Chrysler 79 20hp gearbox help (207H9B)

[mark1961]

I have replaced all bearings in the lower gearcase and have installed all shims/thrust washers in their original places. All other parts appear serviceable. When i consulted the Clymers to look up how to adjust (shim) pinion gear & prop shaft end float i found that it is not addressed in this manual for my engine only for ones over 55hp. I have made a tentative attempt at reassembly but something doesnt seem right, its as if the pinion gear is in mesh a little too far with the forward and reverse gears which is causing the pinion gear to operate roughly. Does anyone have any info on the correct setup for one of these gearcases?

Thanks in advance.

 

[Frank Acampora]

Re: Chrysler 79 20hp gearbox help (207H9B)

If that is the 2 piece gearbox, there is no adjustment on the pinion. It is held in position by the relief its bearing sits in in the upper half.

Is the pinion fully seated in the ball bearing? Is the bearing fully seated in the upper half when the lower unit is assembled? Did you use the correct thickness gasket between the two halves? Are you certain the bearing is fully seated in the prop shaft gland? Is the forward gear bearing outer race fully seated in the casting? Did you true up the mating surfaces of the two halves and take off .010 or more? Any of these conditions will cause the pinion to mesh too far into the drive gear teeth.

Bearings are manufactured to such close tolerances that I would expect new ones to use the same shims with no problems. However, Murphy's Law always takes precedence.

 

[mark1961]

Re: Chrysler 79 20hp gearbox help (207H9B)

Thanks for the reply Frank, it is a two piece box btw, i have been a bit busy with other things to get out to the shed and check on all of the possible issues youve raised, i will get back to you in a couple of days with some answers. Cheers.

 

[mark1961]

Re: Chrysler 79 20hp gearbox help (207H9B)

Okay, i have managed to check out the following,

The pinion gear is fully seated in the bearing with original shims.

The pinion gear bearing does fit into the upper half relief when the two halves are fully mated but DOES NOT fully seat, i know this because i measured how far the pinion bearing outer race protruded above the lower face and also measured the depth of the relief in the upper case, there remains a gap of around .024in, this is what makes me think that the pinion is in mesh too far as it should be much less or next to nil correct? Does a value exist for this?

The gasket between the two is from a quicksilver lower unit seal kit FK1068, its an o ring type and afaik it is the correct part and not an issue.

The prop shaft bearing is fully seated in the bearing carrier/prop shaft gland(?).

The forward gear bearing outer race is fully seated in the casting and the forward gear bearing is fully seated on the gear with original shims.

I did not true up the mating surfaces, they appear to be in good condition are unmolested and mate together perfectly.

I also measured the overall prop shaft end float with pinion removed, dial guage came up with a reading of .017in. Would like to know exactly what this reading should be as .017in seems excessive.

I should also mention that the old bearings were timken and the new ones are nachi although i doubt this would be an issue as all part numbers match.

To me it seems as if the forward or reverse or both gears need to be shimmed closer together which should raise the pinion and bearing up nearer to its correct position and hopefully smooth out its operation?

Any input appreciated.

Cheers

 

[mark1961]

Re: Chrysler 79 20hp gearbox help (207H9B)

Bump

 

[Frank Acampora]

Re: Chrysler 79 20hp gearbox help (207H9B)

The bottom (top) of the upper gearcase half relief for the bearing is radiussed to match the radius of the edge of the outer race of the bearing. If your Nachi bearing has a sharper radius on the edge than the original Timken, this will keep the bearing from seating all the way into the relief thus: 024 difference which is way more than enough to cause rough operation. Even though Chryslers were set up loose, this is still enough to cause problems. Normal rotation of the crankshaft, because of the cut of the gear teeth, will push the pinion and bearing assembly up into the seat. That is why there is no locating land on the lower half of the gearcase. You do not want to relocate the forward and reverse gears closer together. This will aggravate the pinion mismatch. Depth of the pinion engagement is controlled by the pinion bearing, NOT the gears.

So: if the radius on the edge of the Nachi is sharper, mill the radius in the relief to match so that the bearing seats fully.

As far as 017 end float, I am not sure there is a spec for it. The big engines use between .006 and .010, so this does seem excessive, but since it does not affect the spacing of the forward and reverse gear, it probably won't cause trouble.

 

[mark1961]

Re: Chrysler 79 20hp gearbox help (207H9B)

You have solved the problem Frank.

I hadnt realised the pinion/bearing assembly needed to be able to float upwards when running due to the cut of the gears, this would explain the .024 clearance at rest. I have checked that the new Nachi outer does fully seat by placing it in the relief seperately with the upper half inverted, the radius is the same. Fingers crossed the pinion operation during running should be normal.

Thank you very much