Hybrid 620/800 Engine

[tekdo]

Hello gents, looking for a little insight on a project I've been thinking about over the last couple days.

I currently ride an '05 Monster 620 and I'm looking for a little bit more punch (yes I know ridiculous for a beginners bike). As much as I want a second bike, I'm in love with the old girl here since she was my dad's first bike and now my bike.

I happen to have an 800 from an '05 s2r sitting in my garage with a dead main bearing and questionable contamination in the oil. I've been thinking about doing a big bore kit using the s2r jugs and heads since I think it would be fun in a light little 620 with it's super close gearing.

I'm pretty confident that I can get the swap done because I've build a couple engines before but I have a couple questions:
1)is the wrist pin height for the 800 and 620 piston the same?
2)620 or 800 head...or 900 V2?
3)to port or not to port
4)Stock cams or find a bushing to run the 900 cam in a 2 bearing head?
5)What would the static compression ratio of this thing be? I've read that both the 620 and 800 are 10.5:1...would it be different with the 620s 61.5mm stroke?

Goal of the project is not to build a mega fast bike or to prove any points, honestly I just think it'd be fun to build and really fun to ride since it would be like having an oversquare, hi-comp 750.

[Duck-Stew]

Piston pin heights are different as the two engines use the same rods but have different strokes.

800 heads would be an improvement over 620. (Only if increasing the bore to 88mm.)

Both engines run the same transmission.  Are you referring to the final drive gearing as the difference there?

Both bikes and engines weighed almost the same.

800 barrels and heads will swap onto the 620, but you’ll need to find hi-compression aftermarket 750 pistons to make it work.

Porting? All 2V ports from that era are crap.

900 heads/barrels won’t fit due to cylinder head stud spacing being larger.

900 cams run a smaller base circle, so to make them work you have to sink the valves too far into the head which impedes airflow.

I’m not sure about your last question: Are yiu trying to reuse the 800 pistons in your 620?  If so, that’s a whole other ball of wax.

Fast by Ferracci used to make drop-in 11:1 750 pistons. Not sure if they still do.

You’ll need to add an oil cooler and if the 800 has metal bits in its’ cooler, I wouldn’t use that one.

[Duck-Stew]

https://www.ferracci.com/collections/pistons/products/piston-kit-88mm-11-1-750

Seems they still have some…

[Duck-Stew]

Lastly (?):

When adding an oil cooler, do not forget to add the spring steel flapper valve on the underside of the oil filter mounting stud.  Be sure it’s aligned (easy to do).

[tekdo]

Both engines run the same transmission.  Are you referring to the final drive gearing as the difference there?

Yes, the final drive on my 620 right now is 15/46 and the S2R was 15/42 (if I remember what my brother had correctly)

Porting? All 2V ports from that era are crap.

I was asking if it would be worth it for me to port the intake or at least remove all the casting flash or if it wasn't worth the time and effort

I’m not sure about your last question: Are yiu trying to reuse the 800 pistons in your 620?  If so, that’s a whole other ball of wax.

No, I was just wondering what the Compression ratio would be if I used the 800 Barrels and Pistons with the 620 rods and crank.

Piston pin heights are different as the two engines use the same rods but have different strokes.

Good to know, I thought that the increased displacement came from the actual 800's crank throw and not wristpin height.

You’ll need to add an oil cooler and if the 800 has metal bits in its’ cooler, I wouldn’t use that one.

I already have a clean oil cooler on the bike so I'mm not worried about the old one.

[Duck-Stew]

Gearing plays a big part.

Porting is a black art. You can very easily go too far and hurt airflow.  Casting flash? Yeah, if you want to, but I’d focus on the exhaust only.  Small bumps on the intake help keep atomized fuel suspended in the center of the port.  Best to leave the intake as is unless you do research first.  Your time, your effort.

66mm stroke on the 800 & 61.5mm on the 620.  Same deck height and same rods, so the only variable would be piston pin height.  66-61.5=4.5mm.  Half of that is 2.25mm.  That’s how much further down the bore the 800 piston would sit at TDC. So, a serious decrease in compression. Not to mention essentially eliminating all ‘squish band’ from the chamber.

Oil cooler: good. I firmly believe the 620’s & especially the 695’s had the cooler removed to save $$$.

[koko64]

Just spotted this. Nice project.
Used some new Ferracci 750 pistons a few years ago, good to hear they're still available.

Carefully clean/flush out all the oil galleries in the 800 parts.
+1, 800 heads are an improvement, so treat them well. Maybe treat them to a nice valve/valve seat job and light porting job by a professional. A freshen up if you will.

Have you considered your remapping options? Rexxer flash tune or Power Commander and dyno tune are options. A K&N filter and open airbox lid if it doesn't already. It must be tuned right to complete the job.
Depending on the compression ratio used, a change to premium, higher octane fuel may be required.

Gearing. Are you seeking a relaxed cruise in top gear (3800 rpm) or acceleration through the gears and out of corners?
https://www.bikeboy.org/ducgearing.html