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My Guzzi powered "Brooklands" and its "G" Series Gearbox


Peter Johnson

It has been said that opinions are like fundamental orifices - everybody's got one! I have sought and received the opinion of others, mainly on our forum, and am grateful to all of them - even those I don't agree with! Perhaps I should mention that my background is in electrical engineering, although not in any application remotely useful in building cars; fundamentals, however, are still in there somewhere! So there it is; my opinion is not based on any specific knowledge or experience, as will become abundantly clear!

Why a Guzzi?

I was a motorcyclist in the early sixties and grew to love such classics as the Vincent 1000cc 'V' twin and even the occasional Brough Superior. When I wanted a roof over my head I looked towards the Morgan trikes, but, even then, they were out of my price range. In the end I never did get to own a 'V' twin...........until now! Maybe it's the ghosts of the J.A.P.s or Matchlesses out there on the front of the Morgans, but I think this engine so suits the Pembleton - especially the 'round pot' version - for me, there was no other option.

How do you fit a Guzzi engine to a 2CV Gearbox?

"With a little help from your friends", is the quick answer! We have some very clever people on our forum; at least three of which have devised their own adaptor kits. What they all have in common is a very accurately machined plate that bolts to the Guzzi engine flange and to which you can bolt the 2CV bellhousing. In two of the examples, a further pair of round spacer/adaptor plates are used to match the Guzzi 6 bolt crankshaft flange to the 5 bolt 2CV flywheel. The thickness of these plates is such that the flywheel starter ring is accurately positioned for the starter motor mounted on the 2CV bellhousing. In the other case a whole new flywheel was machined and this is the design adopted by Phil Gregory for the Guzzi option that he offers. My kit is of the former type, which I was very pleased to buy from our member who made up several extra kits for his friends. At least three of us are using this kit.

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Any other problems?

Yes - a few!

  1. The carburettors on the Guzzi turn toward the centre as standard. For use on a Pembleton they must turn out to avoid the space frame; you need to make new manifolds.
  2. On the bike, the carbs are gravity fed from a saddle tank a few centimetres above them. In the Pembleton, the tank is at the back and fuel needs to be pumped. You need a low pressure fuel pump for this and you must change the float needles to match the increased pressure.
  3. The standard engine hanger brackets are made with the lighter 2CV engine in mind. The Guzzi is a fair bit heavier and needs stronger hangers with a brace between them to stop the rubber bushes distorting. See photo of my solution and follow the links to Claus's website for his - very elegant - solution. 'Silverfish' has another way - see Dave Parr's construction pics.
  4. There will be other minor things to sort out, such as engine electric's, exhaust pipes, telemetry, control cables and so on, but no more than any other engine installation.
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What about SVA?

In a trike you just take the less stringent motor bike test. On a Brooklands however you must take the full car SVA. The Guzzi engine, as standard, would fail the minimum radius test at a great many places. If I start at the top and work down, these are the points that fail, plus how (I think!) I fixed them. Bear in mind that all this refers to my 1982, round pot 950cc. Other models vary.

  1. Valve covers.

    These have fins on them which do not conform to the definition of cooling fins , are less than 5mm thick, and hence can't be radiused to the 2,5mm minimum requirement. When you have sanded them off, you need to radius the edges of the sanded area, as that too may now be too sharp. The bottom-most lip of the cover is raised and cast at ninety degrees; that too needs radiusing - very time consuming detailed work!

    The oil pipe union and breather pipe union bolts need SVA nut covers as they cannot be changed for dome nuts.

    Some of the 6mm socket screws that hold the covers to the cylinder heads need changing to 6mm studs with dome nuts. I tried button heads but they just fail because the head is not a full hemisphere and goes exactly to the edge of the joint flange.

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  2. Oil pipes.

    The flexible oil pipes from the crankcase union to the cylinder heads are swaged-on leaving a sharp edge to the swage. I have bought replacement pipes which have more modern, edge-free terminals. This also applies to the valve cover breather pipes which used jubilee type clips.

