Chrysler 3 cyl two piece gear cases

[bloodygazza]

Hi all,

Seeking recommendations for the 'best' two piece lower unit combination. I'll explain the reason for my question.

Though I've been a long-time owner of a 1973 70hp with the early short ventilation plate, I have added 3 other 3cyl motors to the 'fleet' recently; a 1970 85hp, a 1974 70hp and a 1975 75hp (basically as parts motors).

Aside from the primary difference in lower units being a change from a short ventilation plate to a long ventilation plate, there are also three different styles of gear case mixed and matched throughout.

The 1970 85hp has a small, dagger-like skeg (fin) on the bottom of the gear housing (1" on the foot) and the early, short ventilation plate; both 70hp's, one being a short vent/plate, the other a long vent/plate, have a larger skeg (2 1/2" on the foot); the '75 75hp has of course the long vent/plate and a large skeg almost the full length of the gear case that leads with a generous radius.

As I have all lower units off and apart for checking from all four motors, I want to assemble the best combination (pinion gear will of course remain married to its original gear set).

My choice is to naturally use a long ventilation plate. I will be using the '75 75hp as my summer engine, which although it already has the matching midsection, it has previously had poor work to remove and replace the lower unit done, destroying several tapped mounting bosses. I will be swapping the 75hp power head onto the other, good, long vent plate midsection from the 70hp.

Although it appears that the later designed motor (the '75 75hp) had largest skeg, I'm wondering as to the reasons behind this. Its apparent Chrysler progressively changed the skeg design from quite a fine entry, dagger-like design to a full-length design.

What are people's theories about this design change? The longer ventilation plate is obvious, the skeg less so.

I propose the use the mid-length skeg. I have a background in sailing performance sailboats and have a rough idea about optimal foil shapes, underwater profiles, etc. What I am also planning to do when one of each type of lower unit is rebuilt and ready to use, is to exchange all three and note any performance and handling differences/benefits.

Images of the skeg types (I shall take better photos and post them when my camera charges up):

Images 1 (not my motor) - early small skeg the same as my '70 85hp:

Image 2: medium skeg as fitted to my '73 & '74 70hp's. Note longer "foot": (this is one of my recent purchases):

Image 3: long skeg; (sorry, impossible to see, I will do better later):

Image 4: (sourced from the web) large skeg on short vent plate!

 

[Frank Acampora]

Re: Chrysler 3 cyl two piece gear cases

Believe it or not, aside from extremely minor turning differences due to skeg size, all three lowers perform just about the same; the main difference is above the cav plate and there is no drag or speed difference. all parts from all lowers are interchangeable with an exception I will note below.

Because it looks better and offends my sensibilities less, I prefer the tapered tail, long cavitation plate lower. The bolt under the exhaust snout is also shorter so you are not threading in a six inch long one to hold the tail of the unit. The longer cavitation plate does make clearance at the exhaust snout for higher pitched props. And it looks like the earliest unit (image 2 ) has the smaller snout. Later units have a longer and more adjustable snout for better scavenging and better torque trim. BUT: Smatter of fact, all three lowers have the same mounting bolt pattern and footprint and will bolt to any midleg--so you can use any one on any engine you have. Image 4 appears to me that someone may have broken a skeg and used a later model gearcase torpedo on the upper half. Notice that the wear patterns do not appear to match. That MAY be a 3 stud gearcase on an earlier upper.

The longer cav plate lower unit has three studs holding the gearcase torpedo to the upper section whereas the earlier units have only two. SO: if for some wild reason you wanted to put it on the ugly lower unit , you would need to drill another hole on the upper section just in front of the water pump, or remove the center stud.

Since the driveshaft lower bearing locates the two halves, the third stud does not really contribute a lot to strength.

As far as I know, all three lowers have the same gear ratio: about 1.72 to 1. If you want to perk up any of those engines a little, see if you can find a gear pack off a 90 HP. This has 2 to 1 ratio gears. The two piece Chrysler lowers are also set up "loose" The forward gear is usually shimmed to the bearing cone and the rear bearing is shimmed into the prop shaft gland. So, If you find a need to mix and match gears, as long as they are the same ratio, it can be done. Be aware that the pinion is mounted on straight splines so proper torque of the nut is essential to proper gear mesh. Also be aware that the two gearcase halves set the pinion depth. SO: if you try to true up the mating surfaces, add a gasket, or change the mating of them, the pinion depth will change and service life will decrease. This decrease will be directly proportional to the difference from original mating.

As an aside: Incorrect shimming of the reverse gear, with insufficient end play, will heat up the lower. When running at speed, the engine will "harden" to a stop, then since the lower unit is in water it will immediately cool. You will be able to immediately start and run the engine, wondering what the hell happened.

As I understand, all three lowers will "blow out" above 60 MPH. I never had any of this vintage Chrysler up to that speed so I must take the word on it. With your engines, I doubt you will go fast enough to test it either.

Anything else, feel free to ask. Frank.

 

[bloodygazza]

Re: Chrysler 3 cyl two piece gear cases

Thanks Frank, fabulous input.

It's been a very worthwhile exercise having the different types of units dismantled simultaneously for comparison.

Strangely, all of my units, 2 long plate, 2 short plate, all have three-stud mounting of the gear case to upper; all fit and seal on all despite the variation of tail shape

The configuration I believe I will settle on for the 75hp is: long ventilation plate, mid-sized skeg, agreeably for asthetics and simplified bolt layout. This will be fitted to a 17'6" half-cabin (sorry, don't know the weight). I currently have a 15p prop on the 70hp, with one of the other motors came a 17p prop which I am interested to try with the 75hp and 1.72 gearing. I have also located a reasonably price 17p stainless prop which I don't think I should pass up even to hang on the wall ready for the 85hp porting project motor mentioned in another thread.

90hp 3cyl's appear for sale not infrequently in Aust, so I might see if I can snap one up even just for the 2:1 gearing for the 85hp/17p combo.

This current project should be finished in the next week or so, I'll report how I went.

Kind regards,

Gary.

 

[Steven P.]

Re: Chrysler 3 cyl two piece gear cases

I have 3 new lower gearhousings for the motors you refered to. If I can help you at any time e-mail me @ bayouboatdoctor@aol.com

Thanks

Steven P.