Topic: cb650, cb750 sleeves and Cb900 pistons?

[tourmax]

Looking to maybe make a 729cc cb650.

650 block, 750 sleeves and 64.5mm cb900f pistons.

Planning to bore out block, press in 750 sleeves, bore to the 900f pistons and address piston height and valve pockets with some machining.

Possible?

Concerns and/or issues?

I may look at having fire ring grooves machined in so I can use copper rings instead of hunting down custom head gaskets. I have custom engine shop I deal with and will have to ask them what they think about that.

This is an occasional "fun" bike (4th bike in the fleet and the FJ1200 handles most rides) for local ripping on sunny Sundays,  so durability/longevity isn't so much an issue.

Not looking to make a "grenade", but it's not going to be beat on a regular basis or go long distances.

Follow on question: how big can you take a cb650 block out? I've read some posts on pressing in 900f cylinders and then what's the upper limit on bore size before you get thin enough to worry about cylinder distortion or otherwise unstable bores?

[tourmax]

Ok.

I may have lost my mind, but just picked up some used pistons from a CB1100F. Measured at 69.98mm and no visible scoring on the skirts.

On the hunt for some CB900 liners now and we'll see if the 650 block can swallow these big boys.

Should work out to around 858 cc from a cb650 block.....oy vey, I think I've gone nuts!

Anyone have an extra cb650 block lying around they'd like to donate to the cause? I may end up "ruining" a couple before it's done......."done" meaning finding out it is or is not possible that is.

Not sure if the 650 intake and exhaust valves are going to be able to feed that much displacement though.....but it should make for one heck of a vacuum signal when the intake valve opens! It might actually make those 36mm downdraft carbs work right!

LOL!

[gschuld]

Before you very carried away, you might want to get a close look at your cylinder stud spacing.  When going up to bigger sleeves/pistons, you must bore out the top cases to accept the bigger sleeve OD.  You can only get so close to the studs before the base around them gets too compromised to keep the studs anchored.

For reference, 73mm bore on a sohc cb750 is getting up to that limit.  Some have managed 74mm in special cases and survived.

I have no idea what your spacing is on a cb650 block.

I’d guess though that you would be pushing it around the 65mm bore vicinity.

George

[MRieck]

Before you very carried away, you might want to get a close look at your cylinder stud spacing.  When going up to bigger sleeves/pistons, you must bore out the top cases to accept the bigger sleeve OD.  You can only get so close to the studs before the base around them gets too compromised to keep the studs anchored.

For reference, 73mm bore on a sohc cb750 is getting up to that limit.  Some have managed 74mm in special cases and survived.

I have no idea what your spacing is on a cb650 block.

I’d guess though that you would be pushing it around the 65mm bore vicinity.

George

Who did 74 George? Building bombs is best left to the military,

[tourmax]

Before you very carried away, you might want to get a close look at your cylinder stud spacing.  When going up to bigger sleeves/pistons, you must bore out the top cases to accept the bigger sleeve OD.  You can only get so close to the studs before the base around them gets too compromised to keep the studs anchored.

For reference, 73mm bore on a sohc cb750 is getting up to that limit.  Some have managed 74mm in special cases and survived.

I have no idea what your spacing is on a cb650 block.

I’d guess though that you would be pushing it around the 65mm bore vicinity.

George

Only one way to find out: buy some cylinders and see whats what….

[tourmax]

Before you very carried away, you might want to get a close look at your cylinder stud spacing.  When going up to bigger sleeves/pistons, you must bore out the top cases to accept the bigger sleeve OD.  You can only get so close to the studs before the base around them gets too compromised to keep the studs anchored.

For reference, 73mm bore on a sohc cb750 is getting up to that limit.  Some have managed 74mm in special cases and survived.

I have no idea what your spacing is on a cb650 block.

I’d guess though that you would be pushing it around the 65mm bore vicinity.

George

Who did 74 George? Building bombs is best left to the military,

Good thing I’m military (ret) then!

[tourmax]

well, seems I got too far out over my skies. CB1100F pistons seem to have a 17mm wrist pin. CB650 rods are 15mm. I've bushed down rod small ends before, but never the piston side. I'm not sure I even would want to try.

