Bore and Stroke 101 question

[snlsmith]

During a recent valve job on my 620 Sport, the mech noticed the smell of gas in the oil. He said I would probably need a rebuild in the next 5k miles, including new rings and having the cylinders re-honed. This is all an educated guess right now since he didn't go any further into the engine than he needed to do the valve job.

My question is, as long as I would have to go through this trouble of having the engine torn down, would I be able to replace just the rods (believe that is the right term) in mine with those of an 800SS? This would change the stroke from 61.5 to 66 and up the cc's from 618 to 664. It seems like this would be a cheaper option than boring the engine. HOWEVER, since I have very limited knowledge of engines, I am not sure if this is even a feasible option of it would include a lot of other work and parts as well. It seems like it were that simple, Ducati would have just done it to begin with.

If this is a viable option, what effect would the stroke change have on hp and torque? It seems logical that they would both increase, but maybe give poor linear supply?

[st2lemans]

Originally Posted by snlsmith

would I be able to replace just the rods (believe that is the right term) in mine with those of an 800SS? This would change the stroke from 61.5 to 66 and up the cc's from 618 to 664.

No, it wouldn't.

Stroke depends only on the crank throw, not the rod length. You will need a new crank to change the stroke.

Much easier (and cheaper) to change the bore.

Tom

[Muschi]

the cylinders are nigusil or nicasil plated, and can't be honed anyway.
i'd change the oil, and have a sniff some longer time later, to see if
it smells like fuel again.
than you could only change the piston rings and be fine with it i guess,
as the plating and pistons themselves should be still okay.
depends on overall mileage of course.

i'd probably wouldn't get the engine repaired at the shop as this could
be rather expensive, but have the bike sold and get a newer and bigger
one for the amount of the repair plus what you get for your old bike.

cheers.

[snlsmith]

Originally Posted by Muschi

the cylinders are nigusil or nicasil plated, and can't be honed anyway.
i'd change the oil, and have a sniff some longer time later, to see if
it smells like fuel again.
than you could only change the piston rings and be fine with it i guess,
as the plating and pistons themselves should be still okay.
depends on overall mileage of course.

i'd probably wouldn't get the engine repaired at the shop as this could
be rather expensive, but have the bike sold and get a newer and bigger
one for the amount of the repair plus what you get for your old bike.

cheers.

never smelled gas before, and it is due for a change anyway, so this will be the first choice. I'll check the smell in 1000 miles or so. I've also considered just doing an engine swap with another 620 (a couple on Ebay right now for less than $500) and then just have a parts engine.

[Bella749]

Originally Posted by snlsmith

never smelled gas before, and it is due for a change anyway, so this will be the first choice. I'll check the smell in 1000 miles or so. I've also considered just doing an engine swap with another 620 (a couple on Ebay right now for less than $500) and then just have a parts engine.

Did the mechanic to a compression/leakdown test to arrive at the conclusion of a necessary rebuild? I would send an oil sample off for analysis before dumping the $$ into it. If you are, indeed, getting excessive blow-by, by the bike is still making good power, you could just change the oil more frequently as a precaution.