Moto Guzzi V50 C

The high-performance (or at least higher performance) V50II (and V65 Lario) Moto Guzzi article

Note: I sold all the smallblock Guzzis at the end of August 2009. Their time had come and gone and it was better that they went to a new home.

By Michael Moore, Copyright 1996-2000

The V50II is another of my (many) hop up projects. At present I’ve upgraded to V50III pistons and valves/springs. Since the later piston uses a bigger wristpin I put in Carrillo rods instead of spending the money on stock rods. I’ve lightened the flywheel quite a bit and had all the rotating/thrashing parts balanced.

Craig has increased the flow in the head a lot (80% over the stock small valve II head if I remember correctly) and I had Megacycle do a regrind on the cam. I think I had them do a lobe profile that they don’t normally show in the catalog (Craig was working at Megacycle at that time, so he could check to see what was available – Megacycle has a lot of profiles not listed in the catalog.)

I’ve bored a set of 30mm Dell’Orto carbs out to 36mm for use on the bike, and I’ll also build a 2-1 pipe. I’ve been looking at putting on a Lucas alternator as I plan for my SP, and mounting a RITA ignition on the cam, as per the III model. This will require an electronic tach, but that isn’t a problem.

The above mods will let me shorten the front of the motor quite a bit, allowing it to be moved closer to the front wheel.

‘ll probably do a different frame. The stock frame design doesn’t impress me too favorably, and the stock forks are junk. A frame is easy to do on the little twins since the swing arm mounts on the transmission, simplifying things noticeably.

I’ve got a set of wheels from a Honda CB-1 400, which are 3/4 x 17, and I keep looking at the cush drive to see if I can adapt them. This will require a new swing arm too, but what the heck, the stock swing arm isn’t particularly wonderful either.

Craig wants to pull the Astralite wheels and short leading link front end off of his 450 Honda single road racer to put on his V50, and I haven’t decided what to do on the front end of mine. I’ve got all the parts for a hub-center front end that I’m trying to get around to building (the Laverda 750 race bike has higher priority at the moment) – maybe I’ll stick it on the V50, or I’ve got all the tubing on hand to build another leading link front end like Craig has.

Then again, I could do a Hossack/Fior front end as used on the Swallower Guzzi race bike. Sometimes, you can have too many options!

Here are some fairly large-scale scans of the the wiring diagram for the V50II (Bosch electronic ignition version) that may be of help to some of you.

New addition as of mid-December 2000

The following is a compilation of a series of posts I made to the Moto Guzzi email list after starting back into the V50 project at the beginning of 2000.

V65 Lario 4 valve engine

I found a 650 Lario engine with dropped exhaust valves on one side that is otherwise supposed to be in pretty good shape. I hope to have that here in a few weeks and will then get a cylinder head up to Craig for analysis. Remember how much more flow the 4v Honda head gave vs the 2v Guzzi head?

There might be some possibilities with a short stroke 500cc Lario.

The Lario engine arrived. I did most of the repair welding on the damaged Lario head after removing the valve seats and Craig will finish the rest (I have a small 50A electrical service into the house and the TIG welder will start tripping the breaker when welding big chunks of aluminum like a cylinder head). He’ll also see about sending the cylinder from that side off for repair/replating.

If you have nigasil/nikasil cylinders that are damaged internally you can get them welded/machined and replated, generally for under $200/cylinder (I think that is accurate – it varies with bore size and amount of welding needed) so keep that in mind.

The cam lobe for the exhaust valves that lost their heads was looking pretty sad. Craig told me that what probably happened was that the lobe went bad and got the valves chattering bad enough that they ended up failing, as opposed to the valves failing on their own. I don’t think that any of the tappets looked really bad so it may have been a soft cam rather than a tappet that caused the wear.

Cylinder heads

I spoke with Craig today and he says the 36mm intake valve does look like it will fit, though it is going to need a new intake valve seat – the V50II intake seat is actually just a bit small for the V50III Monza valve we were going to use, and these new valves are a couple of mm bigger than that. He should get the valves/seats ordered today or tomorrow if no crisis erupts in his shop.

A side conversation has brought up the power potential of the V50, and how unlikely it seems that it could pull a very thin skin from a very small rice pudding.

Herron heads can perform well – the Cosworth SCA, a 997cc 4 cylinder with Herron head from 1964 had these specs (pulled from the Ginetta club website – isn’t the Internet wonderful?):

Bore 81.0 mm (V50 74mm)

Stroke 48.36 mm (V50 57mm)

Rod ratio 2.85:1 (vs 1.86 for the V50 – longer would be nicer)

Compression Ratio 12.5:1

Gross Horse Power 115 bhp minimum @ 8700 rpm

Gross Torque 76 lb ft @ 6800 rpm

Carburettor: two 40 DCM2 Weber Choke size 32mm

36.8mm intake valves and 31.75 exhaust valves with 7.15mm stems.

