valve train is not adjustable" was I not clear about? The 6.2/6.5 diesel does not have an adjustable valve train. I don't know how to state that any more clearly.

If a cam gear is out of round as it spins it won't be spinning the same speed through all 360 degrees. It will slow down and speed up. As a result, some valve timing is retarded and some is advanced. PM me your email address and I'll forward over an engineering analysis that supports this.

Yes, this was a running motor. It had 20K on it when I bought it, and I put another 60K on it.

[Tom]

I had a gear drive on the small block in my 78 when I bought it. Made cool whinning noises to a 16yr old. Eventually the weight sheared the key on the flywheel during a burnout and my valve to piston contact was a bit more dramatic haha. No more gear drives for me.

[Bigdav160]

I don't recognize the piston, What engine is this?

My experience is: if the cam timing moves only one valve makes contact not both intake and exhaust.

Did you float both valves? Not enough spring pressure?

[Marv D]

Agreed! If you advance the cam you open the intake sooner, that would close the distance between the intake and piston just after TDC. And closing the exhaust valve sooner would OPEN the clearance between the exhaust valve and the piston.

Id suspect someone wanted to here the 'power steering pump whine' of the gear drive and over-rev'd the CRAP out of this. With that much witness, you can all but bet you have a bunch of bent valves. You may think they move 'fine' but do a vacuum test on the chamber and see how poorly they are sealing.

If your seeing 8° movement in timing that's only 4° cam degrees. Still sloppy even for a junk gear drive 'take-out' from a sprint car.

[DieselSJ]

Originally Posted by Tom

I had a gear drive on the small block in my 78 when I bought it. Made cool whinning noises to a 16yr old. Eventually the weight sheared the key on the flywheel during a burnout and my valve to piston contact was a bit more dramatic haha. No more gear drives for me.

Sorry but that makes no sense. Flywheel has no key and flywheel is at the rear of the crank. Your valve/piston contact was more likely due to valve float from over-rev.

Originally Posted by Bigdav160

I don't recognize the piston, What engine is this?

My experience is: if the cam timing moves only one valve makes contact not both intake and exhaust.

Did you float both valves? Not enough spring pressure?

6.5 diesel. Everything is 100% stock. Governor in the pump keeps the engine from getting into valve float territory. Since the gear is out of round, the cam timing changes between intake and exhaust. I had hits on multiple intakes and one exhaust. The exhaust was the most dramatic.

Originally Posted by Marv D

Agreed! If you advance the cam you open the intake sooner, that would close the distance between the intake and piston just after TDC. And closing the exhaust valve sooner would OPEN the clearance between the exhaust valve and the piston.

Id suspect someone wanted to here the 'power steering pump whine' of the gear drive and over-rev'd the CRAP out of this. With that much witness, you can all but bet you have a bunch of bent valves. You may think they move 'fine' but do a vacuum test on the chamber and see how poorly they are sealing.

All valves are getting replaced. I'm taking no chances. Piston/valve clearance will be measured on the new engine. Actually with the diesel you can't hear the gear drive at all and the governor in the injection pump keeps it from over-revving.

I'm also not the only person that had this issue. I found another 6.5 owner that also had valve/piston contact with nothing other than a change from a timing chain to gears.

[Marv D]

DIESEL!!,, Ohhhh Very different animal. That fuel is proper for towing the stuff I was thinking of to the track,, and I know NOTHING about them. I mean still advancing / retarding has the same effect on VtP clearance. But still a very different creature with the low RPM they see.

[Bigdav160]

I have had one of those apart but it was 20+ years ago. And I did many of the injection pump campaigns back in '94-'95

What symptoms were you having that you pulled the head?

Another thing comes to mind. Stacking the rod bearing and having the piston come up above the deck surface. Since there is really no combustion chamber those valves would be really close.

[70K10Hunter]

well you didn't state in your post you were dealing with a diesel. the crankshaft gear is keyed and so is the cam, and like I suspected the gear drive set that goes in those engines isn't much different than a small block chevy, I did notice there is an idler gear size difference from one side to the other. you did not state why you pulled the head in the first place. If the idler gears were machined out of round LIKE I said they would bind up, you have 4 gears meshing and the 2 idler gears are mounted to a common bar which has a spindle on each gear they spin on there is no room for either gear to move sideways or any other way without getting in a bind, that is unless you have some sloppy fitting gears. my guess is somehow it skipped a gear or possibly sheared a key on the cam or crank. As you stated "if a cam gear is out of round it will speed up and slow down as it spins" WOW!!! I learned something new today.... IF the cam gear is out of round it will bind. It will rotate like a cam lobe, The crank can go nowhere the cam can go nowhere the 2 idler gears are caged they can go nowhere. SO yes the gears WILL bind. Obviously you are light years ahead of everyone else on mechanical motion. good luck with your engine.

[DieselSJ]

Originally Posted by 70K10Hunter

well you didn't state in your post you were dealing with a diesel. the crankshaft gear is keyed and so is the cam, and like I suspected the gear drive set that goes in those engines isn't much different than a small block chevy, I did notice there is an idler gear size difference from one side to the other. you did not state why you pulled the head in the first place. If the idler gears were machined out of round LIKE I said they would bind up, you have 4 gears meshing and the 2 idler gears are mounted to a common bar which has a spindle on each gear they spin on there is no room for either gear to move sideways or any other way without getting in a bind, that is unless you have some sloppy fitting gears. my guess is somehow it skipped a gear or possibly sheared a key on the cam or crank. As you stated "if a cam gear is out of round it will speed up and slow down as it spins" WOW!!! I learned something new today.... IF the cam gear is out of round it will bind. It will rotate like a cam lobe, The crank can go nowhere the cam can go nowhere the 2 idler gears are caged they can go nowhere. SO yes the gears WILL bind. Obviously you are light years ahead of everyone else on mechanical motion. good luck with your engine.

