150 to 185 Conversion

[dapper]

Have a 150 casing which I'm rebuilding. Have opened up the transfers on the casing and the transfers on the cylinder (cast iron) with my dremel, and made a nice job of it too Matched nicely. Cleaned up the inlet and exhaust ports, but not opened them up in any way yet, just matched the inlet manifold to the inlet port. Am thinking of having the cylinder bored out to 185cc (64mm piston).
Would I need to do anything to the exhaust and inlet ports after rebore? IE, would they need to be reshaped in anyway. Know feck all about port timing Not going down the re-rodding of the GP crank/Wiseco piston malarky. Have drilled and lightened the rear sprocket
Advice???

[tarmac tickler]

reshape the ports after boring??? yes you will mate,., a 150cc piston is 57mm a 185cc piston is 64mm, a lot of material is taken off the cylinder,, you will also need the head profiling to suit the piston
youll be surprised how much bigger the ports will be, especially the transfers
the edge of the ports will be razor sharp and will need rounding off or blunting or they will snag the rings you can do this with a small bent rasp or file, dont use the dremel as its too easy to slip and mark the bore

[dapper]

My reborer is a good guy and knows his stuff. He knows about chamfering port edges to prevent ring snag and checking piston to bore clearance correctly. It was more about the inlet and exhaust ports really, as they will be larger and the inlet will not go so far round the cyclinder once rebored. He can also reprofile the head to suit the piston. So if all the ports, inlet exhaust and transfers, are all bigger after reboring, will they need reshaping?

[nuttyboy]

talk to roger at pro design in littlehampton .he does a great 185 conversion.

[sean brady scooters]

the ports will certainly NOT be any bigger after re boring and will in fact end up being smaller in relation to the new larger bore size ...for eg ,and say following the 65% of bore rule for port widths ,.
a std 150 @ 57mm and having a 65% width exhaust port ,would be say 37mm wide.
where as a 64mm would really require a port 41.6mm wide.
and the same for inlet port width,and transfer widths ...

further complications can arise as regards port timing /durations because of the angles of entry from transfers and inlets into the bore ,because yet again these will typically be reduced when over boring a cylinder that much .

the above will hopefully explain just why a 125/150 barrel thats just bored out to say 175 will never work as well as a proper 175 barrel .