How to measure a Lower Unit Drive Shaft?

[DGMAX]

I have a ~1980 Mercury 175HP V6 with the lower currently removed, (but lower unit is still assembled). Last year the prop caught a submerged limb and it broke a section of the cavitation plate off. I am trying to determine the drive shaft length so I can purchase a spare lower unit while my OEM unit is repaired. But I am not sure where to take the shaft measurements from to match the dimensions provided by sellers. I currently have the water pump assembly completely removed, including the base.

I am getting a measurement of ~25" from the driveshaft stop collar above the case upper bearing to the tip of the drive shaft, (with a half inch of that being what appears to be an alignment pin, see pics).

Is this the proper way to measure the drive shaft with the lower unit still assembled?

Thanks!

 

[Texasmark]

I don't know to start! With that said, I would think that since shaft lengths vary by 5"....15", 20", 25", a 1"variation in the gear teeth length or what have you is negligible.

 

[DGMAX]

I have since swapped some messages with a vendor in FL who sells lower units. He instructed me to measure from the lower unit casing, flat mating surface (the surface which mates to the mid section), to the top of the splined shaft. From those points mine measures ~23 1/4" (not including the small tip he referred to as a preload pin). Based on that measurement he stated I needed what they refer to as a "20" lower unit".

Why they don't just use the dimension from the flat mating surface to the end of the splined shaft is beyond me! Would think that would be the set/critical measurement to ensure the lower unit fits. I do not get 20" from any of my available points to measure from, yet I am told that is what I need.... Go figure.

 

[racerone]

??---Go figure.-----Shaft length has to do with transom height.-----It is not the length of the driveshaft.

 

[DGMAX]

??---Go figure.-----Shaft length has to do with transom height.-----It is not the length of the driveshaft.

I was commenting in terms of finding a suitable lower unit replacement, not in determining the correct overall foot length of a complete outboard to be installed on a boat based on a given boat/transom height.

When someone is "replacing" just the lower unit, the dimension from the flat mating surface to the top of the splined shaft of a removed lower unit is a fixed dimension, which should not really vary. The proper outboard shaft length vs. transom height was already determined when the outboard was selected in the first place, and the proper mounting height set accordingly..

 

[racerone]

Well , yes it does vary!!-----Examples are abundant.----Take a 1963 Johnson 10 hp verses a 1964 Johnson 9.5 HP.---Both short shaft motors.----Gearcases are the same.----Driveshaft diameters are the same.----Only the length of the shaft is different.-----Design of the upper part of the motor and spline position determine all this.----Short shaft or long shaft.----Both have the same amount of lower unit below the hull.

 

[DGMAX]

Well , yes it does vary!!-----Examples are abundant.----Take a 1963 Johnson 10 hp verses a 1964 Johnson 9.5 HP.---Both short shaft motors.----Gearcases are the same.----Driveshaft diameters are the same.----Only the length of the shaft is different.-----Design of the upper part of the motor and spline position determine all this.----Short shaft or long shaft.----Both have the same amount of lower unit below the hull.

Yes they may vary if the shaft length is different between units to begin with, (short vs long vs XL).

"But" when some one seeking a direct replacement for their lower unit, (power block and midsection remaining the same), and the lower unit is removed and the exposed shaft is measured from the upper flat mating surface of the lower casing to the top of the splined shaft, would think one would be looking for the same length shaft and interchangeable case design as a direct replacement. (as in short vs short, long vs long, XL vs XL).

Again, my point is that when someone is "replacing" just the lower unit "like for like", the dimension from the flat mating surface to the top of the splined shaft of a removed lower unit should be the same for the replacement lower unit when the outboard application remains the same also. That was the whole point of this thread and my original question.

 

[jimn]

I have a ~1980 Mercury 175HP V6 with the lower currently removed, (but lower unit is still assembled). Last year the prop caught a submerged limb and it broke a section of the cavitation plate off. I am trying to determine the drive shaft length so I can purchase a spare lower unit while my OEM unit is repaired. But I am not sure where to take the shaft measurements from to match the dimensions provided by sellers. I currently have the water pump assembly completely removed, including the base.

I am getting a measurement of ~25" from the driveshaft stop collar above the case upper bearing to the tip of the drive shaft, (with a half inch of that being what appears to be an alignment pin, see pics).

Is this the proper way to measure the drive shaft with the lower unit still assembled?

Thanks!

