Rotary shaft keeps shredding gear

[mbedford]

Hello All! Great forum and community. This is my first post but I have read tons of information. I need some advice on my 2000 GTX RFI 787 motor. I just picked this thing up from somebody who didn't want to fix it. It has a new battery and turns over fine. So, I went through the process and found that plugs are new and have spark, fuel pressure is good, etc...

Upon digging, I found that the rotary valve was not turning. So, I tore it apart and pulled the shaft and found as suspected, the brass gear was shredded down on one half.

I know, I know, I read so many conflicting posts. Some say with the brass in there, I have to pull the motor and rebuild the lower half. Others have said it is just brass and being a soft metal and sealed away from the cylinder/connecting rods, just clean it out best you can and go for it. I know that it would probably be best to tear it apart but I want this one to be a quick and dirty fix for now. I have a small hose attached to my shop vac and was able to feed it all around and suck up as much as I could. I then poured some oil in there, turned it over and sucked the oil back out as a way to "rinse" it out. This is good for now.

So, then, I bought a whole new shaft which came with gear, seal, bearing, etc... and pressed it in. A new rotary valve plate to go along with it too. Did the timing and put it all together. She fired right up after. Several times of shutting down and firing up, every time, perfect. The next day, I decided to run water through to run it longer. I started her up and turned on the water. She was smoking really bad at first, assumed it was due to all of the work I had done. Ran at idle for about 5 minutes and was running strong. Did some revs, no problem. Towards the end of the 5 minutes, I went to rev and it sounded great but then it started bogging down and died. Went through some things and ultimately found, my new rotary shaft brass gear shredded again, even worse than the original. All the way around instead of half flat.

I researched some more and read that the gear on the crank could be off center but it doesn't appear to be. Seems to be centered right in the hole. Also, the condition of that gear looks perfect. No bent or missing teeth. Also, cranked while looking in the hole and the crankshaft doesn't appear to bent or wobbling at all. So, now I am left with some questions. Why did it fail the first time? More importantly, why did it run for a while for me with a brand new part and then fail again? What could cause this besides the things I already listed which seem to check out?

I will tell you what I suspect. Could the oiling system not working right cause this? I see the two small lines that go from the pump into each port. These must be to do the premix for the fuel? Then there is a large line below this center rotary shaft chamber. I assume this is to oil that chamber? I will tell you, it seemed rather dry in that chamber when I pulled the new shaft back out. The rotary valve itself had oil on it but not much inside of the chamber.

Another theory. Is it possible that I didn't get seated in the rear bearing inside of the engine? I must admit, when I pulled the first/original shaft, I didn't pay any attention to that rear bearing and just jammed the new rotary shaft in there. I assumed it would all line up but what if that bearing had slipped out of the seat when I pulled the first shaft and therefore, the new one was just floating around back in there?

Sorry for the long story, wanted to paint the picture. I can check the oiling system and be extra careful when I re-install with regard to that rear bearing but wanted to get thoughts from all of you. I don't want to put yet another one in and shred it too. Just another side note. The motor has low hours and looks to be in really good shape. Compression in both cylinders is 145 and 150. So, I hate to tear it apart if I don't have to.

Thanks!!!

Mike

 

[AKnarrowback]

The rotary valve shaft chamber should be full of oil, no dry spots on the line or inside at all. It's an oil bath.

You need to take that motor apart, no quick fixes here.

Lack if oil can kill the gear, you might have bent some brass putting the shaft back in, residue of the old brass gear smeared onto the crank shaft gear could have worn down the new gear.

Again YOU NEED TO TAKE THE MOTOR APART to do this job right.

 

[jts32]

Again YOU NEED TO TAKE THE MOTOR APART to do this job right.

That is the only correct answer! You will never get all the brass shrapnel out without splitting the cases, and you need to find the cause of the original failure.

 

[POPPS]

AK and 32 are right on, George Washington spent 4 hours sharpening his axe, but only 2 hours cutting the tree.

 

[mbedford]

Okay, thank you all for the responses. I am fine with that answer if it is the best and only way. Guess I have a job to do in the coming days.

Two quick questions related: First, even if the crankshaft and gear "look" okay, should I plan on replacing/rebuilding it or just clean everything out and replace bearings/seals? Second, if it should be an oil bath, there is definitely a problem there. But, the pump doesn't feed that area does it? Seems to run right from the tank into the bottom of that chamber. Aside from an empty tank (which I don't have) seems to me that the only thing that can prevent oil from getting in there is a clogged hose or something? Is it gravity fed?

Thanks again everybody!

 

[mbedford]

One other thing I am trying to wrap my head around as a clue to what happened. The first shaft had a flat spot on the gear, which seems to me would happen if the rotary valve seized up or was blocked from being able to spin.

However, the new gear was ground all the way around. This tells me the crank was grinding it or ground it down WHILE the rotary valve was also actually spinning. In trying to figure out the root cause, could heat do this or does this point to the gear, while it looks centered in the hole, is actually off?

 

[AKnarrowback]

You need to replace the rotary valve shaft bearings and seals.

The oil bath is not fed by the pump. Thereare two large oil lines that allow the area to fill with oil using gravity since the area is below the tank.

Even though the RV shaft hear is brass it can "smear" onto the steel gear on the crank shaft. This smeared material can make a rough surface and chew up the next brass gear in a short time. I learned the hard way on this one when I thought I had gotten all the brass residue of the crank gear when doing a rebuild, the new brass gear only lasted 15 minutes.

