Motzing Derbi head to Honda bushing kit instalation
Lately, while reading on the Moped Army site, I learned of a peformance modification that I could do to my Honda PA-50II engine. It involved converting a Derbi engine head to fit a Honda cylinder bolt pattern using a easy to install aluminum bushing kit developed by Graham Motzing.
Below is a pictorial guide as to how I installed the bushing kit and the tools I used.
This is what the end result will be. Yes, impressive looking and the huge cooling fins will definatly help keep my motor cool. The only big drawback is the larger head hits the Hobbit gas tank which is slung mounted under the Honda frame. The only way to combat this is to trim off about 3/8ths of an inch from the topside fins, or convert to a top mount tank which is what Ichoose to do. That is another page I have yet to create.
Here is the farm fresh, Derbi variant metrakit 70cc head with decompression, straight out of the mail from a San Francisco parts vendor (Treats). Cost of this jewel is 75.00+ shipping. the cost alone is reason enough not to want to cut the fins and to at least have a working plan for the modifications. I could probably just as easy dremel the bolt holes into an egg shape and It would work fine but I try to hold myself to a standard of building that doesn't allow loose detail.
The bushing kit, pictured above, Made by "The Moped Factory" of Milwaukee, was donated to me when I asked them advise on modding the head. Nicely made pieces and they all measure the same. Also shown are the two bits I used, a 27/64" bit to create the bushing seat hole and a smaller 17/64" bit to chase out the cylinder stud hole.
I used this Tiawan made "Orbit" brand drill press I found in the trash and refurbished. It is not necessary and this can all be done with a hand drill so long as you secure the head to a long piece of 2x4 stud or other means to hold it secure.
Before I bolt my head to the 2x4 I want to point out that the first hole you will drill, the bushing seating hole, will only travel about 1/4" or about halfway into the top of your head. If you cut all the way through, then that would put you tearing into the decompression channel on the sealing face of the head. You do not want that.
Now lets secure our head to a 2x4. I actually removed this sheetrock screw from the bathroom of the shop I was visiting to do this mod (he should have taped and plastered, right?) and matched it to a handy washer.
Now just center the head on the board and lock it down through the spark plug hole.
Now I'm ready to move to my drill press table, or clamp it in a vise, stand on it, you could even park the rear wheel of your car on top of one end of the board.
I like to measure the area of the bushing that will be recessed into the seating hole since the handle of my press has an adjustable stop so I can make all four cuts the same depth. It measures at less than 1/4" so I'll set my stop at 1/4". Hand drill users musy rely on their spider senses to tell them they have cut to about 1/4".
Now you want to place the 27/64" drill bit in the chuck of your hand drill or drill press. This next instruction is for drill press users. Before you start your press and begin to cut the seat hole, it's a god practice to center your work as best you can under the bit. The way I do this is to lower the unspinning bit down til it is lightly contacting the center of the stud hole. Then I grab the drill chuck with my other hand and manualy twist it to the left in the opposite way that it cuts. This action will move the board and head directly under the rigidly held cutting bit and find it's own center. Hand held drill? Spider senses. You will do fine.
While still holding the bit to the hole with the press handle, I grab a clamp with the other hand and clamp the work tight.
Next is just the easy proccess of drilling aluminum and repeating the "bit centering" and clamping for the next three holes.
Here is the finished seat hole. I am pleased
Now I'm ready to position the bushings in place over the seating holes I created. This picture acually shows all four bushings already installed. (I lost some picture files) But you will position the bushings one at a time with the new stud hole pointing outward to each of the four corners.
I recommend you use a flat tipped tool like a socket extension and not a socket to drive the bushings home. I used a socket and it left ugly ring patterns on the top. The bushings fit nice and snug in their seat holes and I'm ready for the last step.
In this last step it is important that your 17/64" bit is brand new or very sharp since you will be what I call "cliff cutting" since I don't know what the real term is called. But it's where half the bit is cutting and the other half is in open air. If the bit is not extremely sharp it wants to walk away from the work and into the open air. Kind of like most moped riders. Luckily the bushings existing stud hole helps keep the bit on track and guides it in a straight cut down through the cliff of aluminum. Just remember to go slow slow when making this cut so that the bit will not walk and start to elongate or "egg" the bushing stud hole.
Just one more shot of the Honda/Derbi build in all it's glory. With the extended cooling fins this motor should stay way cool
and Graham Motzing says that people have been raving about this type of mod which has been done for years now by many.
True that this Derbi head to Honda mod can also be accomplished without this bushing kit yet I like to look in the long term.
There may be a time when I want to sell off this head and If it has egg shaped canyons dremmeled in to it, I just doubt it
it will have much appeal. Kinda like selling a pretty red car with one brown door hung on it.
Many thanks to Graham Motzing of Milwaukee for the bushing kit, and as always thanks to Linda Green for teaching me to use and build Wiki tutorials. I am now done with this page.