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Small block Guzzi conversion


Alan Walker

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Having built and thoroughly enjoyed the creation and running of my 2CV engined Brooklands, I found the need to continue a building project to strong to resist, so encouraged and egged on by Mike Meakin the decision to do a Moto Guzzi conversion was taken. The first step was to source a suitable donor, Ebay was the obvious choice and it wasn't long before a suitable candidate appeared about 30 miles away. Bidding was successful and Mike and I trailed the 750cc SP home after each having a ride on it and enjoying it's performance. Mileage showed at just over 10,000 which, given the performance, sounded about right. The appearance of the bike was spoiled by the corrosion all over the alloy components, otherwise it was in good nick. Fired up with all the enthusiasm of a new project I removed the engine / gearbox assembly from the frame, a straightforward procedure but one unique to the Guzzi.

There followed a deal of heart searching brought on by friends comments of 'vandalism' at scrapping a potentially restorable machine, Mike not helping matters by expressing a desire to take on the bikes restoration if I would part with it .

A chance browse on Ebay revealed a 750cc Strada which had been the donor of an abandoned trike project. Here was the engine I wanted with no recriminations of vandalism, that deed had already been done ! A successful purchase saw Mike and I on a trip to Bromsgrove to pick up my treasure which, much to my surprise, included most of the original bike minus the frame which had been chopped up to form the trike chassis.

Now the project could start in earnest. This engine also suffered from corrosion of the alloy parts. Close inspection revealed that the trouble was caused by deterioration of the powder coating which had been used in manufacture to protect the surfaces. Over time water had got underneath and caused all the powdery white rust. Paint stripper worked wonders and soon the coating wrinkled up and came off in sheets, you have to improvise with a variety of tools to get in between the fins and other deep recesses. With the coating removed the engine still looked pretty untidy so I looked into some form of blasting to get a uniform clean finish. Aqua blasting seemed to be the ideal approach until I found the cost ! My son put me on to a local farmer who used a home made sand blaster driven by one of those huge Council type compressors. A very agricultural solution but cheap so I decided to go for it.

It was extremely important to make sure no sand entered the engine so I took a lot of time and trouble to ensure everything was sealed. The resultant job was very good, with every part a uniform alloy colour, the finish was coarse but perfectly acceptable. Once back home I removed all blanks and sprayed the whole with ACL 50 a product used by the aircraft industry to protect polished alloy aircraft finishes. This also came via good old Ebay.

When I removed the unwanted gearbox from the Guzzi in preparation for blasting I discovered that the electronic ignition triggering was done by lugs on the flywheel and a sensor on the bell housing, and of course these components would not be used on my conversion. After a moment of absolute panic with thoughts of a useless engine with no form of sparks I discovered the redundant housing and half speed drive for the earlier small blocked Guzzi's ignition system above the alternator. It was lucky I had the original SP still in the garage for comparison so could see that I could retrieve the loss by fitting an earlier system. A phone call to Moto Corsa in London soon persuaded me that although possible the cost was prohibitive at over £400 !

Somewhere I discovered that a Lucas Rita kit was produced for the small block 750 Targa, it's similarity to the Strada meant that this could be the answer to my sparks problem. Ebay again came to my aid with an advert for the very item by a breaker in Chichester.

Next step was to prove the engine, the vendor had assured me it was a goodun but I didn't want to wait until it was in the car to come across any major problems. I built an engine stand out of some old 2 inch square stock left over from a trailer build and borrowed a fuel tank from my 1 1/2 Lister stationary engine. The rest of the installation was made from bit box items. Mike unravelled the Rita wiring and we were helped no end by Mistral Engineering who produced the original Rita kits. After some very loud backfires the engine sprang into life enough to know that there were no major defects. After switching off we noticed that the plastic boots around the spade connectors had started to melt and the Rita unit was too hot to touch, clearly all was not well ! A further call to Mistral revealed that the coils, the Strada originals, were of the wrong resistance. Ever helpful, the correct coils plus wiring diagrams were sent up and installed and the engine ran sweetly without threatening to set the garage alight.

At this stage I had both engines out so it was an ideal opportunity to find their relative weights, using the bathroom scales I was pleasantly surprised to find the Guzzi was 92 lbs and the 2CV 91lbs, both engines with oil but without flywheel/clutch.

Now came the interesting bit, the making of the adaptors. Initially I planned to take David Tocher's excellent drawings and dimensions to a local engineer to be made up, but after a deal of thought I felt I could have a shot at it myself, mostly using my drill press. Some professional assistance was needed to produce the two four inch dia. mild steel discs, these were 1/2 inch thick and needed a 22 mm hole at their centre. This done I was able to complete the inner adaptors myself. The reason for the 22 mm hole was that the small spigot on the end of the Guzzi crankshaft and the centre hole on the 2CV flywheel were both this dimension and my plan was to use a 22 mm mandrel to mount the 2CV flywheel and then the Guzzi clutch housing on so I could drill through their mounting holes producing an identical hole pattern in their relative mild steel disc. Searching around for a suitable mandrel I spied the rear axle of the donor Strada, the thick end section was the 22 mm I was looking for. The axle was mounted vertically in the drill press, next a piece of MDF, followed by the disc and clutch housing. Two drills are required, one clearance and one tapping. The 6 Guzzi side disc holes are clearance 8.2 mm dia. As I had decided to use 8 mm Allen key headed high tensile set screws throughout, the next job was to counter bore clearance holes for the bolt heads.