Oh well, 750 liners and 900 pistons looks to be the biggest that may be "workable" on a 650 block......

[Tracksnblades1]

well, seems I got too far out over my skies. CB1100F pistons seem to have a 17mm wrist pin. CB650 rods are 15mm. I've bushed down rod small ends before, but never the piston side. I'm not sure I even would want to try.

Oh well, 750 liners and 900 pistons looks to be the biggest that may be "workable" on a 650 block......

Just use the 1100 rods. They’re  better and the bolts are too..
Some have used them with good results.. AsK Bear. If memory serves…

[gschuld]

Before you very carried away, you might want to get a close look at your cylinder stud spacing.  When going up to bigger sleeves/pistons, you must bore out the top cases to accept the bigger sleeve OD.  You can only get so close to the studs before the base around them gets too compromised to keep the studs anchored.

For reference, 73mm bore on a sohc cb750 is getting up to that limit.  Some have managed 74mm in special cases and survived.

I have no idea what your spacing is on a cb650 block.

I’d guess though that you would be pushing it around the 65mm bore vicinity.

George

Who did 74 George? Building bombs is best left to the military,

I agree, it’s getting sketchy.

Jon Weeks stroker automatic dragbike and Brian’s Australian stroker crank sidecar both successfully ran 74mm bores, based on their own accounts.

The Australian period 4 road racer RPE “Hulk” that Hugh Robinson did the machining on went WAY out there and claimed 1204cc with a factory stroke.  That works out to 78mm bore.  Custom cylinder, offset rods, etc.  Not sure about the fine  details and Hugh wasn’t very open to divulge when we spoke a while back.  I’m confident he ran slipper skirt pistons and severely cut the sleeves to notch around the  stud locations (as in, cutting the sleeves flush with the upper case deck other than the 1.5” sections front to back between the cylinder studs). It’s the only way to leave enough material at the studs.

Oh, there are plenty of 70/71mm aftermarket cb750 pistons around. 15mm pin, 2V, etc.  1100 rods use larger (cb750 spec) big end rod journals than a cb500/550 crank.  I assume cb650 rod journals are the same. 

George

[gschuld]

This is the only method I can think of to do it with a stroker crank. 
This engine was reportedly 1324cc.  That works out to 73mm stroke, 76mm bore.

Notice the giant depth add on stroker plate.  This, IMHO, served one critical purpose.  To raise the cylinder block high enough to allow the entire 73mm stroke, plus slipper piston skirt depth, to be above the upper case deck.  That way, you can leave full support at the studs.  The downside is you need rods made like 7/8” longer than stock, and the cam chain gets about 4 links longer.  I applaud the pure insanity of the effort.

I’ve held a 73mm stroke welded stroker crank in my hands. And yes it cleared the cases with work.  But it flunked the magniflux test spectacularly 🤣. It’s been donated to the Bill Benton sohc4 museum collection.

Sorry, this is a bit off subject for the OP, but it gives an idea about a serious issue when going WAY bigger than factory bore on our sohc4 engines. There are creative work arounds, but the time and money spent starts getting out of hand really fast.  And you may well end up with something of a time bomb anyway.

Cb750 is 61mm bore.  73mm is about out realistic limit.  That’s 12mm over stock bore
Cb650 is 59.8mm bore. Plus 12mm is 71.8mm.  So who knows, you ….might, get away with it.

George

[tourmax]

This is the only method I can think of to do it with a stroker crank. 
This engine was reportedly 1324cc.  That works out to 73mm stroke, 76mm bore.

Notice the giant depth add on stroker plate.  This, IMHO, served one critical purpose.  To raise the cylinder block high enough to allow the entire 73mm stroke, plus slipper piston skirt depth, to be above the upper case deck.  That way, you can leave full support at the studs.  The downside is you need rods made like 7/8” longer than stock, and the cam chain gets about 4 links longer.  I applaud the pure insanity of the effort.

