DIY 620 to 750 Big Bore Conversion Questions

[2 Wheel Wanderer]

Hi All, I have an 05 Monster 620 and would like to do a 750 conversion on the cheap. I have eyed some parts, 03 M800 cylinders and brand new M750 Pistons.

I just wanted to know if the cylinders will work for this application. I think it will but wanted to confirm. I wanted to make sure the 620 heads will seal on the 800 barrels due to the barrels being a bit wider.

I'll be doing all the labor and the parts will cost <$300.

Thanks for your help!

[Speeddog]

Not sure about 620 head to 800 cylinder fitting.

Check piston to valve clearance, valve position on 620 head may be a bit different.

[2 Wheel Wanderer]

Would 750 cylinders work better?

[caperix]

No  FHE but I have heard of people putting 620 heads onto a 750 due to better flow, so it should work.  How much do they charge for an overbore & replate, I did not think it was too expensive from ferrachi.  High compression pistons would be a good idea as the 750 ran pretty low compression so you may not see much power improvements with stock 750 pistons.  I have a set of 695 cylinders & heads in my garage that one day I will get around to installing on my 620, Im hoping the bigger valves & cams of the 695/800 heads will put the power close to a 800.

[Slide Panda]

Don't forget to gather up what's needed for an oil cooler.

[2 Wheel Wanderer]

Thanks caperix,

From my research I seem to remember that boring and plating from FBF costs about the same as the big bore pistons . I didn't want to spend $419 for the high comp pistons from FBF but you are right about the extra power they would give. I'll keep looking for different cylinders.

Thanks Slide Panda! That was my next search.

[Duck-Stew]

88mm barrels from an '02-'07 bike will work (the '03 M800 you're looking for fit this).  There was some monkey-ing around by Ducati w/the alignment dowel between the cases & barrels around 2000-2001 but anything 2002 & up will have that changed out.

Fit the oil cooler.  Even 620's could use them.

The 620 head combustion chamber should work for the 750 pistons as I've done this conversion to an '05 M620 motor using some 2002 M750 barrels w/the FBF drop-in hi-comp pistons and it ran quite well for what it was.

Best pre-696 small-case Ducati cylinder head though is the 695's.  Many years back, Speeddog & I did a set of FBF drop-in pistons onto a 2001 M750metallic with some 695 heads.  Bike ran quite well for what it was. 

[caperix]

88mm barrels from an '02-'07 bike will work (the '03 M800 you're looking for fit this).  There was some monkey-ing around by Ducati w/the alignment dowel between the cases & barrels around 2000-2001 but anything 2002 & up will have that changed out.

Fit the oil cooler.  Even 620's could use them.

The 620 head combustion chamber should work for the 750 pistons as I've done this conversion to an '05 M620 motor using some 2002 M750 barrels w/the FBF drop-in hi-comp pistons and it ran quite well for what it was.

Best pre-696 small-case Ducati cylinder head though is the 695's.  Many years back, Speeddog & I did a set of FBF drop-in pistons onto a 2001 M750metallic with some 695 heads.  Bike ran quite well for what it was. 

Do You remember what compression ratio pistons you ran?  Im looking to do something smiilar but am not sure how much the larger combustion chambers of the 695 heads will effect the compression ratio.  I think FBF has 10:1 or 14:1 available.

[Speeddog]

Do You remember what compression ratio pistons you ran?  Im looking to do something smiilar but am not sure how much the larger combustion chambers of the 695 heads will effect the compression ratio.  I think FBF has 10:1 or 14:1 available.

They were FBF 11:1 pistons.

750 cylinders and 695 cylinders have the same 100mm diameter 'snout' that fits the head.
Since the 800 shares the same 88mm bore, I would assume that it would have the same snout.
Interpretation of 'assume' per standard practice. 

Not relevant here, but tangential info, M400 cylinders have 95mm snout, 900 cylinders have 104mm snout.

[2 Wheel Wanderer]

Great info, thanks very much you guys!!!

[koko64]

Subscribed, cool project.

[caperix]

Thanks Speeddog