GuzziJohnO's Diahatsu Convert

[GuzziJohnO]

First some prelude. I grew up on a farm around diesel equipment, so they're nothing new to me. In the 1990's I became interested in biodiesel and started making my own. I eventually bought a well used VW Jetta turbodiesel from a U-of-Idaho grad student, who'd accumulated over 100,000 miles on experimental biofuels made by the University. After commuting in the VW for a few years I realized that I wasn't riding the 1971 Moto-Guzzi much. It got the same mileage as the VW, but burned the wrong fuel. The solution was obvious - I needed a (bio)diesel motorcycle, and since I couldn't buy one, I'd have to make one.
Examples of successful diesel bikes were hard to find. The few found on the web back then (1999 to 2003 or so) weren't impressive, either rat bikes that looked cobbled together, or clean but under powered tiddlers. Somehow I managed to find Dave Hubbard (BMW guy) and contacted him for lessons learned. I chose the Diahatsu engine because of his success, and because it looked like it would fit a modified Guzzi frame. Choosing the Guzzi was easy, I'd been riding them for decades and a chat with the club expert (the late, great Bob Nolan) decided the model (Convert) due to the removable sub-frame, which would simplify frame mods, the torque convertor, which would simplify joining the engine to the (2-speed) tranny, and the tallest rear end gearing of the marque (at the time).
A club member had a running 1976 Convert available, and another member wanted the gas engine, so a 3 way deal made us all happy. I hauled the chassis home the weekend the club had a private viewing of the just released movie "The World's Fastest Indian." Perfect inspiration!
The engine was ordered from Tulsa Engine Warehouse the same week. That was 2005. The first ride was in 2013. What follows are the steps, problems encountered and solutions.

[coachgeo]

Thank god....... thought I was only one with a near 10yr build

[GuzziJohnO]

Planning actually started a couple years before. Besides, the streamlined fairing isn't done yet, either, so arguably it's still in-work. But that's just nitpicking.
I'm away from home until next week, and that's where the build pictures are located. I'll post some then.
In the mean time, here's a slightly different view that was on my tablet, showing the first time it was assembled after modifying the frame.

First assembly

[GuzziJohnO]

I found another photo on my tablet that might be instructive. It's of the front chassis removed (for paint). Making it removable has proven to be a good idea, as it makes access really easy. During the design and build process, the major components have been apart and back together dozens if not hundreds of times. Also, the valves will need periodic adjustment, and the valve cover can't be removed otherwise:

Chassis front, removed

[GuzziJohnO]

Another build photo found. This was taken just after cutting the top tubes. It took several months of pondering and planning before I got the courage to take the band saw to what was up til that point a perfectly good stock chassis. Was I about to make a horrible mistake? I'd made a cardboard template of the motor silhouette, and fabricated the motor/tranny adapter, but couldn't actually test fit until after chopping the chassis.

Rear chassis after cutting the top tubes

[GuzziJohnO]

Here's the homemade adapter, and the homemade flywheel. It was the only way I could figure out how to mount the torque convertor. The tranny adapter couldn't be designed until the location of the torque convertor was finalized. Still, that was easier than fitting a clutch. The torque convertor robs some power and RPM's and fuel economy, but it was the shortest path to getting on the road.

[GuzziJohnO]

The stock exhaust manifold was cast iron, with restrictive flow paths and an exit that pointed right at my shin. I made the new one from welded pipe fittings, tucking it in where it follows a rather traditional route to the "Triumph" muffler. The shiny exhaust pipe was a stainless steel bathroom grab handle - cheap at a second hand store.

[pietenpol2002]

I can appreciate the ambivalence over dicing up the frame. Daunting indeed. But, I do like the "detachable" front end. And the exhaust manifold is a work of art.

[GuzziJohnO]

The stock Tonti-designed chassis had three top tubes under the tank running up to the steering head. The Moto Guzzi V-twin cylinders straddled those tubes, but the Daihatsu upright engine interfered with them. My solution was to cut them out and replace them with a V of Chrome-Moly tubes raised above the valve cover. I also added a pair of large gussets to make up some of the lost rigidity around the steering head. The Tonti frame is entirely made of triangulated tubing shapes, and I didn’t want to introduce any additional flexing, especially around the steering head. The new tubes end in those tubing joints mentioned previously, allowing the tail to be removed as illustrated here. The aft chassis also bolts to the rear of the cylinder head, effectively incorporating the engine as a (lightly) stressed member. The chassis feels very tight on the road. The longer wheel base and taller CG make the bike feel top heavy, especially at parking lot speeds where it feels as though it wants to turn in too easily, but on the road it tracks like it’s on rails.