If you have the serial number from yours, you should be able to find the correct part using its serial number- ALL OEM part replacements use serial numbers to find the production timeline and era. If you find parts that don't come from a complete outboard/sterndrive that have been disassembled, be patient and find something that can be verified as correct.

 

[DGMAX]

If you have the serial number from yours, you should be able to find the correct part using its serial number- ALL OEM part replacements use serial numbers to find the production timeline and era. If you find parts that don't come from a complete outboard/sterndrive that have been disassembled, be patient and find something that can be verified as correct.

I have the Outboard S/N, and have been able to determine it is either a L (Long) or XL (Extra Long) per the online Mercury Marine parts catalog. It is a ~1980 model (S/N 5489172) outboard but there are no visible numbers stamped on the upper end of the driveshaft, (above the stop collar at the upper bearing).

I have determined this much from the serial number:
The gear ration is 1.87
The driveshaft has 8 splines.
The shift shaft has 6 splines.
The prop shaft has 15 splines.

The upper end is still assembled, as is the lower unit, the lower end is just separated from the midsection so I could do seasonal water pump rebuild and to get the shaft measurement so I could pursue purchasing a complete lower end to setup as a spare/backup, (and use while I have the broken cavitation plate repaired on my OEM unit).

Easiest way I see to pursue the replacement is to compare applicable case year and HP range, including the shaft measurements/specs, spline count, etc... Seems there are plenty of used ones available, including some new aftermarket units. I'm just trying to ensure I target the correct version!

Thanks!

 

[jimn]

I have the Outboard S/N, and have been able to determine it is either a L (Long) or XL (Extra Long) per the online Mercury Marine parts catalog. It is a ~1980 model (S/N 5489172) outboard but there are no visible numbers stamped on the upper end of the driveshaft, (above the stop collar at the upper bearing).

I have determined this much from the serial number:
The gear ration is 1.87
The driveshaft has 8 splines.
The shift shaft has 6 splines.
The prop shaft has 15 splines.

The upper end is still assembled, as is the lower unit, the lower end is just separated from the midsection so I could do seasonal water pump rebuild and to get the shaft measurement so I could pursue purchasing a complete lower end to setup as a spare/backup, (and use while I have the broken cavitation plate repaired on my OEM unit).

Easiest way I see to pursue the replacement is to compare applicable case year and HP range, including the shaft measurements/specs, spline count, etc... Seems there are plenty of used ones available, including some new aftermarket units. I'm just trying to ensure I target the correct version!

Thanks!

What letters are ahead of the serial number- the Mercury Parts catalog requires those.

 

[DGMAX]

What letters are ahead of the serial number- the Mercury Parts catalog requires those.

There are no letters in my S/N, "5489172" is it. Per the Mercury Marine online parts catalog, it would appear to fall in the "4301235 Thru 0C100560" serial number range for 175HP Carbed Outboards.

 

[jimn]

There are no letters in my S/N, "5489172" is it. Per the Mercury Marine online parts catalog, it would appear to fall in the "4301235 Thru 0C100560" serial number range for 175HP Carbed Outboards.

That's what I get for entering the serial number at the top before checking the tree view showing the HP.....

I'll assume the gear ratio is 1.86:1 since they don't show anything with 1.87:1, but they make the distinctions using the number of teeth on the pinion gear, not the driveshaft, shift shaft or prop shaft. The last three have that number of teeth in most/all models.

How long is the shaft from end to end?

 

[DGMAX]

That's what I get for entering the serial number at the top before checking the tree view showing the HP.....

I'll assume the gear ratio is 1.86:1 since they don't show anything with 1.87:1, but they make the distinctions using the number of teeth on the pinion gear, not the driveshaft, shift shaft or prop shaft. The last three have that number of teeth in most/all models.

How long is the shaft from end to end?

The lower unit is still assembled, just removed from the midsection & power block, so I don't have the exact "end to end" length.

What I do have is the measurement from the stop collar on the shaft right above the upper case bearing, which is about the same as the measurement from the water pump base mounting area up to the splined shaft end, ~24 1/2" (without the preload pin).

I also have the measurement from the case to mid section mating surface to the splined shaft end, which is ~23 1/4" (also without the preload pin included). This, I am told, is the proper way to measure the still installed shaft to determine what lower unit is needed, see attached pics. Per a vendor of lower units, from this measurement I have been told I need a "20" Lower Unit".

