1969 70 HP Lower Unit Gears

[cclovern]

New to the site , I have found a lot of good info here while working on my newest project 1976 Sea Nymph. I have disassembled my lower unit and found the gear set destroyed. The previous owner included another complete 70 HP Chrysler in the deal. I pulled it apart to find it was used in salt water and the case was badly pitted. I only used the gears from this lower unit.

I installed new bearings and seals in both the gear case and upper portion , I installed all shims from the spare set of gears as they were removed. The gear case and upper section spin fine until they are bolted together , then the unit binds up. I have set the gear case .010" thrust as described in the manual, and tightened the input gear to the drive shaft nut until I felt a slight drag. I took everything apart again to check for debris behind the bearing cages and shims. The only item I changed besides the gears was the upper portion of the lower unit housing . Is it possible the dimensions are different in the upper section ? If so is it worth trying to repair or just find another lower unit ?
Any insight or help is appreciated.

 

[Frank Acampora]

Re: 1969 70 HP Lower Unit Gears

The drive shaft is carried in two tapered roller bearings and the pinion nut needs to be torqued to a specific value--offhand I forget what. Top bearing is located on the drive shaft by an external circle clip. The pinion is slip fit on the shaft splines and the nut controls both the pinion and the compression between both bearings..

Be certain both races are fully seated in the casting and be certain the circlip is in the groove on the shaft. Sounds like the pinion is too deep into meshing with the drive gears. It needs to be set so that there is about .005 backlash and this would be set by the top besaring and shims.Since every case is slightly different, you may need to re-shim the top bearing.

 

[cclovern]

Re: 1969 70 HP Lower Unit Gears

Frank

Thanks for the info , I have checked the circlip and the upper bearing is against it. The SELOC manual recommends clamping the input shaft in a vise and slowly tighten the pinion nut until slight drag is felt on the shaft while turning the case, I have reset that twice.
I will check for shims under each bearing and on the drive shaft.
I checked both bearing seats and they were clean and appeared to seat well.

 

[Frank Acampora]

Re: 1969 70 HP Lower Unit Gears

Yeah, that slight drag they mention is actually pre-loading the two bearings. Run the nut really tight, and the shaft will turn hard. Run it tighter yet and it will jam.

Again, the shims under the top bearing are the ones that set the pinion depth and backlash. You need to change them until engagement is correct. There is a "sweet spot" of .005 backlash, but I have been told by old time Chrysler mechanics that the lower can be set up "loose."

You can feel it by hand, but be aware that a slight error will eventually ruin the gears. Example: on my first engine, when I was still wet behind the ears, to even the case and make the O ring seal better, I sanded about .010 off the joint between the bullet and top portion of the lower unit. This did change engagement and the gear set failed in one season.

If you eventually find an old junker 3 or 4 cylinder engine of the late 70s to early 90s, they have the "newer" one piece lower unit. It is a superior design to the two piece and runs about 3 MPH faster--and--- it is easier to get props for it. Everything from your existing engine will bolt right onto the midleg.

 

[cclovern]

Re: 1969 70 HP Lower Unit Gears

Frank

Thanks for the help on this issue , I will keep my eyes open for a lower unit. I have found a 105 lower unit in my area in the same vintage. You stated the newer designs will bolt right up ? Are there specific years and is there a gear ratio difference between horse powers ?

 

[Frank Acampora]

Re: 1969 70 HP Lower Unit Gears

Chrysler and early Force only used two ratios: 1.78 and 2 to 1. The 90hp almost always had 2 to 1 and most others had 1.78 to 1. The 2 to 1 will give better performance but may need a pitch change from, for example, 17 to 19 pitch prop..

To find out which ratio it is, simply turn the drive shaft in gear. If the prop only turns one rev for two of the shaft, it is obviously 2 to 1. 1.78 will turn about 1 1/4 turns.

The engine mounting bolt pattern was never changed so yes your block and all its goodies can bolt right up to almost any midleg and lower unit. - that is, any three and four cylinder midleg.

 

[Frank Acampora]

Re: 1969 70 HP Lower Unit Gears

I should clarify though, that the newer one piece lower unit will not bolt up to the older midleg. The two lower units use a different mounting pattern and the one piece is much larger. Thus, you must swap the block onto a newer midleg if you want to upgrade to a one piece lower unit.