[GuzziJohnO]

Raising the cruising and top speeds of the Moto Guzzi Convert Tranny, or any in-line, shaft-drive bike, can be done by putting a larger gear on the main input shaft. The Moto Guzzi Convert has a very simple transmission, perhaps the simplest of any 3-shaft design, and only one gear needs to be changed to allow higher speeds in both high and low “gears". Since the gears are an odd metric helical pitch (here in the US), the only matching gears available are spares from an identical tranny. Here's how I did it.
For those unfamiliar with helical gears, each meshes with a gear of opposite angle; left-hand meshes with right-hand, etc, so the only gears to pick from must be from one-shaft-over. I was surprised to find that a T-3 tranny uses different pitch gears that don't mesh with a Convert tranny gears. (Anyone want a set of T-3 gears?) This is where Moto Guzzi’s Convert tranny offers a limited, but useable, variety of options with relatively simple machining.
The stock input shaft is a machined forging ending with a left-hand 19-tooth gear. It normally would mesh with a right-hand 18-tooth gear on the aft end of the intermediate shaft, also a forged unit. The forward end of the intermediate shaft ends in a 22-tooth right-hand gear. The output shaft had 2 free-spinning left-hand gears that constantly mesh with the right-hand intermediate shaft gears – one a 22-tooth, and the other a 24-tooth. A dog-clutch determines which gear sends power to the rear wheel. Either of those 2 left-hand output gears might be used as a main-shaft input gear, since they mesh with the right-hand intermediate shaft gears, potentially changing the overall gearing. To keep the shaft spacing consistent, and wanting to switch only a single gear from the limited choices I had on hand, I chose to use the 24-tooth gear, giving the 26% overdrive. By reversing the intermediate shaft, the overall overdrive bumped up to 33%, giving an 86mph top speed at 3600 rpm. That looked pretty good, considering the limited number of motorcycle-sized engines available here in the US, and the proven power available.
I bought a “scrap” used tranny to experiment with, and concluded to use the 24-tooth “loose" gear from the output shaft to replace the “fixed" 19-tooth gear on the input shaft, raising the output rpms in both high and low by 26%. The two “loose” gears on the output shaft are the only ones that shift, engaging one or the other through a dog-clutch. There is no neutral between gears, so it will become apparent that one must be aware that a Convert is always “in-gear".
The 19-tooth input gear was turned off the input shaft, leaving a fat boss to accept the 24-tooth gear with a light press fit. A generous weld prep groove was ground, the assembly was preheated to 400F, and welded all around on the output end, avoiding a heat affected zone that could fatigue and break at the input end. Concentricity indicated less than 0.0005 runout after welding, happy to report. It's worth mentioning that the torque converter eliminates impulses that might fatigue the welds, so I'm hoping they hold for a while.
The next steps were to relocate the intermediate shaft bearings, to properly mesh with the larger input gear. Aluminum plugs were turned to fill the original bearing pockets, so that their positions could be accurately located on my crude x-y machine, and new offset pockets bored. There wasn’t enough room to keep the original roller bearing in the aft cover, so a plain bronze bearing is used, but the forward end had enough room for a roller bearing. Both bearings are submerged in oil, so lubrication should be sufficient for the plain bearing, considering the reduced power being handled.
The Convert doesn't have a “neutral “ between high and low. Top speed with the diesel engine in low, before modification, was about 48 mph at 3600rpm, and high was about 65 mph. That’s not fast enough to keep up with traffic here in the US, besides wasting fuel. The bike would accelerate up a steep hill near home to 64 mph, proving that it had plenty of power available to utilize taller gears. The new taller gearing should give a top speed in low of about 60mph, and in high about 81mph up my test hill. These gearing choices don't require expensive new gears to be made, only old-ones to be repurposed. YMMV.
The torque converter (the factory inspiration for the “Convert" name), is rated to provide a 1.6:1 maximum torque conversion ratio, happily at 2400rpm, which in “low” gives a maximum speed under maximum load of just 30 mph, assuming the 3600rpm diesel engine input, up a very steep hill. OK, I'm willing to accept such performance, since it isn't much different from the stock performance. It can accelerate up our driveway in high gear, so a higher low gear doesn't create a problem. More importantly, the bike needs to be able to keep up with realistic traffic speeds here in the US, i.e. 75mph at least, so a gearing change is necessary.

Comments on in-line vs transverse engines.
An engine mounted with crankshaft perpendicular to the direction of travel needs to be relatively narrow, or "short", to fit between a rider's knees. It has to allow enough room for a “primary" drive system, like a harley system, usually sticking out to the left. A short (one or two cylinder) engine that can fit transversely allows easier gearing modifications than an in-line, shaft-drive system. Here in the US the options are more limited than over-seas. Our most powerful 2-cylinder diesels don't provide enough power to keep up with freeway traffic, which is suicidal. Therefore it is preferable to choose from the physically larger engines that offer more power. Also, industrial engines here only offer fixed injection timing, reducing efficiency at variable loads and speeds. Unfortunately, 3-cylinder engines are all too wide to conveniently fit across the chassis and leave room for a primary drive system. We have no small diesel cars to use higher output/higher rpm engines from, nor any suitable industrial diesel engines. My 950cc Diahatsu is only offered here as an industrial engine with fixed injection timing, unlike an automotive version that would vary timing with load. It's found here in grass mowers, as used on golf courses and municipal grassy areas. Having fixed timing fuel economy will suffer at partial throttle. Wouldn't it be nice if I could convert the IP to a variable timing system? The IP castings clearly indicate such an option, but I've been unable to get a response from the manufacturer to investigate the option. That's a future project, and it's on “the list”. For now, I just want to finish the tranny modification, the Vetter nose and tail, and the other bits that remain “on the list". My goal is 100mpg, which doesn't seem to be out of reach, judging by other folks who've gone before.