For the 2CV flywheel side a change of disc and drills to 6.8 mm tapping for the 5 holes, I nearly made a mistake here and started to drill clearance, just remembering in time so no harm done ! There is a sixth hole for a roll pin which I did drill but on reflection I think its unnecessary.

Now that the two discs could be fastened to their respective sides it was time to drill the holes to fasten them together. Six holes, clearance with counter bores on the 2CV side disc and tapping on the Guzzi side now need to be bored. I spaced mine 60 degs apart in a circle approx half way between the mounting holes and the disc edge. I used a M8 x 1.25 second tap throughout.

So much for the inner adaptor, now to the outer. A one foot square 15 mm alloy plate is required and its cost may make your eyes water at £50 plus ! particularly as the centre disc you remove is surplus. My idea to get an accurate drawing of the two outer sides was to fasten a sheet of thin stiff plastic to each face and then tap out the mounting holes and their centres in the same way you make a replacement card gasket, using a ball peined hammer. Using the two plastic template centre holes as a guide place them on a drawing sheet and transfer first the six Guzzi holes, drawing a vertical centre line through it. Mark a similar vertical line on the 2CV plastic template and align the two centres and the centre line before transferring the position of the four 2CV mounting holes to the drawing sheet. Both sets of mounting holes, the six and the four are in perfect circles about their respective centres, my aim was to get the Guzzi engine sitting horizontal against the 2CV box.

The marking out onto the plate was fairly straightforward but I did find you need to concentrate to get it right, the Guzzi holes are 8 mm clearance with two 12 mm counter bores for the top and bottom holes which have locating tubes in them. The holes are not symetrical about the centre. The2CV holes are 10.5 mm clearance with two 14 mm counter bored for their two locating tubes. My plan was to bolt the engine and gearbox together using 10 mm x 600 mm bolts rather than relying on a tapped hole in the alloy plate. A small amount of alloy fin needs to be removed to accommodate the bolt head on one side of the lower attachment. Remember to place the bolts in position before attaching the plate to the engine. One other hole is required, that is the access to the engine plug and is easy to mark out when the plate is loosely fitted to the engine face.

David suggests you also remove an area of alloy for clearance of the starter motor nose. I did this on my plate but found on final assembly that it was not needed. I drilled an 1/8 inch hole in the plate to act as a centre, this came in useful when I cut out the disc for the flywheel clearance with a router. Cutting in from both sides using the 1/8 hole as a guide centre. the outer shape was done with a jig saw and then both cuts were tidied up with a drill and flap wheel which produced an excellent finish.

Before a test assembly it was important to remove the knob end from the g/box shaft, this was another useful tip from David, otherwise it will prevent the surfaces mating up. I used an angle grinder and added a taper to ease final assembly. The trial fit was without a clutch and tail bearing and was successfully made so I proceeded to make the final component -- the g/box shaft bearing. Using Oillite bushing I managed a reasonable bush on my old converted wood lathe, after all, I told myself it only functions when the clutch is disengaged !

Final assembly at last. The inner adaptor set screws had Locktite applied and were torqued up to 30 lbs/ft. The Oilite bush was pushed into the centre 22 mm hole of the assembled adaptor. A new clutch plus thrust bearing installed and to my delight the whole thing came together without any undue pressure.

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As my Brooklands had started life with a 2CV setup it had a very nice SS exhaust system as supplied by Phil which I needed to retain as much of as possible. Using my aforesaid stiff plastic sheet I contrived a likely shape to link the Guzzi exhaust ports to the existing system. Luckily the 2CV engine side with its balance pipe had joints where it met the long side pipes so I needed something akin to a hockey stick shape. Using the template, a drawing was produced, this together with the two Guzzi exhaust port flanges removed from the donor Strada pipes were taken to Competition Fabrications at Attleborough, Norfolk, a local firm. They produced an excellent pair of adaptors which fitted with very little adjustment. Please note that the drawings I made had to be modified slightly by Competition Fabrications as the bend was too tight, A larger radius was used and the pipe then turned back to match the drawing. The overall result was quite pleasing.

Another important modification necessary with this conversion is to swop over the carburetor manifolds so they face outwards otherwise the carbs will not fit. The air intakes are still a little tight, particularly the near side. I used cut down Piper Cross foam socks to protect the induction.

I am happy to report that today, 1st April 2009, the Moto Guzzi engined Brooklands passed its MOT and is now up and running for the year. Initial reactions are WOW, it certainly does fly and with a very healthy bark, I shall have to watch the loud peddle in town ! One further modification was necessary, the Facet pump put out too much pressure for the Dell Orto carbs so I fitted a pressure regulator --- from Ebay, of course. This cut the pressure down to 1 lb/sq in and that seems fine.

I have written this article to give any like minded builder an idea of how I did it, it is not meant to be a definitive guide and probably still contains some hiccups as memory does not serve as it used to !! Its been great fun and an absorbing winter project.

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