125 hydro dragster

Wed Mar 16, 2011 11:10 pm

Hi all. Just found this site and have been enjoying combing through the old posts. Lots of good info and interesting builds. I am currently working on a 125 project that has become a dragster instead of the parade only tractor it set out to be. The history on this actually goes back to a 125 my dad had since I was about 9 yrs old. By the time I was 12 I was mowing with it, but by the time I was 17 that job fell to the neighbor kid as I was working in my dad's garage. Well the little Kohler developed an oil leak which the kid didn't notice and it didn't take long for the 30 yr old engine to lock up. A replacement engine was hard to come by used and too expensive new. As we lived in Maine it occurred to me that there were lots of horizontal shaft engines to be had cheap...snowmobile engines. So we scored a 440 Rotax for $100 and I grafted it in. It was pretty easy and the hydro doesn't care where your engine makes its power just keep your revs up and modulate hydro input. So we went from 1 cyl to 2, cast iron to aluminum, 4 stroke to 2 stroke, tired 12 horse to tired 30ish? (guessing on that last one) It worked out ok, but drove my dad nuts because he kept trying to lug the engine like it was still a 4 stroke and consequently fouled the plugs pretty fast. I actually mowed the lawn with it a couple times without a problem (the neighbor kid didn't work out) and it would pull a wicked wheelie. After I went off to college dad just hired someone else to cut the grass with their own mower and it didn't get used any more. I got married and moved down to Florida, Dad sold off the garage and let the 125 go with the property. Well, when I heard that I got all indignant that he would let my creation go so easily and decided I needed to build something similar. I had scored a military surplus 4a084 opposed 4 air cooled 84 ci engine off ebay earlier intended for an unrealized generator project . (apparently any generator I build needs another generator as a backup just in case mine doesn't work when needed, according to my wife....you know, a backup for the backup. So I just cut to the chase and bought a "real" generator) I found a 125 locally for $200 and decided to put the two together. It was just supposed to be a fun parade-type tractor but I recently found out that a local dragstrip has lawn tractor drags twice a year and I couldn't resist. So the plan changes slightly and I start looking for more top end. Sorry for the long post. I'll let the pics take it from here.
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Re: 125 hydro dragster

Thu Mar 17, 2011 12:06 am

Lookin good. Grump

Re: 125 hydro dragster

Thu Mar 17, 2011 4:55 am

Looks like a cool project. Good luck with it. Keep the photos coming. Oh yea, when it's done maybe a video of it going down the dragstrip.

Re: 125 hydro dragster

Thu Mar 17, 2011 6:57 am

Nice project! Like Jim said - keep the pictures coming and a video would be great!

Re: 125 hydro dragster

Thu Mar 17, 2011 7:37 pm

got my 12v starter installed today, the original was a 24v and I didn't have room for two batteries. I still have to correct my driveshaft angle. I'm hoping by lowering the rear of the engine and rolling the axle up I can get a workable angle. Currently it is just too steep.
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Re: 125 hydro dragster

Thu Mar 17, 2011 9:40 pm

All I can say is WOW! Neat project. Really look forward to more pics as you go. Don't forget the wheelie bars. :lol:

Re: 125 hydro dragster

Thu Mar 17, 2011 11:43 pm

I'm contemplating a chain drive to replace the transfer gear set that drives the pinion in my hydro. I was looking at some weld-a-sprockets and #50 chain at tractor supply today. Is #50 suitable to transfer up to 60hp? I'm not up to 60hp yet, but who knows where this project will end up. I just don't want to have to redo it later.

Re: 125 hydro dragster

Fri Mar 18, 2011 6:17 am

You may need a chain tensioner on your project. As you can see I don't have one on mine. There is a little slop in the chain but it is pretty close to being tight. I don't run it hard, just at show speeds. I tried different sprockets until I got one that fit as tight as it could without a tensioner. You can tell it is a little loose because it will rock back and forth a little while it is parked in gear (about an inch or so).


The bar in the housing is an oil slinger because the bottom sproket does not get into the oil to keep oil running up the chain to oil the top bearing (homemade item). I have ran it around for two summers so far with no problems let (knock-on-wood).

PS: Where did your u-joints come from and do they have 5/8" holes in them. I used a snowblower drive shaft between my two trannies on Artie and they have 3/4" holes in them so I had to buy a couple of bushing for them. Got the bushings at TSC.


Re: 125 hydro dragster

Fri Mar 18, 2011 8:01 am

the rear yoke is a #1510 from napa and has a 5/8 bore with a keyway and a set screw. I had a 1/4 inch hole drilled through it on center of the bore and tapped it on one side so I can use a retaining pin from a automotive axle as my drive pin. The other three yokes I got from www.driveshaftparts.com and are 3/4" bore with a keyway. I made the front one into a flange. Both u-joints are #338 pto style joints. I wish it would have occurred to me to just use a bushing to take up the difference because the 5/8 yoke was almost 3 times as much as the 3/4.

Re: 125 hydro dragster

Mon Jul 25, 2011 7:58 am

I finally got around to fixing my driveshaft angle. I lowered the rear of the engine an inch
and rolled the input of the transmission up an inch. So instead of the 25 degrees the shaft was
at, now it's 10 degrees which I think will be fine. Compare this pic with the earlier one.
What do you guys think? Also, I'm looking for a round gas tank for this project. Something
7-8" round by 10-12" long. Any ideas?