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  3. Dynamo mounting bed.

    Early manifestations of this engine had a dynamo mounted between the cylinders on top of the crankcase. Even very modern engines, like 'Silverfish's, still have the mounting beds cast into the crankcase and timing cover. I think it would be very hard to radius them as they are curved to fit the dynamo body. I just ground them off.

  4. Timing cover.

    While you are grinding off the dynamo bed, you need to go round all of the top part of the timing cover with a detail sander - it just fails the radius test. Like the valve covers, you must change the socket screws that hold the cover on for 6mm studs and dome nuts. The tachometer drive cable exit needs a through bolt with dome nut if not used for you tachometer.

  5. Crank case.

    Grind off the dynamo bed as mentioned above. Put a nut cover on the oil pipe union bolt and another, with a small hole in it, over the oil warning sender. The hole is for the sender wire. Use 8mm studs, with dome nuts, in the original Guzzi starter motor mounting holes. On my engine, these two studs thread into the adaptor plate.

  6. Manifolds.

    Use 8mm studs with dome nuts for all fixings.

  7. Carbs.

    I'm unsure what to do about these. There are certainly several projections that fail the minimum radius test but most of the bolt heads are too small for nut covers. Maybe a dollop of mastic on each one would do it!

  8. Adaptor plate.

    On my kit, the adaptor plate front edge can be contacted by a 100mm ball on each side and hence must be radiused. Be sure to radius only the areas between the fixing bolt holes.

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The "G" Series Gear Box

Why? There are many Pembletons in use today with big BMW boxer engines, of up to 75bhp, and original 2CV gear boxes; Phil Gregory's Brooklands demonstrator is a prime example. The owners all seem very happy with the rig. The 2CV gear box was designed for an engine of about half that power - so what's going on here? Is the 2CV box up to it? When I asked Phil about it, he emphasised the fact that the Pembleton was very light compared with the car the gear box was designed for and hence did not present such a load to the transmission. Further, he said that if you use the cross ply tires that he recommends, any over-powering would be "spun off" in wheel spin. The cross plys also have a larger profile and hence raise the ultimate final drive ratio by something like 10%. I have the utmost respect for Phil and accept everything he says on this subject.

So why do I have to be different? Good question! There have been many times over the past year or so when I have wished I'd stayed with the 2CV box! However, now that I've solved nearly all the problems of fitting a "G" series box, I'm very glad to have done so. Here's some thoughts on why.

Jeroen Cats, the late doyen of the 2CV world, reckoned that the 2CV gearbox is good up to 40bhp. See Links page for Jeroen's site. If I don't want to use cross ply tires, and want all my welly to propel the car, then the gear box has to take whatever my Guzzi gives it. I do not want to max out on revs at 2CV speeds and do not like the option of raising the 2CV box ratios by installing a Dyane CW+P. Swapping the CW+P, in my view, is not the panacea that it seems - nothing's for nothing! Lengthening the legs by 7% in this way simply slows the drive shaft speed for any given engine speed without compensating for the increased load. In effect, it weakens the gearbox, probably by 7%, and thus lowers it's power handling capability. I'm not a mechanical engineer but, in my view, there is a clear electrical engineering analogy. It goes like this: if you study the components of the drive chain, from clutch to front wheels, you see that, as the speed of rotation is reduced by gearing, so the components get more and more bulky - just compare the first motion/primary shaft with the drive shafts for the overall picture. The primary shaft is spinning at engine speed and has a smaller section than the main shaft which in turn is much lighter than the crown wheel......My electrical analogy is the basic formula, Watts = Volts X Amps, i.e., raise the volts and the current goes down commensurately, just transpose volts for RPM and your amps - conductor size/shaft section - goes down. Slowing the drive shafts in this way must reflect an increased load up the drive chain. Your gearbox that was good for 40bhp is now probably weakened by the same percentage change in the overall reduction, 37.2bhp. I must re-emphasise that I have no specific knowledge in this area - the deductions I'm making just seem logical to me. I'm sure we have some real experts who could comment more authoritively.

