GX 240 shaft questions

[hitman]

Hello all,
I spent a pile of hours trying to figure out how to take apart the stupid trash pump thats on the side of my gx 240. I final got it apart and I find this for the shaft. The treaded end is 3/4, and the rest is one inch. There is no key way what so ever. What do I need to do so I can use this big hoss on the boonie? Do I need to take the entire engine down and take the crank to a machine shop so they can not only turn down the crank but to also cut a 1/4 inch key way into it?
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[minidragbike]

Yes the crank will need to come out of the motor and then be turned down. The key way area is gonna be the hard part.

 

[jeep4me]

That's a major bummer!! I found a single cylinder 7hp diesel engine, but the last inch of the crank shaft is tapered from 1" to 11/16". I really wanted to get this but I had to turn it down because of that damn taper.

 

[Yellowhand]

Just bumping this thread so I can add to it later (soon)...

Hitman, your crank arrived in Nebraska...I'll start work on it (probably) this weekend...

...and I'll try to post some pics...

 

[Yellowhand]

This particular engine has popped up in a few different threads...

I tried to buy this engine from hitman when he put it up for sale down in the OldMiniBikes classifieds, but he really wanted to get the crank machined to fit a clutch, rather than sell the engine...but couldn't find a cost-effective method to get it done...

Since I can fix the crank problem, I'll help him do it so he can use the engine...along with a few other things I was going to do to the engine if he had sold it to me...

I get a number of PM's inquiring about doing custom machine work for members, many of them for the same task, so I spoke to Hent about doing a few threads on the subject, rather than duplicating the same info several times in PM's...

Here's the first thread...machining a non-standard (tapered, oversize, threaded, etc.) crankshaft so a centrifugal clutch or torque converter can be used...

 

[Yellowhand]

1- hitman disassembled the engine and mailed me the crankshaft and intake manifold adaptor plate...cost was $17.10...the package must be insured and trackable...there is some liability involved in doing machine work on existing parts...it may get lost in transit, or I may accidently damage the part while machining it, or I may perform some modification that wasn't wanted or doesn't work by mistake or miscommunication, so there needs to be some precautions taken...

2- The crank, as it looked on arrival...

3- Output shaft OD is .980" for the unthreaded portion...

4&5- Threaded portion measures under .748"...the size we want when finished...

 

[Yellowhand]

The threaded portion of this shaft will not cleanly machine down to .748", so this portion of the shaft is unusable, and needs to be cut off...

1- The usable portion of the shaft is 2.110" long, and needs to be turned down to .748" OD...

2- Standard length for a Honda 3/4" crank output shaft is 2.250", so the usable portion of hitman's shaft is 0.140" too short, requiring an extension...

3- Rough drawing for proposed extension/retainer...the piece will add the required length to the shaft as well as act as a retainer for the clutch...I want to fab it from silicon bronze for strength and corrosion resistance...this piece doesn't need to be as strong as the material the crank is made from...it merely acts as an alignment bushing, and isn't driving the clutch...

4- The extension/retainer will be held on by a 5/16" x 24 TPI bolt, standard size for 3/4" cranks, so hitman's crank will need to be centerbored and tapped during the lathe operation to reduce the OD...this pic is a factory Honda 3/4" crank...

5- hitman's crank, currently not tapped for a retaining bolt...

 

[Yellowhand]

So...if it's OK with hitman, I'll get started on the crank...

The drawing of the extension/retainer is not to scale...0.140" is not nearly as long as it appears in the drawing, and the piece won't need a keyway cut in it...most brands of clutches are relief-cut along the key for about 3/8" back from the end of the output shaft...

 

[Yellowhand]

1- Had I bought this engine, I was going to fab an aluminum intake and mount this 24mm Mikuni SBN carb...

2- I got the carb for free...I can't sell it because the original owner altered the mounting holes, trying unsuccessfully to mount the carb to a manifold made for a Tillotson carb...but the carb is unused and is complete...the SBN series carbs have jets available in sizes from 30-260, so they work well for a variety of applications...

3- hitman's intake port appears to be about .940" and round...

4- Nominal venturi size of the carb is about .960"...should be a good match for the GX 240...