 

[jimn]

The lower unit is still assembled, just removed from the midsection & power block, so I don't have the exact "end to end" length.

What I do have is the measurement from the stop collar on the shaft right above the upper case bearing, which is about the same as the measurement from the water pump base mounting area up to the splined shaft end, ~24 1/2" (without the preload pin).

I also have the measurement from the case to mid section mating surface to the splined shaft end, which is ~23 1/4" (also without the preload pin included). This, I am told, is the proper way to measure the still installed shaft to determine what lower unit is needed, see attached pics. Per a vendor of lower units, from this measurement I have been told I need a "20" Lower Unit".

Is your OEM gearcase at a local shop? Can you go there and measure it? What are they doing in order to repair the cavitation plate, just welding new material on, or disassembling beforehand?

Since it's a 1980 unit, it might not be a bad idea to remove the bearing carrier, so you can inspect the gears, replace the O-ring & seals (or replace the whole thing since a new one comes with the bearing and seals). At that point, you're just one (pinion) nut away from holding the pinion gear and shaft in your hands. You can buy Mercruiser specialty tools on ebay and if you decide that you don't want to keep them, they can be resold.

 

[DGMAX]

Is your OEM gearcase at a local shop? Can you go there and measure it? What are they doing in order to repair the cavitation plate, just welding new material on, or disassembling beforehand?

Since it's a 1980 unit, it might not be a bad idea to remove the bearing carrier, so you can inspect the gears, replace the O-ring & seals (or replace the whole thing since a new one comes with the bearing and seals). At that point, you're just one (pinion) nut away from holding the pinion gear and shaft in your hands. You can buy Mercruiser specialty tools on ebay and if you decide that you don't want to keep them, they can be resold.

Naw, I still have the unit at the house, probably wait till the end of season to do a repair now. It is still usable as is and all the internal mechanics seem to be working fine boat still gets on a plane fine and I haven't noticed any major increase in cavitation. It is just a section out the left plate back area. May be able to do the repair without complete disassembly. I may even try to weld up a scab plate to it and dress it down myself if I can ever find the time.

I'm looking at available new and used units, just haven't committed to a purchase yet. I will be sure to get however the seller is to confirm the dimension from the mating surface to the end of the splined area of the shaft, along with the other specs. First vendor I was talking with just didn't seem to want to tell me where to take the measurements from, another didn't even respond when I asked. That led me to post the question here, hoping someone had been through the same issue before when replacing their lower unit. But after posting I had a vendor out of FL who was clear on where to measure from, but it still seems the figures are generalizations and/or approximates.

 

[jimn]

Naw, I still have the unit at the house, probably wait till the end of season to do a repair now. It is still usable as is and all the internal mechanics seem to be working fine boat still gets on a plane fine and I haven't noticed any major increase in cavitation. It is just a section out the left plate back area. May be able to do the repair without complete disassembly. I may even try to weld up a scab plate to it and dress it down myself if I can ever find the time.

I'm looking at available new and used units, just haven't committed to a purchase yet. I will be sure to get however the seller is to confirm the dimension from the mating surface to the end of the splined area of the shaft, along with the other specs. First vendor I was talking with just didn't seem to want to tell me where to take the measurements from, another didn't even respond when I asked. That led me to post the question here, hoping someone had been through the same issue before when replacing their lower unit. But after posting I had a vendor out of FL who was clear on where to measure from, but it still seems the figures are generalizations and/or approximates.

If the cavitation plate is damaged at the rear, I'm guessing that whatever hit it was pushed upward by the prop- if you haul the boat out after using it, spin the prop while it's in neutral and check for wobble in the hole on the propshaft and make sure the prop isn't damaged. The good news- if the propshaft is bent, a machine shop can straighten it.

You could contact the boat manufacturer- give them the Hull ID for the boat and they might have the build sheet.

 

[DGMAX]

If the cavitation plate is damaged at the rear, I'm guessing that whatever hit it was pushed upward by the prop- if you haul the boat out after using it, spin the prop while it's in neutral and check for wobble in the hole on the propshaft and make sure the prop isn't damaged. The good news- if the propshaft is bent, a machine shop can straighten it.

You could contact the boat manufacturer- give them the Hull ID for the boat and they might have the build sheet.

