'84 Mercury 115 inline six, Long to short mid unit swap question

[gomopar440]

Weight is a big issue with the Dillon Laker 14 tunnel hull boat I'm currently building. There's a link to the build thread in my sig line below for anyone wanting to take a look at it. I have an '84 Mercury 115 HP inline six "Tower Of Power" for the motor. This particular Tower is an ELPT (Elec start, Long shaft, Power Trim) that weighs 315 lbs, which is 15lbs over the recommended max motor weight for this hull. The recommended power range is from 60-80HP, so I'm a bit over there as well. To compensate for the extra weight and HP I'm scaling the boat up from 14.5' to 16.5' and adding reinforcements to the transom. I've already received the designers approval for these changes, as well as a few other tips via email. Even though he's OK with the changes, I'd still like to see if I can get the weight of my motor down to around 300lbs or less. I don't want to jeopardize any reliability to do so though, so hack mods are NOT an option. One idea I came up with is to use an older inline six short drive shaft and mid section to replace my current long shaft setup. I found a white Mercury 70six motor on Craigs List that appears to be a short shaft. The white Mercury is listed for $100, so that makes it pretty appealing to me for the parts alone, as long as I can use them on my 1984 motor. There's not much of a description with the CL ad, so I have no idea of the condition until the seller contacts me back.

My motor:


White Mercury 70Six:


So, my questions are:
1) Would this idea work?
2) What is the weight difference between the short and long parts?
3) Is the mid section and drive shaft from the white motor all I would need to convert mine?
4) If not, what other factory parts would I need?
5) Are there any other safe and reliable mods I can do to further reduce the weight of the '84 Mercury 115 Tower?

Thanks in advance for your assistance.

 

[jimmbo]

I doubt there any parts, well some nuts and bolts will, swap between the two engines. I was going to say the gearcase might swap, because it is not the original gearcase but a later one. Notice the backwardly installed trim tab... It is from the original gear case and has the water inlets in it. whether of not the spines on the driveshaft are same as the 115 is highly unlikely. There is no way on this earth the midsection will bolt onto the 115.

As for making the 115 lighter? The only parts you could remove are the power trim components, but you would need to install several parts to keep the engine locked to the tilt pin for reverse operation. No Stainless props

Just put a bit more fiberglass in the transom to bottom and transom to side areas. Better yet a bit of Kevlar or Carbon Fiber in these areas and the transom will be good for far more

 

[Faztbullet]

One idea I came up with is to use an older inline six short drive shaft and mid section to replace my current long shaft setup.So, my questions are:
1) Would this idea work?
2) What is the weight difference between the short and long parts?
3) Is the mid section and drive shaft from the white motor all I would need to convert mine?
4) If not, what other factory parts would I need?
5) Are there any other safe and reliable mods I can do to further reduce the weight of the '84 Mercury 115 Tower?

1) Sorry it wont work as that's a 10 bolt power head and yours is a 8 bolt and the exhaust adapter is designed different. You will need to find a 1976 or newer short shaft for this to work. You will also need to modify trim system as the new style 3 ram trim wont fit a short midsection..
2) about 35lbs when trim removed
3) No see #1
4) see#1
5) Yes...convert to old style short midsection,add single ram thru transom trim,use mercruiser trim pump and add 12v lawn battery so 24v can be applied to down solenoid if boat gets flighty. This is what the sport c race tunnels use ...

 

[gomopar440]

Thanks for that info. I am especially grateful to hear about what works on similar hulls (sport C), as that's what this boat design is based off of. I'll be keeping an eye out for '76 and up short shaft parts. If I can't find them by the time I'm ready to build my transom, I'll just stick with the long shaft and build it for that. If I come across the short shaft parts later on, I can shorten the transom to fit as needed.

BTW, what year range does that white Mercury I posted look like it's from? I'm also looking for a mid 60's Merc for a 1964 Silver Line Rambler I have. That won't be getting restored until after I'm done with the Laker, but I'm still keeping an eye out for a period correct motor for it though. A 65HP is what I'd rather have, but that 70 would probably look good on the Rambler as well IMHO. The transom on it looks like it's made for a short shaft motor, but I need to rebuild it, so I can go any way I want with it really.

Here's a pic of the Rambler as I was prepping it for winter storage. I still have all the factory parts I pulled off of it. Those long white side panels on top are from a bigger Larson boat that I might use on this one to clean up the insides.

 

[Faztbullet]

1962 or earlier.....

 

[gomopar440]

Thanks again. It's a little earlier than I was hoping for, but it still looks like it would be a nice match for the Rambler. I'm not up to speed on old Mercury values, but does it look like it's worth $100 to you? I know it's kinda hard to give a value with so little info, but that's all I know from the CL ad. I'm expecting it's most likely going to need a complete going over if I pick it up. I have a tiny endoscope I can use to check out the cylinder condition though the spark plug holes. I didn't see a motor stand in any of the CL pics so I'm not sure if I can try to spin it with the starter motor or not. Maybe if I bypass the tilt switch? Other than that, I'll just have to use my best judgement, like on any other used purchase.