One problem you have to consider now is if the RV shaft seals on the crank shaft are damaged now from brass shavings. The only way to put new seals in is by rebuilding or replacing the crank.

 

[mbedford]

Okay, thank you for all of the great input. Here is what I am looking at and if I have this wrong, please somebody feel free to correct me.

Since the motor seems to be in great shape and does have good compression, I am going to leave the upper alone. I am going to buy a rebuilt crank and new RV shaft. I am going to tear apart the bottom end and replace the crank and clean out everything. I will also replace the two RV chamber crankshaft seals while I am in there to be safe.

Finally, as for the root cause of the issue, I will make sure that the lines are not clogged and that the tank feeds oil by gravity through the line and into the engine.

Sound like a plan?

Thanks!

 

[mikidymac]

You need to find out what caused the first one to strip.
It is almost always a bolt falling down the intake and locking the rotary disc. Something caused the disc and shaft to stop turning.

You should also replace the rear shielded rotary shaft bearing.

The new crank comes with new inner crank seals, they are not something you can change and are part of the crankshaft.

Finally the rotary gear cavity is fed from gravity out of the tank, the pump has nothing to do with it. Oil comes in from the tank on the carb side and flows up the hose on the exhaust side to the level of the tank. The oil does not circulate, it just stays as full as the tank and should never be dry. IF you pulled the shaft with the hoses attached and tank full it would dump the entire tank volume out of the shaft hole.

 

[GGuillot]

Okay, thank you for all of the great input. Here is what I am looking at and if I have this wrong, please somebody feel free to correct me.

Since the motor seems to be in great shape and does have good compression, I am going to leave the upper alone. I am going to buy a rebuilt crank and new RV shaft. I am going to tear apart the bottom end and replace the crank and clean out everything. I will also replace the two RV chamber crankshaft seals while I am in there to be safe.

Finally, as for the root cause of the issue, I will make sure that the lines are not clogged and that the tank feeds oil by gravity through the line and into the engine.

Sound like a plan?

Thanks!

I'm not sure of the RV shaft bearing numbers if it's the same on the carb 787. When replacing the one pressed in the case, the smaller one, I heat the oven to 300 degrees and drop it in for 10 minutes and it makes it easy to remove and replace.

I don't see how you did this job with the engine in, as mentioned above, pull the engine and strip it down completely to do it properly. I'd wash the cases to make sure to get all the shavings out.

 

[mbedford]

Well, while I already said I am going to tear apart the lower half and I still will, that will mostly be for cleaning at this point. I believe I found the root cause of the issues and you guys are never going to believe what I found (or maybe you will believe it). I have attached some pictures. Somebody capped off the oil feed!!!! I took the hose off and oil started pouring out, which is good. But then, under the line, I found this cap. In other words, the cap was on the elbow first and then the hose over the cap. I confirmed the chamber was pretty much bone dry, as expected.

I also confirmed that the return line, on the opposite/back side of the motor is NOT capped off, so at least this is good.

I have no idea why somebody would have capped this off. When I assembled the new gear, I used engine assembly lube on the gear. So, this explains why it ran for a while and then probably at some point started eating away at the gear until it ground it down all the way towards the end. I guess this also points to how critical this oil is to the brass gear and the fact that the engine will only last about 5 minutes before the heat and friction destroys it.

What sucks is, if I do the leak test and it is good, which means my seals are good and the rest of the motor is good (compression, etc...) that I will be tearing this thing apart just to clean out the brass shrapnel. I know that is the right way, for longevity but this really sucks after I found the smoking gun.

Can you believe this? Any possible reason somebody has to explain why this might have been capped off?

 

[POPPS]

I can only, some how think, that the inner oil seals leak, and this move, reduces the leak

 

[AKnarrowback]

I have a 97 GTX that has a leaking inner crank seal. It is not uncommon for these seals to leak. My GTX would leak enough oil, over the course of a week, where I would have to pull the spark plug and crank the motor to clean the oil out and even then it smoked like crazy for a minute. I did a thick oil / very liquid grease experiment on GTX with, so far, good results.

You can do a quick leak test on the RV shaft chamber before you take the motor apart to see if the crank has bad seals, you won't be able to tell after you take the motor down.

 

[mbedford]

@AKnarrowback so, are you suggesting that you are doing an experiment where one might cap off the oil feed, like mine was and semi-permanently seal the RV cavity with fluid that is thick enough that it doesn't easily leak through the seals? This is similar to the concept of an oil pan on a 4-stroke. What type of oil are you using just out of curiosity?

 

[AKnarrowback]

Doing an experiment, yes.

Do I suggest others do it, no.

I am putting a "band aid" on something that needs to be fixed properly, a crank replacement. I'm willing to take the risk because I'm familiar with motors, have access to oem parts at a discount and have access to all the tools I need to do a rebuild when it fails. I'm just curious to see how much longer I can get out of it.

As for what oil I used? It's sort of a mix of very light weight grease and heavy oil, no one makes it, that I mixed up on my own. Just a mix of experience, guess work and some luck.

In your situation it sounds like there has already been enough damage to require a new crank since it appears the seals might have failed on someone in the past. If the RV shaft chamber was run out of oil I would call the seals shot since they were run dry with metal shavings in the area. Do not think you can re-use your current crank and have a reliable motor.