Connecting rod length and crankshaft diameters?

[soos]

Does anyone know the lengths of these for the 650?

And more to the point, are there any other cb's out there I could grab the connecting rods off of that are 1-2mm shorter?
I need to drop the compression for a 65mm overbore I am slowly doing.

l8r

[Ibsen]

The first thing that came to my mind is the 500/550 rods. What's different on the CB500/550 engine, the length of the rods or the crankshaft itself? And what about the rod journal diameter?

[soos]

I know the cb550/500 have shorter strokes than the cb650, but as far as big end journal size for the connecting rods...
I dunno.

I have finally removed a rod from my cb650 crankshaft, How are they properly measured?
I'm assuming from the center of the big to the center of the little bore.

I will be measuring the journal diameter of the crankshaft that the rod rides on soon...

I believe the cb550 and 650 crankshafts are interchangeable.
Paulages over at SOHC4 is making a 550/650 hybrid motor(with a 64mm bore at that)
If I remember right it is a 550 case with a 650 crank, head, cam, and 650 cylinder jug.

I know the stroke on a 650 is 55.8mm, not too sure about the 550's stroke...

Anyone know the stroke of a 550 offhand?


l8r

[Pinhead]

What is the compression, that you need to drop it?

[soos]

Thats half of the problem.
I am not 100% sore on the comp. ratio yet.

The 65mm's were at least 11:1's...for a cb750.
Either that or a set of pistons for the '77/78 cb750 heads.(they came out of a '74cb750)
And even then i'd say a 10.5:1 set then.


I have mocked them up in a 650 motor(minus sleeves) and it will not hit the head, but it's MIGHTY close.
2mm more room would give a serious ease of mind... as it is, without something to compensate for the high dome they have, i'm afraid of blowing head gaskets.(or hitting valves)

According to some rudimentary testing(play dough and measuring spoons) The depression left by the 65mm pistons comes to about 8-10ml. looked it up, and ml converts evenly to cc's 1:1.


In a post here on head volume for the cb650

If compression is 9:1 then the total combustion volume should be 17.4cc. With a head gasket of 1.25mm thick that's 3.5cc, leaving 13.9cc in the deck and combustion chamber combined.

If thats true, than with lets say 8cc taken up by the dome on the 65mm pistons, that leaves 9.4cc's.

So with bore/stroke of 59.8 x 55.8mm that gives 156.7cc per pot (627cc overall).

So 17.4-8=9.4's
Unless my math be WAY off, 156.7/9.4= compression ratio right?
that gives 16.6:1....

But to compensate for the added cc's, 2mm shorter rods would allow gaining back 6.6cc's, so the CR would be more like 9.8:1.(calculating cc's added by the 2mm more of 65mm bore if I can find shorter rods)
3mm would add 9.95cc's, completely negating the dome of the piston's added volume.
Or more.

That would be more what i can live with.
And on pump gas.

I have already cut down the deck height on the 65mm pistons as far as I dare, about 1.5mm.

Doubling the base gasket will give me an addittional 1+mm, dropping the CR to about 12.3:1.
A 2mm one(metal) would suffice, getting into a more "safe" CR zone, but how much of a shim on the bottom can the cam chain handle before needing a softlink.
I would like to avoid having to add one.
Anyone have any ideas as to how much can be shimmed without needing a softlink in a camchain?
And if you are wondering, the cam chain I intend on using is from a motor with about 12,000 miles, so it is worn in, but not too loose.



l8r

[soos]

Well... today I went out and bought one of those medicine syringes, with ml on the side...Had to be more accurate than using measuring spoons.

I thought I'd check the dome displacement of a std cb650 piston dome, because without that to compare the 750 pistons dome... the compression ratios are off from what i though they would be.

Using the medicine syringe, the 650's dome displacement is about 7cc's and the cb760 pistons are 14cc's
So there is a difference of about +7cc's.

So still a 2mm shorter rod would be the perfect answer IMO... and hopefully not custom rods.

17.4-7=10.4cc 156.7/10.4=15:1
So the CR should be 15:1 with no additional gaskets, and stock length rods.
17.4-7+3.3=13.7cc 156.7/13.7=11.4
With 1mm added(extra base gasket) the compression ratio of about 11.4:1
Still higher than i want...
17.4-7+6.6=17.0 156.7/17=9.21
2mm would give 9.2:1 CR... much more street usable.


The other use i could see for the shorter rods, would be to drop the compression ratio to be able to use a turbo kit effectively without low comp. pistons.


l8r

[Pinhead]

Honestly, milling down the dome of the pistons would be your best bet. You don't want to kill what little semblance of squish that is there. You should try to keep the squish clearance down to about 0.040" which will greatly aid in detonation tolerance. Adding thicker gaskets will only eliminate squish/quench and therefore add detonation tendencies. I've seen numerous engines with higher compression be less detonation prone due to the tighter squish. The same engine would detonate at 9.5:1, but with tighter squish and resulting 10.2:1, the detonation was eliminated.