 

[merc850]

$100 is about all I would pay for a 1961 model, that is missing? the tilt tube and the rubber grips on the clamps; it also has a dual point, dual coil ignition system that uses a degree plate to setup the timing. I would get the 1967 and earlier Mercury Manual to help you restore it; to get an idea of parts etc. check out this site http://store.oldmercs.com/category_s/811.htm
This site might be able to help http://johnsoldmercurysite.com/phpBB3/index.php they work on those motors all the time.
I have a six like that and they are a good running engine.

 

[merc850]

Correction it is 1965 and earlier manual - old part Nr. C-90-25500. That shifter box with Quicksilver on it is for the Direct Reversing motors 1957 to 1961 that's why it only has one cable (throttle) the motor ran forward or reverse by pushing the button on the top of the handle.

 

[gomopar440]

Are you referring to this old single cable control box (on the left) I have?


If so, thanks for that bit of info. I never got any responses in my build thread to help me figure out where or when that thing came from. Google wasn't any help with trying to ID it either. I was planning on just using it on my Laker to control the gear shifter and a separate foot throttle to control the throttle on the 115 Tower. Since I had no idea what the button was supposed to do, I was just going to repurpose it for something else like a trailer tilt switch. The key switch in it doesn't push in like on my other control box in the pic, so I wanted to try to see if I can adapt a "push-in to choke" type key switch to fit the older control box. If not I'll just mount the key and choke in the dash or the console.

BTW: I took a closer look at the pic of the 70 six and I was able to read the words "Full Gear Shift" under the powerhead. The profile of the mid section matches the one in the link you provided of the '61 70HP Full Gear Shift model. There were two others listed with that one, but the back side of the mid section on both of those was rounded where the 61 FGS model had a more defined bend along the back edge of the mid section.

 

[merc850]

Here's a diagram for the single cable that shows how the switches are wired



there are 2 positions in each start mode; push the button straight down to activate the starter and forward and down to activate the choke and starter. That box is a collector's item for old Merc guys.

 

[racerone]

Those older 6 cylinder Mercury motors can be made to run realy nicely.-------------But they must be totally torn down and refurbished to get that result.--------A new battery and some carburetor cleaner won't do the trick.

 

[gomopar440]

I guess I lucked out when I found it just laying on the floor in that Larson All American I picked up from a CL free boat ad. It looked like it had been laying in the bottom of that boat for at least a couple of decades out in the weather when I got it. Nothing a little elbow grease couldn't fix though. The rarity of it must be why I couldn't find any info on it before now. BTW, that wiring diagram is a great help and I'm sure it will come in handy. Does anyone still make a control cable for these control boxes or do I need to modify one to make it work? I'm a retired Navy machinist (MR) with a big mill and a little lathe in my shop, so I can mod just about anything as needed to make it work correctly. Finding the correct cable for it would be the easiest way to make it work though. The cable that was attached to it was so badly mangled and bubba'd up that I just cut it off, leaving a foot long piece of cable sticking out from the control box.

I'm still kinda on the fence about picking up the white 70HP motor (wrong year for the boat). If I do, it will be gone through thoroughly before putting it back in service. All maintenance will be done with as per a genuine Mercury factory service manual that I will pick up for it (if I get the motor). I'm not a fan of aftermarket service manuals at all, especially ones that are about 20 years old or newer. Seems like the most common quote I see in those newer aftermarket service manuals is "Take it to a dealer for service". If that's all they're going to say, why should anyone buy the manual in the first place.:der:

 

[jimmbo]

The single lever, well the one that says Quicksilver is for a DockBuster. No DockBuster ever had 'Jet Prop' exhaust. It is of no value to either engine

 

[gomopar440]

No value in it's original configuration maybe. This Quick Silver control box does have value to me though. It's looks are something you don't see everyday and I really like it a lot. I don't have a problem reworking it to fit my needs.

 

[jimmbo]

It is kinda of cool. A Trophy/reminder of one of Carl's worst/dumbest engineering decisions he ever made during his ownership of Mercury.

 

[gomopar440]

I'm not too well versed in Mercury/Carl's history, but even I've heard to stay away from those engines labelled as "Dockbusters". And, Yea, I've heard he was a bugger to work for. From what I read about the history of the Tower of Power, they were kept a secret from him until they had a working prototype. Only then did the designer present it to Carl to show him what it was capable of. He was impressed with it and ordered production of the Tower of Power engines to start immediately.

Good or bad, at least it looks like Carl probably learned what DIDN'T work from his mistakes by the successes of the Mercury company we have today. Or was that due to whoever took over after he left the helm? Like I said, I'm not to knowledgeable about Mercury history, but I still find it neat to read and hear about it.

I think I hijacked my own thread here. :lol: It's all good though, I'm learning a lot with each new post/reply that comes up.

 

[jimmbo]

If you ever get a chance, read Iron Fist, if you already haven't. It is an interesting read

 

[merc850]

I have a 1960 Merc 700 dockbuster and it pulled tubes/other boats/skiers etc.; ran it for hours and 15 years and didn't "bust" any docks - of course I know how to operate a boat, used a good battery and changed spark plugs once in a while! It was retired running 6 yrs.ago and it now sits in my living room.
Prior to that motor I had a Mark 78 DR motor for a few years and they both started and ran in F or R easily.