Technical Engine CC Enlargement

[Bleeding Knuckles]

Greetings, I've been considering fitting some bigger cylinders to my engine and think that 800cc (86mm bore) would suit rather than going up in smaller steps. My engine is presently 704cc (80mm bore) and I don't believe that the cylinders would tolerate a rebore to 86mm. I have been scanning the net for information and have found at least four suppliers mainly in Europe. One thing I have yet to easily obtain is the size of the recommended crankcase modification for the cylinder spigot to sit in. Has anyone anyone here had the mod done??? and if so could you furnish me with the mod measurements please, and does the mod size differ with supplier???? Thanks.
Ian.

 

[the hobbler]

Ian; I don't know the crankcase aperture size for 800cc, but for the "Athena" 740cc kit (739.94cc to be pedantic) the aperture has to be opened up to 89.25mm (+/- 0.1mm). Having had this done for somebody, i can assure you that the enlarged 'hole' is getting close to the head-stud threads. The bore for that engine is 82mm--you might be able bore out that size of barrel to give you a bit more cc, but I think that size of aperture is about as large as you want to go. The difference in crankcase aperture size between the '695'engine and the '740' engine is 3mm. Also be aware, the 800cc engines have a habit of running a bit rough and a bit on the hot side.

 

[Toshi 975]

I never made any measurements when I rebuilt the 1980’s Alquati conversion engine a couple of years back. There was always speculation that it was based on half a VW 1600 engine but the final conclusion was that it used a purpose built 85mm bore kit that originated in Italy with Assi pistons. Because the sealing land on the crankcase for the cylinders was much diminished they compensated for this by making the cylinders a press fit into the crankcase. So much so that I was not happy trying to remove one cylinder so left it in place.

 

[Toshi 975]

Also be aware, the 800cc engines have a habit of running a bit rough and a bit on the hot side.

I did see signs of overheating on no 2 piston, broken lower ring and a little piston scoring. However I also discovered that the oil pickup oil seal had been badly fitted which may have contributed plus the original owner was an RAF Test Pilot and amateur racing driver , I know he had a need for speed so I am sure that he would not have spared the horses in its short life.
It took me some time to get replacement piston rings, eventually getting some old Subaru ones from an outlet in Greece. They were a better design than the originals so hopefully about those along with some Mobil 1 10/60.
With the Alquati camshaft, stage 1 head & lightened flywheel I expected the engine to be lumpy and a bit noisy with the modest sport exhaust but on start up it purred like a kitten.

 

[Toshi 975]

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