PM me your email address and I'll send a document from an engineering firm that did an analysis on the set of gears that came out of the other engine and you actually will learn something today. Then you can come back and tell me I was right. I'll be waiting for your email.

[DieselSJ]

Small excerpt from the report -

I installed my DSG set during reassembly of my engine last year and degreed the cam to determine the intake and exhaust lobe centerlines. I then removed the gear set and installed a stock timing chain and sprocket set and repeated the process. The DSG set put the exhaust lobe center at 125° BTDC and the intake lobe center at 97° ATDC. The stock timing set put the exhaust lobe center at 120° BTDC and the intake lobe center at 100°ATDC. This means that the DSG set has 5° of advance on the exhaust side and 3° of advance on the intake side. At first, it seems impossible to have different amounts of advance between exhaust and intake. I did the measurements three times. The reason that this is possible is the low quality level of the gears, especially the runout. There is 110° of lobe separation in the cam (due to errors in the DSG set, the lobes appear to be 111° apart with the DSG set installed). This means that the cam gear is meshing 111° apart at each lobe centerline as shown in Figure 4. The cam gear has over .0025 in runout. When the peak eccentricity is passing through the mesh with the main idler, the cam will be advanced from its normal position. When the nadir of eccentricity is passing through the mesh, the cam will be retarded from its normal position. This accounts for the variation in advance for the intake and exhaust lobes. This is for cylinder #1. The advance will vary from one cylinder to the other depending upon where the mesh position of the gear is when the lobes for a particular cylinder are at peak lift. I believe this advance and variation explains why I found evidence of piston to valve contact on some cylinders and not on others. The #2 piston from my engine is shown in Figure 5. There is clear evidence of near catastrophic piston to valve contact. I think that the advance is the result of a design error and is not intentional.

The cam is speeding up and slowing down due to the gear being out of round, which is why the amount of advance changes.

[Marv D]

Now wait a minute,, 0.0025" (two and 1/2 thousandths, but lets assume the typo and use 0.025) runout in the cam gear isn't going to make SQUAT. Simple geometry,, Even if it were a 10" diameter! the added radius of 10.0025 ,, at 180°
The total circumference in (exaggerating to 10.025 assuming a typo) 31.4945" in 360° vs 31.4159" for a perfect 10.000" diameter.
Lets throw this 0.00786" diameter difference ALL at 180°

The angle subtended by 0.00786" on a 10" diameter is 0.8981 degrees. Those are cam degrees so lets double it and call it 1.6 crank degrees. And that's a 10" diameter. It is less the smaller the diameter. NO WAY is this diameter difference making the claimed 4-6 degrees.

NOW, a crap off-shore machined gear that the teeth are not all ground at EXACT degree intervals WOULD,, but certainly NOT a difference in diameter of 0.00025 or 0.0025 either one. Knowing now gears are cut and fitted it would be very ODD for the teeth to be cut at uneven intervals,, but I have learned to never say anything is impossible when dealing with imported crap. It can be tough to convert the 400gon metric circle to our 360 degree circle for a left handed 7 year old machinist' in some of those factories.

[70K10Hunter]

the average human hair thickness is between .003 and .005 for comparison.
The OP never did state why He pulled the head.

[Ironangel]

Why a gear drive instead of the chain? I ask this because a friend just rebuilt his Olds 455 and installed a Cloyes Duel Idler gear drive timing set. What are the pros and cons to this mod other than the noise they make?

[DieselSJ]

Originally Posted by 70K10Hunter

the average human hair thickness is between .003 and .005 for comparison.
The OP never did state why He pulled the head.

The reason I pulled the head is totally irrelevant to the discussion.

But if you really must know it is because the engine always was mechanically noisy and it developed a bad tick in the valve train that got worse over a very short period. I pulled the valve cover and found a couple bent pushrods.

[DieselSJ]

Originally Posted by Ironangel

Why a gear drive instead of the chain? I ask this because a friend just rebuilt his Olds 455 and installed a Cloyes Duel Idler gear drive timing set. What are the pros and cons to this mod other than the noise they make?

Because on the 6.2/6.5 diesel the injection pump is driven off the cam, which puts way more strain on the chain than in a gas engine. Chain stretch is an issue. As it stretches it retards the cam and the injection pump timing. The gear sets were developed as a solution to that problem.

[Tom]

Originally Posted by DieselSJ

Sorry but that makes no sense. Flywheel has no key and flywheel is at the rear of the crank. Your valve/piston contact was more likely due to valve float from over-rev.

lol your completely correct. Key on crank snout sheared and stopped spinning the gear drive, freezing valvetrain. No idea why I wrote flywheel.

[garyd1961]

I'm not a master mechanic by any means but I have been working at dealerships for a long while. A gear would have to be drastically out of round to change the timing enough for a valve to hit the piston. To me it sounds more like it jumped a tooth or jumped timing as some would say.
I talked to one of our master mechanics today and you may want to pull the oilpan and check the rod bearings.