Topic: 550/650 hybrid or 650 big bore HOW TO

[razor02097]

Welcome to the forums Seeker 

[esw07]

Wow just read through this complete thread for about the 10th time and am thinking of tracking down parts for when the snow starts to fly here in PA.

So I have a '76 550k and wanted to make sure i got this all right. I'll need a 1979-1980 650 crank, head, cylinders and a 650 big bore kit as well as some 750 carbs right?

Amazing bike by the way, and is there any vid's of the 718 in action. I'd love to hear her

[paulages]

Wow just read through this complete thread for about the 10th time and am thinking of tracking down parts for when the snow starts to fly here in PA.

So I have a '76 550k and wanted to make sure i got this all right. I'll need a 1979-1980 650 crank, head, cylinders and a 650 big bore kit as well as some 750 carbs right?

Amazing bike by the way, and is there any vid's of the 718 in action. I'd love to hear her

you'll have a hard time finding a "650 big bore kit"... 750 big bore kit might be what you mean. the pistons have to be reworked, but that's the way to go, along with the remachined 750 liners. DOHC will work too.

no video of the bike now (the engine is actually back apart now due to a broken cam flange), but i'll try to do so when it's back together. it sounds and runs like a champ. been piddling around on a stock K4 cb750 for a couple weeks, and it's made me miss the power and acceleration of the 718..

[Ecosse]

gotta say again thank you for all the work you put into the build up documentation, paulages.

i checked out the spot on the website and it's great! no question, one day i'll do this and your great work covering the... uh, work, is a huge help. certainly a motivator.

btw: sorry if i overlooked it but what sprocket #s are you using? i recall hondaman suggesting moving up a few in back for 550's but i imagine this hybrid may change things.

i should look up 650 gearing too.

[esw07]

Wow just read through this complete thread for about the 10th time and am thinking of tracking down parts for when the snow starts to fly here in PA.

So I have a '76 550k and wanted to make sure i got this all right. I'll need a 1979-1980 650 crank, head, cylinders and a 650 big bore kit as well as some 750 carbs right?

Amazing bike by the way, and is there any vid's of the 718 in action. I'd love to hear her

you'll have a hard time finding a "650 big bore kit"... 750 big bore kit might be what you mean. the pistons have to be reworked, but that's the way to go, along with the remachined 750 liners. DOHC will work too.

no video of the bike now (the engine is actually back apart now due to a broken cam flange), but i'll try to do so when it's back together. it sounds and runs like a champ. been piddling around on a stock K4 cb750 for a couple weeks, and it's made me miss the power and acceleration of the 718..

Haha yea i meant 750 but you said a DOHC would work too? Is that just in DOHC 650 cylinders or head as well... Just trying to imagine a SOHC 550 with a DOHC 650 head on it.

[paulages]

gotta say again thank you for all the work you put into the build up documentation, paulages.

i checked out the spot on the website and it's great! no question, one day i'll do this and your great work covering the... uh, work, is a huge help. certainly a motivator.

btw: sorry if i overlooked it but what sprocket #s are you using? i recall hondaman suggesting moving up a few in back for 550's but i imagine this hybrid may change things.

i should look up 650 gearing too.

Thanks! Just trying to give back to a place I've learned a lot from. You know, I'll have to double check, but I believe I'm running 17/45 for the final drive gearing. I went big on the rear, figuring that swapping the front for a different ratio is quick and cheap, especially since I'm using sprocket specialist sprockets, with the clip-style retainers for the front. As is, it's very quick on the takeoff, but probably can't quite hit it's potential top speed, though I've not had the opportunity to try...

[paulages]

Wow just read through this complete thread for about the 10th time and am thinking of tracking down parts for when the snow starts to fly here in PA.

So I have a '76 550k and wanted to make sure i got this all right. I'll need a 1979-1980 650 crank, head, cylinders and a 650 big bore kit as well as some 750 carbs right?

Amazing bike by the way, and is there any vid's of the 718 in action. I'd love to hear her

you'll have a hard time finding a "650 big bore kit"... 750 big bore kit might be what you mean. the pistons have to be reworked, but that's the way to go, along with the remachined 750 liners. DOHC will work too.

no video of the bike now (the engine is actually back apart now due to a broken cam flange), but i'll try to do so when it's back together. it sounds and runs like a champ. been piddling around on a stock K4 cb750 for a couple weeks, and it's made me miss the power and acceleration of the 718..

Haha yea i meant 750 but you said a DOHC would work too? Is that just in DOHC 650 cylinders or head as well... Just trying to imagine a SOHC 550 with a DOHC 650 head on it.

I guess I should have been clearer: you can use DOHC CB750 liners as well.

[CrisPDuk]

Afternoon all, it's me again, with yet more questions

I've taken the heads & barrels off both the 550 I seized, and the 650 I bought that was stuck, and have found the following;

On the 550,

A/ One of the rods has properly stuck itself to the crank, and another is stiff, and

B/ The barrel of the cylinder with the seized rod has split, around the base and vertically to about halfway up the bore

C/ Everything else from the primary chain back seems OK, but I'm going to split the cases this weekend to check.