That had purpose built heads designed for racing, but Craig has been doing a lot of reshaping/filling in the V50II heads which will have valves nearly as big but with 1.65mm smaller stems (less restriction).

So if, in 2000, we can come close to matching the bhp of a 26 year old race car design (and I bet a lot of the internals were a lot bigger/heavier than the V50 parts), then we are looking at a possible 57 bhp, which is pretty darn good.

We’ll have to spend some time on the dyno (Craig is working on getting one set up) but if we can get only(!) 50 bhp at the rear wheel that would still be comparable to a bevel 750 GT/Sport Ducati which weren’t exactly layabouts (not to mention a lot heavier than a V50).

The flow bench records show the modified V50II head with Monza valves is doing 87.5 cfm @ 15 of water at .400 (intake) valve lift. I didn’t write down what the stock II head measured out at, but I think the max flow was around 50 cfm, so big progress has been made (I don’t know what a stock V50III/Monza head does). Craig doesn’t plan on doing any welding in the exhaust port – he says that side is doing quite well as it is.

He is going to get a 36mm intake valve and seat to put in, and thinks that 95 cfm is pretty likely, and an ultimate 100 cfm may happen.

36mm Dell’Ortos will be sufficient for that level of flow.

For comparison a very nice MX BSA B25 head was doing 96 cfm at .400 lift, and a STOCK XL250 4/v single head does 115 cfm with a pretty nice XL250 flat tracker head doing 124 cfm.

Obviously, more valves are handy to have.

I made a small fixture for the Lario heads for screwing the valves open on the flow bench and we did make several test runs on the good stock Lario head. Craig’s comment was that the same person who did the ports on the 2v head must have done the Lario as well, as both flow pretty badly stock.

The first run on the stock head (15 of water vacuum) showed:

You’ll note that the exhaust flows more than the intake for the first .200 of valve lift and is nearly the same as the intake for the rest of the lift – bad, bad, bad.

The stock Lario cam gives right at .300 valve lift (which is where the stock port flow flattens out), and since the port doesn’t flow appreciably more after that point there isn’t much reason to put a hot cam into a Lario with a stock head. The Megacycle cam (which I’m not going to investigate further until we get the head working a lot better) will give about .380 valve lift.

The stock valves (27/24mm) are smaller than the stock valves in a 1972 Honda XL250 4v single (same bore/stroke as a V50). The stock seats will allow a bit bigger valve though there may not be enough room, even with new seats, to put the size valves a powerful 325cc single (650 twin) would want.

Craig then did a little bit of grinding – less than 5 minutes. He removed the ridges around the valve seats in the combustion chamber (bad for low lift flow) and relieved the intake ports a bit in the area around the valve guides, as well as smoothing out a noticeable bump/dip combo in the port.

The second run showed:

As you can see, that little bit of work made some significant improvements in air flow. These were not big changes in the ports and could have easily been incorporated in the casting from the factory. The intake at low lifts is still not sufficiently better than the exhaust flow so more work is needed to bring things into balance.

The balance between the ports at .250 and higher is looking better. The Lario head still needs to pick up

15 cfm on the intake side to match a stock XL250, and 25 cfm to get close to a nice race XL250 head.

Craig is of the opinion that for this year we should just stick with the 2v heads, especially since we’ve got the valves/cam already in hand for that. We’ll work on the Lario heads as a possible upgrade for the future.

The V50II heads came back with the larger diameter intake valve seats. Craig also has the new stainless steel valve blanks that are

2mm bigger than the V50III/Monza valves so once he gets a chance to shorten those and cut the valve seats in he’ll be able to put the 2v heads back on the flow bench. He’s expecting a fair increase in flow from the bigger valves, plus the old valve seats were too large an inner diameter for the valve size, and that has been corrected with the bigger valves/seats.

The V50II cam with new Megacycle 630-x1 grind has .308 cam lift, 106 degree lobe centers, and 19/51, 51/19 timing.

We’ll be running shortened Precision Machining valve guides (nice AMPCO 45 bronze parts) and I think the valve springs we’ve decided on are R/D Spring kits for an XR200 Honda single.

Cylinders, pistons and bottom end

Pistons – A rough burette check of the Monza piston makes it appear that it will give about 11:1 compression. That is presuming no more than .040 squish clearance. CYCLE magazine listed 10.5:1 for the III/Monza so that seems to check our rough numbers. Craig says that should be fine for the street, but that we’ll want to have new pistons made for Bonneville to help compensate for the high altitude.

He had an opportunity to talk to Keith Duckworth of Cosworth about the Cosworth SCA engine (1 litre four with Heron heads) and was told they ended up with a flat top piston with two pockets for the valves and a little pocket under the spark plug – all of those being connected. Duckworth said they maximized the SCA compression ratio – probably in the 14:1 area or thereabouts.