Some Background

The "G" series box was first used on the early LN, LNA and Visa models. The two pot engined versions used a 2CV type engine of around 650cu with several refinements; Teflon type coating on the bores and computerised ignition etc. This engine is interchangeable with the 2CV engine but has more teeth on it's starter ring; the starter motor needs to match the engine. Unfortunately, the LN, LNA and Visa models did not have inboard front disc brakes. The gearbox casing is however drilled and tapped for the larger calipers later used on the GS and GSA models. The drive flanges are 2CV type, as are the drive shafts; unfortunately, you can't simply fit 2CV discs because you can't fit 2CV calipers, and GS/GSA calipers don't match 2CV discs!! At least one of our number is working on an adaptor plate so that 2CV calipers will fit. So, until that is available, this manifestation of the "G" series box is of no use to us. What is useful - indeed essential - is the bellhousing, as it takes all the "G" series boxes on it's aft end and 2CV engines on it's front end.

This would be a good point at which to explain the evolution of the Guzzi engine adaptor kit . From the previous paragraph you might be wondering why one would want to fit this gear box to a 2CV engine! Early Guzzi installations in 2CV based kits used the 2CV gearbox, so the kit basically makes the Guzzi engine look like a 2CV engine to the bell housing. Using the LNA/Visa bellhousing, with a Guzzi engine adaptor kit, now allows us to fit "G" series gear boxes to Guzzi engines. "2CV" has thus become an "interface standard" only!

"G" series boxes used on GS/GSA models all had different bell housings which differed from the LNA/Visa ones in that they had no provision for a starter motor and were of different depths, fore to aft. To cater for the different depths, Citroen made the input shafts in two sections; a first motion shaft which splined to the clutch plate and which in turn splined onto the input shaft proper just behind the crown wheel. This means that the first motion shaft must be matched to it's bell housing! i.e., you need both the LNA/Visa bell and it's first motion shaft.

The five speed GS/GSA gear box is much too long for the Pembleton so I will discuss only the four speed one in this article. (Claus Gartner, who made my adaptor kit, has used a five speed with a Guzzi engine in his Lomax. See Links page for Claus's site.)

GS four speed gear boxes have drive flanges which are drilled for GS/GSA discs - 3 X 2 holes - and, as mentioned above, cases which take the large GS calipers. Again, adaptors are required to mate the 2CV drive shafts to the GS discs. These adaptors are also needed as spacers as the "G" series gear box is thinner crosswise Than the 2CV one. (The drive shaft adaptors are commercially available in Germany).

So now we're getting close to a full kit! We have an LNA/Visa bell housing and matching first motion shaft, a Guzzi/2CV adaptor kit and a set of GS/2CV drive shaft adaptors. Time to choose our gear box!

The four speed box comes in several flavours ratio wise and these are well tabled on both Claus's website as well as that of the late Jeroen Cats. (See Links page) .Basically it comes in short and long legged versions with the longest legs coming from the GS model that was made in eastern Europe, known as the Axel, which gives a fourth gear speed of 28kph per 1000rpm of engine speed. I was lucky to get an Axel gear box, complete with calipers and discs, which David Stevenson sourced for me in southern France. So look at the tables and take your pick!

What's Next?

Well, we're not quite ready to to put it all together; there's still a little work to do! We'll start with the bell housing from the LNA or Visa. Remember, these cars did not have inboard brakes, so there is no provision to accommodate the discs. We need to cut away a little of the bell on either side to stop the discs fouling the bell housing before we mate it to the box. Have a look at my pics to see what's involved. Claus also has some on his site.

