Honda PA-50 and Hobbit cylinder stud upgrade

Honda PA-50/Hobbit cylinder stud upgrade

Soon after completing a Honda Hobbit rebuild using a Athena kit and custom Derbi head, I found it was neccessary for me to repeatedly remove the kit and piston to look at heat patterns. It made me nervous threading the steel bolts into the softer aluminum case over and over so I decided I wanted to install 7mm studs instead of having those long 6mm Honda bolts. I never liked the Idea of torquing down on the thin Honda head bolts time after time, and always felt like they would pull out the case threads. I felt that the thicker 7mm permanet studs would be stronger and worry free.

7mm1.00 tap

I got this 7mm 1.00 tap off the shelf at Napa Auto, cost 7.50. The handle I had already. Do not attent to do this mod without a T handle tap tool. If you try to just clamp the tap in some vice grips and start cutting in you wont be able to tell if your straight and your stud will wind up leaning and you could lose your valuable case.

the players

When I tap threads, I like to keep grease on the tap as it will collect all the cuttings so they dont pile up in the hole. Note that I have placed tape over my bearings to keep the cuttings out. Also, I'm in my living room and don't have compressed air on hand to blow them out. Plus, New Englands playing Denver on TV and I wanna watch.

check depth

Before I start to cut the threads, I first check the depth of the holes and make sure they are all the same. I find that honda case holes seem like they often have a 1/8th" or so of dirt and grease packed in the bottom. This needs to be removed so that it's all metal to the bottom. A good way to do this is just use a smaller than 6mm drill bit, twisted back and forth in your fingers will cut that gunk out. When using a tap, you dont count the first 1/4" of the tap as workable threads, so if your hole is packed with dirt, the tap will bottom out before you have actually cut enough thread to work with.


Since I wrote this page I have done about 20 of these stud upgrades and found soon that you will want to pick up a 15/64 drill bit and chase out the M6 threads all the way to the bottom. This will remove material that will be cut by the M7 tap anyhow and make The tapping process much easier.


Dip the point in the grease and then place it as straight up and down as you think is square and vertical in the existing hole your replacing. Two things work in your favor. One is that your cutting aluminum, and the other is that the threads your replacing are 6mm 1.00. The threads your cutting in are 7mm 1.00. The thread pitch is the same so it tends to almost walk in. Now give the handle a 1/4 twist to the right, then back to the left. Then advance another 1/4 twist, then back 1/4. Next try and advance 1/2 twist and then back a 1/4. Every round or so hold the case out at arms length and rotate it around so your eyes can tell you if your going true. Keep up this monotenus process until you have advanced all the way down the hole, until the tap bottoms out and you cant go any deeper.

studs in

You can now spin the tap out, and spin in your 7mm Peugeot studs and check for alignment. Mine were good and straight and my kit slid on nice. I then took the studs back out and put red threadlocker in the holes and twisted them in for good with vicegrips.

check alignment

I decided at this point to part the cases and slide the kit onto one half of the case so I could see the actuall alignment of the transfers that I had ground out to match earlier in the build. I certainly liked the scene.

fit the kit

At this point New England was way ahead of Denver, so I gathered up my crank and some bits and hung the motor back on the bike. Like I said, the studs were straight and the kit slid on nice. Not to blow my own horn, but if you not comfortable with hand tools, don't attempt this mod. I am a vetran of many custom mods, both cars and mopeds.

torqued head

Finish up with the Derbi head and torque the head nuts and Wow, looks great and feels more solid when I twist on the nuts. I noticed when bolting the head on that the profile of the cooling fins made it very hard to start the nuts onto the studs. I had to use needle nosed pliers to reach into the recess with the head nut. Instead if using regular 7mm nuts I built 1" long head nuts out of thread couplers that I drilled out then tapped to 7mm. Now it's no trouble starting the head nuts and they touque nice with a 13mm socket. I am now done with another Wiki page. If you want to ask me anything get me "cheetahchrome" through Moped Army.