I’ve held a 73mm stroke welded stroker crank in my hands. And yes it cleared the cases with work.  But it flunked the magniflux test spectacularly 🤣. It’s been donated to the Bill Benton sohc4 museum collection.

Sorry, this is a bit off subject for the OP, but it gives an idea about a serious issue when going WAY bigger than factory bore on our sohc4 engines. There are creative work arounds, but the time and money spent starts getting out of hand really fast.  And you may well end up with something of a time bomb anyway.

Cb750 is 61mm bore.  73mm is about out realistic limit.  That’s 12mm over stock bore
Cb650 is 59.8mm bore. Plus 12mm is 71.8mm.  So who knows, you ….might, get away with it.

George

Interesting you would post the spacer plate pictures as i was thinking about using one to compensate for some (or all) of the difference in the piston Ch differences. Cam chain is the obvious issue, but might be oil passages as well....

[tourmax]

well, seems I got too far out over my skies. CB1100F pistons seem to have a 17mm wrist pin. CB650 rods are 15mm. I've bushed down rod small ends before, but never the piston side. I'm not sure I even would want to try.

Oh well, 750 liners and 900 pistons looks to be the biggest that may be "workable" on a 650 block......

Just use the 1100 rods. They’re  better and the bolts are too..
Some have used them with good results.. AsK Bear. If memory serves…

yes, but are we talking sohc 750 or sohc 650?

Not sure a DOHC 1100 rod will fit in a 650 lump......

[gschuld]

Assuming the cb650 rod journals are the same as cb500/550, the 1100 rods certainly will not fit on a cb650 crank.  They will fit on cb750/900/1100 cranks

George

[bwaller]

650 and 550 journals are the same.

[grcamna2]

subscribed.    Yeah !

[scunny]

the vultures are circling I'm in for the entertainment.

[tourmax]

Well, expectations are coming down pretty quickly. It is looking like the absolute maximum bore for the 650 block is 64-65 mm. After some measuring and eyeballing, a sleeve big enough to accommodate 66+mm pistons will break through the cylinder block in multiple places.

So that means displacement is likely to top out around 718-740 cc's.

I still need to get a few more components in my hands to measure, but 740 is what it's looking like as a maximum and that means thin sleeves and a thin block as well....

[gschuld]

Ok, my offer to send a set of free cb750 sleeves is still good.  65mm is about the limit on OEM cb750 sleeves

George

[tourmax]

Ok, my offer to send a set of free cb750 sleeves is still good.  65mm is about the limit on OEM cb750 sleeves

George

Sure, I’ll take em if you're offering. I’ve got a 750 block here, but for some reason the liners are refusing to drop out with just heat. Looks like I’m going to have to make a fixture to press them out.

We can figure out shipping costs and I can paypal you the amount.

[Tracksnblades1]

well, seems I got too far out over my skies. CB1100F pistons seem to have a 17mm wrist pin. CB650 rods are 15mm. I've bushed down rod small ends before, but never the piston side. I'm not sure I even would want to try.

Oh well, 750 liners and 900 pistons looks to be the biggest that may be "workable" on a 650 block......

Just use the 1100 rods. They’re  better and the bolts are too..
Some have used them with good results.. AsK Bear. If memory serves…

yes, but are we talking sohc 750 or sohc 650?

Not sure a DOHC 1100 rod will fit in a 650 lump......

Oops. Looks like I went off on a tangent 😁

While on the tangent though and since youre considering a one off build, Perhaps a cb750A crankshaft 63mm stroke with a hyvo chain sprocket. Turn the mains down only 1mm, if the oil holes don’t wonder off to far, Or line bore the cases to 39mm… One could then choose from std length 750 sohc, shorter 750 dohc, and the longer 900-1100 dohc rods.. Bragging rights for a stroked, long rod,  big bore 650 😜

[scottly]

I'm in for the entertainment.

Hey Scunny, I was just thinking of you when I remembered the biggest earthquakes I experienced, a 7.3, followed a few hours later by a 6.5, were 30 years ago next month. I take it things have settled down there, more than 10 years past your big one?
PS Pass the popcorn.

[scunny]

Yeah things have settled down the last 5 or so years, still lots of broken buildings and homes.