Re-bored to 0.25mm oversize after releasing seized pistons from barrels. Inlet valves 1 and 2 were replaced due to heavy pitting, otherwise OK.

Bead blasted top end.

Known problems:-

Broken cooling fins – The most obvious – have all the nuts been removed?, don’t forget the two nuts by the spark plugs on cylinders 2 and 3, these live outside the engine and will be heavily corroded. One of these refused to move for me and rounded off, I hammered an undersize socket onto the remains and luckily it came undone. I use a hard wooden batten to spread the impact, running the full length of the cylinders ‘tap’ free the cylinder head with a mallet and again with the cylinder barrels, if stuck (usually the barrels) try applying even heating (to avoid warping the components) around the fins with a hot air gun or similar. Persevere with the mallet blows, it will give in eventually! Take every care as a broken fin ruins the apperance of any engine and do not try to pry apart with a screwdriver.

Camshaft wear occurs if the engine has been thrashed from cold or regular oil changes neglected. The oil ways to the head are very small and may be blocked.

Cam chain problems – new tensioner blades and cam chain at around 30,000 miles. More

Pistons are reputed to wear at 20,000 miles. This may be attributed to neglected oil changes or general thrashing to the red line, I’ve even heard someone blame electronic ignition – who knows?. It seems to start at the little ends causing pistons to move about, this in turn cracks the piston rings, then as they disintegrate they destroy the cylinder bores – beware!!

NOTE: Piston bore wear occurs mostly at the top of the stroke, check top & bottom with a piston ring. If the end gap is equal top & bottom but outside the given tolerance of 0.7mm maximum then replace the ring set, the cylinder bores are OK, otherwise have the cylinders re bored to the next oversize, i.e. whatever size you have been able to source as parts supply dries up. Check the little ends very carefully if only replacing piston rings, any doubt then replace the pistons, piston pins & the 8 clips.

Piston rings – take care when feeding them into the barrels. One engine of mine the previous owner had bent the bottom most oil ring out from its seat and it trashed the liner when run forming a deep groove. I presume the engine must have smoked a little?. Do make sure the the top and second rings are 120 degrees apart and facing forward (a rough guide is they will be facing the two studs in front of the piston), also make sure the two oil rings are at least 30mm apart, if these are too close the engine will burn some oil. Piston ring compressors are not essential but it is easier to keep the rings where you wanted them. Here I’ve used two baked bean cans and zip ties as my homemade solution.

Exhaust valve guides wear around 60,000 miles. This is a job for a specialist, have the valve seats reground for the new valves at the same time for a perfect seal. There are 8 o-rings (one for each valve) included in gasket sets, use them to avoid oil dribbling into the cylinders, this gives for a smokey warm up.

Leaking head gaskets are usually for the following reasons:-

1    Cap nuts not torqued to the correct 2-2.3 Kgm or 14.5-16.6 lb ft.

2    Rubber o-rings in the camshaft oil feeds have hardened with age.

3    Some castings were porous from new.

4    Poor quality pattern gasket set, try to use OEM parts if possible.

5    Engine revved beyond its red line.

6    Warped cylinder head due to incorrect tightening sequence, see workshop manual.

7    After at least one hot/cold cycle of the engine the cylinder head nuts will require one more tighten.


    On early F models up to s/no.1057306 slightly shorter cylinder studs were used, this allows very easily stripped threads on the cap nuts, first clean the threads with a tap & die set and have some spare cap nuts to hand before starting. I also like to use a gasket compound like Wellseal by Stag Ltd; or Copper ‘spray a gasket’ by Permatex on the head gaskets. When torquing down the head always double check the torques are correct by making further passes of the sequence, ideally the next day (use a 3/8″ digital torque wrench if available).

    The CB350F cylinder head is not interchangeable, differs in appearance, mountings & valve seats.

    Gasket material is hard to remove. Block all openings to the crankcase before the next stage. Use a gasket scraper or with care a blade of some sort. Scotchbrite any last remaining adhesive, use with a solvent if your paint is resistant.

    Tip – If re-using cylinder head, or any copper washers for economy, anneal them by heating to cherry red and then quenching in cold water.

    Tip – I torqued my cylinder head nuts to a higher 19ft-lbs and so far (3,750 miles) no oil leaks.

    Tip – after replacing head gasket, use old rocker cover gasket. The old gasket will seal better if boiled in water for 10 minutes. Keep the new gasket for use after the first hot/cold cycle and subsequent re-torquing of the head nuts.

    Tip – if running the engine hard is to occasionally snap the throttle closed, this will allow some oil to be sucked up the cylinders.

    Tip – before attempting to remove exhaust studs, apply penetrating oil for a few days. Now heat the stud with a blow lamp and remove using suitable tools. If particularly stubborn, heat again to cherry red, sometimes the aluminium of the head comes out with the stud but it is preferable to heli-coil rather than drill out a snapped off stud. Studs always break flush with the aluminium and are at an angle which makes drilling out difficult. If any still break off it is useful to have someone sight the angle against an adjacent stud while drilling. In some cases a nut can be welded on to the remaining material and can then be removed using a socket set.

    Tip – removing seized in engine dowels. First try heating the entire component in an oven (avoid localised heating as it may warp the component) and while at 100 degrees C try again. Next I tried inserting a suitable size of allen key and then crushing the dowel onto the key with mole grips. This allowed a twisting motion to pull out the dowel, a replacement will be required with this method. Don’t forget to apply copper slip to your replacements when re-building.

    WARNING – Be very careful NOT to drop any screws, washers or tool bits while working on the topend. The two drain holes under the breather cover lead straight down the cam chain tunnel to the bottom end. If you are lucky you may be able to retrieve the offending article via the sump pan, if not a complete bottom end strip. Rags placed in strategic areas may give some protection, just remember to remove them afterwards. Having a check list like a Surgeon would use for swabs, instruments, needles etc. is recommended.