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Now is also the time to decide how to install the gear box in the frame. There are two options. The four speed "G" gearbox is about 25/30mm longer than the 2CV one - depends where you measure. You could just install as is and accept the fact that your engine will stick out the front of your car by that amount, plus the thickness of the adaptor plate(8mm), more than if you had used a 2CV box. Or you could modify the end plate at the aft end of the box to allow it to sit a little further back. I chose the latter, but there is a price to pay! The mod requires that you remove the speedo cable drive and the scroll nut scroll part on the aft end of the main shaft. This in turn gives you room to cut a step into the end plate which allows the gearbox end to sit nicely over the track rod nuts. Have a look at the picture which makes this clearer. It's pointless to try and move the gear box further back than this as the drive shafts will foul the space frame. The price you pay is that your gearbox is now sitting about 20/25mm higher than the 2CV one would. No problem except that there is now no room under the light bar for the starter motor solenoid!! I overcame this by using a remote solenoid and a cable operated starter cog engagement pull wire!!

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So those are the options; back and high or low and forward; take your pick! Either way, you must now make a mounting for the aft end of the box, and a pair of hangers for the normal Pembleton way to hang the engine/gear box from the light bar. Look at Claus's site for his very elegant solution and then David Parr's way on Silverfish. Oh, one last thing! You need to change all the rubbers in the calipers to LHM compatible ones.


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Preparing the Space

My gearbox rear mounting arrangement takes up a little more room than the 2CV one. To make room for it, I cut a small "alcove" into the bulkhead - a little like one of the footwells in the floor plan.(see pic.). The only other mod to do before installation is the position of the cross member mounted hand break lever. I moved mine as far as I could toward the battery; in fact, the lever moves just a couple of millimetres off the battery side. (see pic.). It's best to trial fit the gearbox and callipers so that you can position the handbrake lever optimally and plan the battery area.. A good time also to check your mountings fore and aft. In light of the above, it's best not to buy your battery, or to make the battery tray, untill you have planned the gearbox and brakes installation. I had already bought a standard 2CV battery, but with hindsight, I can see that a better arrangement would be possible with a differently proportioned battery - possibly a motorcycle one. Mine does all fit nicely, but it was a struggle! You could also consider mounting the battery in the area under the spare wheel, as David Stevenson has done.

Getting it in

The weight of the engine, flywheel and adaptor plate is 65kg dry; the gearbox 28kg. I didn't fancy trying to manhandle the engine onto a preinstalled gearbox, so assembled the pair on a table at the right height to slide into the car. It worked well. Once the basic engine/gearbox lump is in place and bolted, you can install the hand brake lever, brake discs, calipers and drive shafts.

Final Thoughts

Moments For my own interest, I took moments about the axis of the 12mm hanger bolts.. The aft moment is roughly 900kg/cm, and the forward one 1200kg/cm.

Engine Support As mentioned earlier, the combined weight of the engine, gearbox, callipers, discs and half of the drive shafts, exceeds 100kg; and it all hangs on two 10mm bolts! I don't know if that's a good idea or not, but it looks a little "skinny" to me. The 100kg load is not static either, it must increase dramatically on fast corners and when bouncing up and down on rough surfaces.. At the time of writing, I'm making a pair of tie rods to support the front of the engine. I'll post a pic when finished. I'd be interested in hearing from our experts on this point.

Gear Change You may recall from the forum that I tried to engineer a 2CV type gear shift mechanism to operate the "G" series box. Unfortunately I have failed. As David S. pointed out, if you use a 2CV rod directly to the gearbox, you end up with 1st and 3rd towards you, and 2nd and 4th away - a quirk too far perhaps! I've ended up using the floor shift from the original LNA that I dismantled to get my bell housing. It looks good and works well! Pic when finished.

Registration I'm aware that, in using a non-2CV engine and gearbox, I run the risk of not having enough parts from the doner car to qualify for age related tags. It might hang on whether I mention that the front brakes are not 2CV! However, as I intend to "import" the car to France, it's not a big issue.

All the best,

Peter J.

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