5- I'll fab a 3" intake manifold like this 2" model, and send it back with the carb and crank...if it works, hitman can keep it...

 

[Yellowhand]

hitman...

The holes aren't centered around your intake port...

I need to know which way is up, so the carb isn't upside down when I fab the intake...

Are the two clips on the adaptor plate up or down when it's bolted on the engine?

 

[hitman]

Sorry about not getting on here sooner. My oldest son turns 11 tomorrow ( the one in my picutures with the boonie) and my youngest son turns 5 on the 20th. Needless to say, we have a few things going on here. Enough of my excuses...the plate shown in your first picture is exactly how it came off the carb studs. The funny question mark machined around the hole matches a similar question mark machined into the carb. Your second picture shows the vague outline of the gasket where it mounted directly to the head. I hope this clears up any confusion.

 

[hitman]

um..yeah. Just went back thru and read your last question and realized I may not have answered your question. The clips go up. They hold the plug wire, to keep it from rubbing on the engine.

 

[Yellowhand]

Excellent...I'll post up a pic of your crank mounted in a bandsaw pretty soon...

My son turned 21 on the 11th...you'll be amazed how time flies when yours grow up...

 

[hitman]

Depress me more...

Excellent...I'll post up a pic of your crank mounted in a bandsaw pretty soon...

My son turned 21 on the 11th...you'll be amazed how time flies when yours grow up...

Yeah, my wife and I were talking about that very thing last night. I can't believe that we have a 5th grader. Seems like yesterday that I was holding him in the hospitial. He is one of the main reasons I got into mini-bikes. He aboslutely loves them, and finds it odd that his friends don't have them. :smile:

 

[Yellowhand]

'Down the street' at midwestx's shop...

There are three different ways to mount a crankshaft into a lathe to be turned down...

A live or dead center can be mounted in the tailstock, or a rest can be mounted to the lathe bed...

All three ways are shown here, and they each work well...the important thing is to eliminate runout when mounting...the crank must revolve true to the bearing surfaces when cutting the new OD on the output shaft...

We used a machinist's parrallel in the chuck and a dial indicator on the opposing bearing surface to adjust the runout to less than one-thousandth of an inch before cutting the new .7485" OD on the output shaft...

If the crank also needs to be bored and tapped for a retaining screw, the lathe bed rest method should be used to mount the crank into the lathe so the end of the output shaft is unobstructed...

 

[Yellowhand]

Yep, that's Crisco being used as as lube/coolant...

...if you want to play some subliminal mind tricks on your friends, and other people who just hang around your shop on the weekends, use 'Butter Flavor' Crisco as a lube on all your machine tools...

..as that stuff heats up, the whole shop starts to smell like the concession stand at a movie theater, and all those 'lurkers' will start to get the munchies and experience a sudden, unexplainable desire to go see a movie...

...and they'll all leave so you can finally get some work done...

 

[Yellowhand]

Whoops...pics are out of sequence...

We cut the threaded end off with a bandsaw, and THEN bored/tapped the crank for a standard size 5/16" 24 TPI retaining screw...

 

[Yellowhand]

Cutting the keyway...

The DRO's mounted on these machines make it much easier to locate where to cut the keyway...

The KURT vise is zeroed and permanently mounted...the crank is sitting on machinists parallels to precisely position it in the vise...the DRO quickly finds the center of the width of the shaft, reading away from the non-moving vise jaw...

3/16" end mill mounted, and touched off to start depth...a few passes with the power feed later, and we have a .115" deep keyway...

 

[Yellowhand]

Now we just need to turn a retainer from bronze or aluminum to finish the crank re-sizing job...

...the last pic is a couple of round aluminum billets to make some intake manifold prototypes, courtesy of midwestx...

 

[hitman]

I'm so exicted, and worried all at the same time. The only thing I can compare it to is when they took my sons to go get circumcised in the hospital. I'm a nervous wreck over here just watching you work your magic.
Its is certainly strange watching this all happen. Thank you for all the pictures. I am currently gathering parts for the rest of it. I have dipped into the world of quarter midgets and JR. Dragsters to get the parts I need.