This actually happened last spring. I think the prop just picked up a submerged branch and hammered it into the plate, breaking it off. Luckily I was just idling slow when I felt it pop. I checked the prop and everything else and all seems to be fine. We actually used the boat all last summer since the place I was going to get it fixed at was already backed up with business. I may run it again this summer and just focus on the repair at the end of the season.

The Merc is on a Checkmate hull, pretty sure the dealer installed the outboard at the shop. That dealer is long gone now tho. At this point it is just a metal casing repair, complete casing replacement, or entire lower unit replacement. My current plan was to purchase a complete lower end but still fix my original one, so I would then have a complete backup, "just in case". We live on a deep water lake and the lower end seldom encounters any obstruction, just the occasional log, branch or stick.

 

[jimn]

This actually happened last spring. I think the prop just picked up a submerged branch and hammered it into the plate, breaking it off. Luckily I was just idling slow when I felt it pop. I checked the prop and everything else and all seems to be fine. We actually used the boat all last summer since the place I was going to get it fixed at was already backed up with business. I may run it again this summer and just focus on the repair at the end of the season.

The Merc is on a Checkmate hull, pretty sure the dealer installed the outboard at the shop. That dealer is long gone now tho. At this point it is just a metal casing repair, complete casing replacement, or entire lower unit replacement. My current plan was to purchase a complete lower end but still fix my original one, so I would then have a complete backup, "just in case". We live on a deep water lake and the lower end seldom encounters any obstruction, just the occasional log, branch or stick.

I doubt the case will be different from one unit to another- mostly, as mentioned, the shafts are different but if the top of the transom isn't extremely close to the water, the OB is just a standard, off the shelf model. Have you checked your gear oil, yet? If you see water, do a pressure test. A 44 year old drive is due for some updates, IMO- I was going to replace the bearings and seals on the bearing carrier in my boat but I saw the price for a new one and did that instead- the only thing I needed to do was remove the Reverse gear and install it in the new carrier, lube the bearing & seals and install it.

The 1st photo is the bearing carrier retainer removal tool- for the price on Amazon, it's worth buying if you know a lot of people who own boats and want to save money by doing some of their own repairs.



The 2nd photo is the pinion nut wrench- new, it's pricey but I got mine when I was doing some repairs for someone else- they offered to buy the tools and let me keep them. A box wrench that has been ground thinner will work. You will need the tool for turning the driveshaft- it can be bought separately, or in a kit like the last photo. Last, I would recommend buying the service manual- it has a lot of info that will save time, head scratching and on-water failures. It specifies the proper lubricants (you'll find other uses for 2-4-C) and it has torque specs. I bought my last manuals on ebay and I don't think they cost more than $18 each.

 

[DGMAX]

Thanks for the links! Wife says I need a few more tools!! LOL!

I have a couple seals coming for the water pump base, was waiting to get everything in hand before checking the gear box. I hope to have everything early this week and will go from there. I'm thankful we live on freshwater, not as harsh on all the metal stuff and corrosion typically stays minimal. My Checkmate has a pretty shallow hull, and based on the transom measurement in addition to the foot/shaft measurements I'm now pretty certain it is the long (L) shaft, which the vendors seem to be referring to as the 20".

 

[jimn]

Thanks for the links! Wife says I need a few more tools!! LOL!

I have a couple seals coming for the water pump base, was waiting to get everything in hand before checking the gear box. I hope to have everything early this week and will go from there. I'm thankful we live on freshwater, not as harsh on all the metal stuff and corrosion typically stays minimal. My Checkmate has a pretty shallow hull, and based on the transom measurement in addition to the foot/shaft measurements I'm now pretty certain it is the long (L) shaft, which the vendors seem to be referring to as the 20".

Save some money- buy used tools. They might not be as clean and shiny as new ones, but they clean up pretty well and the discount puts gas in the tank.

If it has the original raw water pump housings, I would consider replacing the whole thing as a kit- the plastic deforms when it overheats and it's hard to know if it hasn't, especially for a second/third owner. The pinhead who owned my boat before me re-defined 'overheat', so yeah, I replaced a lot of parts that shouldn't have been needed but he killed every piece of plastic and rubber past the exhaust (Mercruiser sterndrive).

Yeah, definitely be careful if it goes on brackish water- even tea-stained is worse than clean-ish water.

 

[faztbullet]

I doubt the case will be different from one unit to another-

There are differences in machining of gearcases, thats why you HAVE to shim a MERC . You cannot just swap parts to a new/used case