There are dozens of small parts on the Twin. None of which are difficult to make, with the exception of one, the gear case. It’s not for the faint at heart. Precision is a must. If you’re going to make the Twin, make the gear case. After you complete the case and cover, go on with the rest of the Twin. It’s downhill from there. Don’t get discouraged. it took me four tries to finally get the case right. Pay particular attention to the depth of the gear pockets. Their depths are each measured from the top surface of the case, not the bottom of the 2.250 dia. x .270 deep cut out. The dimensions can be confusing so read the construction notes first. Note: The holes are first drilled on the “O” ring side, then turned over to bore the pockets.
Several gears will be needed; fortunately, you won’t have to make them. All, including the tail shaft and oil pump gears, can be purchased very inexpensively on eBay; however, they will require some simple modifications. The Howell drawings specify 14 ½ degree pressure angle. The PA of the gears I purchased was not specified, but they seem to work perfectly so far.
Before drilling the holes in the gear case, test drill the holes in a flat plate. Put the gears in place on the plate to verify the dimensions to assure smooth rotation. You may have to make minor adjustments to achieve minimal backlash.
To make life easier I used a 1” ID DOM Steel Round Tube for a cylinder liner (available on line). The tube required modification of the cylinder as designed but it resulted in a perfectly round, and non-tapered inside diameter. No lapping is needed as the inside is perfectly polished. In addition, I’m going to try to use Viton "O" rings instead of cast iron rings. More on that latter.
I intend to add more as I proceeded, so stay tuned. And please, feel free to email me [firstname.lastname@example.org] with any questions.
A minor point concerning the oil pump and that's the gear offset from the center. There is no indication on how that gear is held in place. Just press a .125 dia. x .25 post into the center and place the gear over it. The gear will rotate around the post.
LOCKTITE 640 is specified in several places in the construction notes. DO NOT substitute another product. This stuff is incredibly strong up to 400 degrees. A small tube, although not cheap, will last a long time.
V-Twin – Pistons & Rings
On virtually every engine I built in the past, I had problems with compression. However, that problem is no more since I started to use DOM tubing as the cylinder liner. For the V-Twin, I am using 1” inside diameter DOM tube (from On Line Metals). As close as I can measure, the ID is spot on, 1.000" with no taper. The inside wall is polished smooth as glass. Following is how I make the piston, cylinder and rings:
- Make the aluminum cylinder body first but with a 1.190” bore.
- Start with a 2” length of 1” DOM and turn the OD to 1.188” for a light press into the cylinder body, and then cut it off to length. Press it into the aluminum body.
- Make the piston blank by turning a 4” long piece of 1” diameter cast iron (which is always oversize) just enough so it is a tight sliding fit into your aluminum cylinder body.
- Polish the piston to achieve a sliding fit into the cylinder.
- Bore an .910” diameter hole ¾” deep in one end of the piston blank, actually the ID of the rings.
- Part off six rings from the piston blank (I usually cut off a few extra) each about .045” wide.
- Now, cut the ring grooves as specified on the drawing and complete the two pistons.
- Using fixture A, sand (150 grit) the rings just enough to fit the width of the piston grooves.
- Using a diagonal cutter cut the rings forming the ring gap.
- Using a very fine file, lightly file the two ends of the gap.
- Place the ring on fixture B.
- With a torch, heat the ring opposite the gap until it turns red for about 20 seconds.
- Remove the ring (let it cool). The gap will remain open but will close with a little pressure then spring open when pressure is removed.
- Now it’s a matter of putting the rings on the piston and inserting the piston/ring assembly into the cylinder. That may take a little effort.
You’re done! It took me about five hours. Hope this helps. Note: Email me for a sketch of fixture A&B.
I found machining the V-Twin carbonator quite challenging and one of the more difficult parts to make next to the gear case. So much that it took me three tries to get it right. In my opinion, the original drawing is just too confusing and that lead to the errors in machining the part. As a result I redrew the carbonator body using conventional drawing and dimensioning practices. If you would like the drawing send me an SASE and I will gladly send it to you, but ONLY if you purchased the V-Twin drawing set from Howell. Email me --- email@example.com