Who do yo measure the LU drive shaft?

[175checkmate]

I always wondered about this. What is the proper way to measure the shaft to tell if its 20" 25" ect.

[Raceman]

It's a very broad question, and since it's not brand specific, requires a broad non-answer. 20" mid actually means a mid section that properly places the propshaft for a boat with a 20" transom height. That assumes the clamp bracket is mounted flat on the transom and the cavitation plate (anti ventilation plate to be correct) is approximately even with the bottom of the boat.

Since different manufactureres have place their powerheads at different heights in relation to the clamp bracket over the years, and not all lower units are the same height from the mounting surface on the mid to the center of the propshaft, there's no exact measurement.

In the case of Mercs, the inlines and V6 engines have approximately a 3/4 inch different lengths from clamp transom brackets to centerline of propshaft, and if that doesn't complicate measurement enough, a 12" mid is 5" shorter than a 15" mid (must be some of that new math) which is 5" shorter than a 20.

So.......................................... how's that for sayin' a lot without sayin' anything?????

[175checkmate]

Sorry about that. A bit more detail.

On a merc V-6 How can I tell what size shaft I have. So If I want to buy a replacement do I get a 20" or 25".
I guess that is the measurement I am looking for. I think mine is a 25" but I want to be sure.

[JustMercMe]

Most Mercs are 20's unless it's offshore. Then you get into the 25's. The 15's and 12's are used mostly on tunnels and other high performance or race boats.

[crazy horse]

Post a photo of your engine and I bet most of us can tell by looking at it. Raceman, I have a couple of old mercs that measure 17 in. are they old enough that they were built before the short shaft / long shaft thing ? When was the first years that they had two different shaft length ?

[175checkmate]

Here is the motor on the boat.

[175checkmate]

Here is just the motor minus the LU.

[crazy horse]

It looks like a standard 20" shaft to me;).

[Raceman]

Dave, I'd have to measure some old engines to say for sure, but I think the short shaft/long shaft difference in the inlines was always at least 4" and I don't think anything was ever made in between.

Here are some approximates, BASED ON MEASURING 1 DRIVESHAFT, THEN DOING SOME ESTIMATING, so don't do any driveshaft cutting from this.

Inline 6 shortshaft driveshaft is VERY CLOSE to 18"

That would make the longshaft, commonly referred to as 20" inline VERY CLOSE to 22", because it's actually only 4" longer.

That would also make the 15" V6 in the range of ABOUT 19", a little less actually, and the 20 inch ABOUT 24" since they're a true 5" different between these two.

The 25" V6 would then be ABOUT 29".

All of these numbers are estimates, based on adding to an old inline case which I happen to have laying here and should be roughly within 3/4", but certainly close enough to differentiate between longshaft, shortshaft and extra long stuff.

The reasons that the math doesn't work out exactly when changing from inlines to V6 Mercs is that the crankshaft spline in the inline is at a different height in relation to the bottom mounting surface of the powerheads. Also there was 4" difference between what Merc called a longshaft and shortshaft inline, while 5" difference between the longshaft and shortshaft V6's. There were also some differences of about 3/4" in the relationship between the propshaft and gearcase mounting surface between the inline (which is the same as the early XR4 V2 case height wise) and all the other V6 cases.

[halveb]

Perhaps measure the midsection and then compare to someone's measurement. As long as they are both Merc's that should tell you. If no one has replied when I get home later (and if my internet access is working) I will post my midsection's measurement. I have a 20" driveshaft and it looks like you do too but I couldn't swear to it.

Anyone have their mid handy?

[crazy horse]

RM, I think I still have a owners manual around here from my 1956 mark30 and seems like back then it said that you were to mount the engine so that the cav. plate was at around 1" below the bottom. I can't find the manual at this time, But does that sound right? Would the center sections be alittle bit longer if that was the case? I remember even with those old engines buried in the water you had to be careful if ya made a hard turn under load. I still have my first brass prop I ever bought for that old mark30:D.

[175checkmate]

I went out and measured the mid section. From the top where is bolts to the upper half to the bottom is 20 1/4"

[175checkmate]

I also measured the shat form the top of the water pump housing to the top of the shaft 22 1/2"

[crazy horse]

It's a 20" center section.

[props4u2]

That's a 20" mid. & unit.

Happy Holidays to all,
Lee

[175checkmate]

So if I am looking to buy another lower unit then I would be looking for a 20" right?
Cool.
Thanks guys.

[props4u2]

You got it, 20".

Lee

[Raceman]

Check, you better re read that tape. If it was around 12" it would be a champ. From looking at your picture I can see that it's definately 20".

I based my "about" measurements on the gearcase mounting surface to the top of the splines, so if you take your waterpump to driveshaft measurement into consideration it throws it in that ballpark.

[175checkmate]

Thanks raceman, did not notice the error, its fixed now.
Thanks for the help.

[Dadsambition]

I have a 98 250EFI offshore that I advertised on here for sale but reading some of the other comments about the shaft size I am a bit confused.According to the original spec sheets this motor was only available in 25" and 30".I measured from the top of the transom to the top of the cavitation plate and it measures 20".Is this the correct way to measure or is the measurement supposed to be from the top of the transom to the prop shaft or is the way I measured it correct?I don't want to upset somebody that would be my potential buyer in giving him the wrong information .Is it possible that Mercury had the 20" available as a special order?I am certain that the motor was all original and rigged for the boat without modifying the L/U and I purchased the motor with the boat New.please help me with this info for I want to direct my ad on here towards the right direction.