Arias will make any piston as long as 4 are ordered, so after we can get a head/engine roughly assembled we’ll fill a piston with clay and find out the minimum clearances we’ll need. In any event the bowl in the Monza piston is likely to disappear. Thinner rings will also be sought at that time, along with a tapered wall wrist pin.

With the high compression I’ll have to be careful about not applying full throttle at lower RPM so as not to induce detonation.

The Lario engine has the 15mm rotor oil pump and there are some dimensions different for chain sizes, offsets of the sprockets from the base of the pump etc that will have to be looked at if we decide to upgrade the V50II to the later pump. However, Craig feels that the stock oil pump, when combined with some streamlining in the rest of the oil system and the oil control sheetmetal we’ll put in the crankcase, may be sufficient. Also it will be a bit less prone to cavitation at high speeds(that is a reason that the Butler Smith BMW race twins went with a smaller than stock oil pump).

Moto Guzzi V50 C

I’ve been hearing stories about the small twins being prone to blowing their oil out the breather, and a quick look inside the crankcases reveals a likely reason – a complete lack of any semblance of oil control mechanisms.

The poor oil probably stays mostly wrapped around the crankshaft, being beaten to a froth by the rods. It looks like the main way oil would actually get back into the sump would be from the cylinder head drains.

I’ll be constructing a scraper assy for the crank – this is NOT just a flat plate sticking out next to the crank. Craig made sure I understood just what to do for a F1-style system which should see the oil removed from the crank while not causing any drag to things that are rushing around in the crankcase.

Some additional holes will probably need to be drilled in the floor of the crankcase to help the oil back into the sump. I’ll have to also deal with a method for securely fastening the scraper system in place – it needs to fit quite closely to the crankshaft in several locations.

Our usual exhaust and check-valve assisted crankcase breather system will be installed to help pump down the cases.

Eliminating the internal pressures and battering of the oil will recover some wasted energy.

Clutch and transmission

Clutch – Craig has concerns about the V50 clutch taking the increased power, especially at high speeds where the drag is greatest. He’ll be contacting a manufacturer of race car clutches this coming week to get some drawings/data on 4.5-5 clutches. A small clutch like this (and that is the OD of the basket) will take about 200 bhp from a 4 cylinder car, but we may need a little more to accomodate torque spikes in a twin.

When I talked to Tilton some years ago about clutches they said that on the Wittner race bike they had to go to a bigger clutch than was needed for the bhp to deal with the torque spikes. I think they ended up with a 7 clutch, though I can’t recall if it was a single or dual plate unit.

Craig says the clutch guys will usually install whatever spline is needed in the clutch plates, though we could always rob the centers out of V50 plates if necessary and rivet them in ourselves.

Many of the clutches are now available with a push type of heaver, as used by the Guzzis. We’ll also look to see if there is enough room for an annular hydraulic pusher as used by many race cars.

However, the V65 clutch is a fall-back option (though it might need the V65 transmission to go with it).

Gearing – the Monza primary ratios may prove to be the best choice – and at this time it isn’t known for sure if the taller Lario gears will fit or can be made to fit. I may have to have different rear wheels set up with different OD tires to use to make minor gearing changes. The II/Monza/Lario primary ratios make some pretty big jumps in gearing.

A chain final drive would certainly be handy!

This is what I was able to extract from my V50 manual.

Monza primary drive – 1.4666

5th gear .909

rear drive 3.875

overall gearing – 1:5.1661

A typical Avon 110/90×18 rear tire is

26.2 OD which would have a circumference of 82.31

Some rpm/speed combos would then be:

8000 rpm = 1548.557 rear wheel rpm = 120.70 mph

8500 rpm = 1645.342 rwrpm = 128.24 mph

9000 rpm = 1742.126 rwrpm = 135.79 mph


As mentioned before, the 2v heads won’t outflow the 36mm Dell’Orto carbs, so that’s what they’ll end up with. The carbs will be mounted on different manifolds as the stockers are quite small in diameter.

I’ll build a 2-1 exhaust to Craig’s specifications – they work well on all the other 90 degree twins he’s built them for (600-950cc Ducatis, 500-1000cc Moto Guzzis) and I have no doubts it will work well here too.


Chassis – I’ll have to try and get a roller mocked up so we can start looking at that. I’ve got a set of 35mm Kawasaki KR250RR fork legs and both Ceriani and Guzzi T3 triple clamps to play with. Right now I’m thinking about cutting the steering head off entirely and putting a new one in that will have a much lower top. If I don’t do that I’ll just shorten the top of the stock steering head and make new bearing cups to take taper roller bearings.

The forks will also be shorter – we want to make the bike as low as possible and reduce the frontal area. Back to the home page #169 1996-2009 Michael Moore, last update for this page 01 September 2009

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