Rebuild of Benelli 750 Sei engine

My father and I have a few 70’s Italian motorbikes, one of which is this 1973 Benelli 750 Sei.  Sei in Italian means six – the bike has a straight six cylinder engine with accompanying 6 exhaust pipe – its madness – hideously over engineered but at its heart it is very similar to a Honda 750 from the same era but with an extra 2 cylinders.

We have owned it for about 4 years, bought it not really running and unregistered.  Registration of it was a bit of a drama but it was eventually achieved, however, it has always smoked from number 1 cylinder (the left hand side one).

We pulled the head off about 3 years ago thinking it was valve stem seals or something at the top of the engine leaking oil – replacing of all valve guide seals and a new head gasket didn’t fix it and it got progressively worse so that now 1,2 and 6 are blowing smoke.

There was nothing else for it but to pull the thing apart again so about 3 years after the first pull apart off we go again.

Its turns out the piston rings, in particular the oil rings, were all odd styles and were not doing their job, of the 6 pistons there was about 4 different oil rings being used, 3 of which were not tight enough in the bore to do anything really and one of which was badly damaged – no wonder it blew smoke like a two-stroker.

The problem with having such an obscure bike is that the suppliers of parts are fairly limited so it was to the rescue with a new set of piston rings, head and base gaskets and various o-rings that we may as well replace while we have it open.  We also purchased a tool that compresses the piston rings so that they will slide into the bore.

After having the barrels honed and the whole cylinder block bead blasted it was time to put the new rings on and “try” to get the 6 pistons into their cylinders.  To say it is a little fiddly is an understatement.  There are 6 cylinders, all at various heights going into a cylinder block that is one piece.

After about 2.5 hours of fiddling we had managed to get cylinder 2, 3,4 and 5, however, 1 and 6, because of the clearance between the bottom of the barrel and the top of the crank case, were much more difficult they need to go up into the cylinder but to do so required the rotation of the crank which resulted in pistons 3 & 4 coming out of the bottom of the cylinder so in the end at 1:30 in the morning we called it a night and decided to take the cylinder block off again and start again another day.  Never mind, nothing broken at least which is the biggest problem with things like this when you are frustrated and tired.

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