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ASSEMBLING CARBURETED SMALL-TWIN TOP ENDS
Throughout assembly make sure all reused parts are sorted by their respective cylinder. This rule also applies to newly bored and new plated-bore piston/cylinder sets. Begin assembly with all parts clean and ready for assembly. Newly bored or honed cylinders should be washed in warm water and soap and then very lightly coated with oil to prevent rusting.
Check the ring end gaps by placing the rings one at a time in their respective cylinder’s bore. It helps to use a piston to square the ring in the bore. Measure the ring gap with a feeler gauge. Rings with excessive gap (beyond the specified wear limit) should be replaced. Rings with insufficient gap should be carefully filed. Make sure you remove all small burrs. Once rings have been gapped to a particular cylinder don’t mix them!
Install the rings on their respective piston. Many rings have orientation marks but many aren’t too clear. The rule is that any stamped words or marks go up. Install one wristpin clip in each piston on the intake side (the arrow on the piston crown points to the exhaust).
Begin by installing the base gasket, making sure that any oil return holes line up with holes in engine case. Warm each piston by either placing it in a 200º oven or simply warm the piston pin bosses with a propane torch. Put a little assembly lube or at least some oil on the pin bores of the pistons and rods. Attach the pistons with the arrows on the crowns pointing towards the exhausts (forward) and fit the remaining wristpin clips after first damming off the engine case so that errant clips don’t fall in. Position the rings so that their end gaps point in different directions. There’s no need to get too technical on this point - there is no particular order that must be followed.
Slide the cylinder over the long studs and over the top of the piston. It isn’t necessary to use a ring compressor on Moto Guzzi rings although some folks prefer them. We just use a small, non-sharp screwdriver to encourage the rings into the taper provided for that purpose at the bottom of the cylinders. If a ring doesn’t fit into its groove easily there’s probably a good reason. Using force here will accomplish nothing positive.
Once past the rings, slide the cylinder down close to the base gasket and repeat assembly to this point on the other side. When both sides have been brought to this point, place the new o-rings into the base gaskets and carefully lower each cylinder down against its base gasket without dislodging the o-rings.
The V50 II doesn’t seem to require a sealant on its head gaskets but it’s mandatory on all other 500s and all 2-valve 650s in spite of the fancy new no-leak head gaskets. The Lario isn’t as prone to leaking but they’re still not as oil tight as we would like. We recommend the use of Permatex Super 300 on all small twin head gaskets just to make sure. It’s good enough for diesel head gaskets so it’s good enough for Guzzis! Apply a light coat to the gasket surface of the cylinder and head. Put a dab of anti-seize or at least some grease on the threads of all the studs. As Super 300 doesn’t require a set-up time, go ahead and fit the gasket and the head to each side.
Refit the rocker support apparatus along with the various head nuts and washers. Using a torque wrench, draw the head nuts up to 20 foot pounds starting with the four long studs in a crisscross pattern, then the short stud. Repeat bringing the torque up to 25 foot pounds.
If possible, leave this part of the assembly overnight before continuing. The head gasket will compress over time and after several heating and cooling cycles. Torque the nuts again up to 25 foot pounds and then torque the long studs’ nuts up to 33 foot pounds. Assemble the remaining components as they came off but leave an extra .002” valve clearance so that the compressing gasket won’t leave the valves too tight.
After several hundred miles or at least a half dozen heat and cool cycles (bringing the engine up to operating temperature and then cooling it completely) the heads must be retorqued and the valves set to their standard adjustment. We mention the heating cycles because it may not be sufficient to just drive several hundred miles all at once and figure that the gaskets are fully compressed.
As any fastener takes a set that makes it seem to be torqued higher than it actually is, it is necessary to loosen all head nuts before bringing them all up to first 25, then 33 foot pounds for just the four long studs’ nuts.
Regarding base gasket leaks on carbureted models- The common problem here really isn’t the base gasket. On some bikes the inside stud’s threaded hole broke through into a pressure oil passage.
Moto Guzzi suggests an update that involves machining a groove and installing an o-ring (90706076). That’s money and down time and necessitates a new o-ring any time the top end is removed. The more permanent quick fix is to remove the stud, clean the threaded hole, and refit the stud with blue LockTite. The trick is how to clean the hole of oil so as to insure the LockTite will seal. We’ve had perfect success by blowing it out with compressed air, spraying it with contact cleaner, then blowing it dry again with air.
Although Moto Guzzi indicates the leak is possible from either side of any small twin, we’ve only seen it on the left side of V65s. Still, because of their warning, we believe it’s a good idea to update both inside studs of any small twin whenever the top end if off.
Lubrication updates - For the two-valve small twins, Moto Guzzi has introduced rubber caps that are installed open-end up around the top of each pushrod to hold a puddle of oil between the pushrod and rocker arm. Part number is 27045420 (each, four required).
Updates have also been called for to the dowels between the cylinders and the heads. For the 500s, the oil passage through the dowel is increased from 1 to 3 mm (.118”) or updated dowels can be purchased (19021321). A similar update for the early 650s increases the hole diameter from 2 to 3 mm with the use of another unique dowel (19021360).