On the 650,

A/  The barrels, apart from needing a hone seem to be in pretty good condition, but,

B/  The pistons have quite a bit of scoring around their skirts, and one of them has a few cracks in the skirt.  The rings are stuck solid into them.

C/  One of the valves is stuck down in it's guide, which is possibly why the engine wouldn't rotate, but I don't think it's actually been run like that, as there is no damage to the piston crown.

I've scrapped the 550 barrels completely as quite a few of the fins were bent or broken, and I've got a decent other set anyway, and I think the 650 pistons are scrap too.

I also have, in my collection of gathered parts, a set of std 750 pistons with rings that are in what I'd call almost new condition.

Finally after all that we get to my question;

Do I need to replace the liners in my 650 barrel casting with 750 ones to take these pistons, or would the standard 650 liners be able to take that sort of overbore, and run reliably?


With respect to my camshaft question, I measured all 5 across the peaks, and what I presume to be the base circle, and what I have are two that have aproximately 1mm more lift than the other three.  Are these two likely to be 650 shafts?

[Soos]

Must have missed that Paulages!!!


Sorry to hear the great beastie is down for a while!!!




l8r

[paulages]

Do I need to replace the liners in my 650 barrel casting with 750 ones to take these pistons, or would the standard 650 liners be able to take that sort of overbore, and run reliably?


With respect to my camshaft question, I measured all 5 across the peaks, and what I presume to be the base circle, and what I have are two that have aproximately 1mm more lift than the other three.  Are these two likely to be 650 shafts?

i can't say for sure, but i think you might be able to use the 650 liners with the std 750 pistons. Soos will know for sure, as i believe he runs these. as for the cams, if you can post accurate measurements,we should be able to tell you what you have.

[CrisPDuk]

The 750 pistons are 61mm diameter, the liners at the base (which I assume is where they are thinnest) are 64.4mm diameter on the outside, which would leave an average wall thickness of 1.7mm.
Since this is thicker than we used to end up with when overboring old VW flat four motors, I think I'll be OK, but what I don't know is what the profile of the liner is like inside the aluminium casting? Does it go thinner?

On the camshafts I wrote their sizes down, but unfortunately lost the piece of paper I had it on 

I'll measure them again tomorrow

[paulages]

The 750 pistons are 61mm diameter, the liners at the base (which I assume is where they are thinnest) are 64.4mm diameter on the outside, which would leave an average wall thickness of 1.7mm.
Since this is thicker than we used to end up with when overboring old VW flat four motors, I think I'll be OK, but what I don't know is what the profile of the liner is like inside the aluminium casting? Does it go thinner?

On the camshafts I wrote their sizes down, but unfortunately lost the piece of paper I had it on  

I'll measure them again tomorrow

besides the taper at the base, the liners are uniform through the block. PM Soos if he doesn't reply here.

[Soos]

Yes 61mm pistons WILL work in a stock liner for a cb650.
I got a set of '77 high domed cb750 61mm pistons in my '79 cb650 now.
God I love 'em!

Actually 62mm is the limit.
But I have been stumped as to where to get 62mm pistons....
Hey.. 1mm over cb650 pistons... Duh.(brain fart)
I wonder if DSS has any left.......
I have thought of the KZ650 pistons(62mm), but have not bought a set due to lack of availability of rings.
No point in trying them if I can't seal the cylinder properly in the end!



But anyways... on those '77 cb750 pistons I used....
I guess I bumped the CR mabey .25 to .5 higher with them.
not sure offhand.
I know the cylinder pressure has leveled off at around 180-190 psi with the pressure tester I have been using.




The '69-'76 pistons would actually DROP the CR quite a bit I would think.
Mabey good for a turbo application? Dunno... got a set of (?'72, '73?) pistons I am thinking of checking clearances for and doing that in the future though.....
Gotta find a good turbo first though.. and finish my 66mm cb650 first as well....
(My "wish list" for my cb650 motors is quite long.)






The cb750 pistons I used needed a bit of modding though.
I had to re-position and deepen the valve pockets, and cut a bit off the outter edge to re-establish the deck height to what the stock cb650 pistons were.
Beyond that, not a lot.
I think I re-contoured the dome (without removing any height) to look blend into the land I had cut for the deck height.


Sorry, no measurements offhand.
Still can't find those sketches.




However I recently got a job again, WHOHOO!!!@!
(after nearly 4 months off work!!)
I will be  modding a few sets of cb750 pistons in the near future.(for me and others)
Sooooo....the "print" (poorly done sketch with measurements) will be scanned so I can post it on here for all to see of the mods I make.
Might not give everything, but I will add notes that SHOULD give enough info to a competent machinist to copy the work I have done.


I might even make the measurements readable to other humans!
hahaha....
You would have to see my box of "prints" for all my custom doo-hickeys I have made over the past 10 years to really understand.....
I have to decipher them at times.



Head up, rubber down, balls out!



l8r

[CrisPDuk]

Thanks for that info Soos, I'm not sure what year my 750 pistons are from, I'll try and take a picture of them, maybe you can tell me

Anyway, I've got a few more questions after a weekend spent stripping & cleaning my various engines;

I took the engine cases apart on the seized 550, and apart from the seized big end bearings and a sump full of the remnants all seems OK, so I think that will be the one I shall use for my 'long stroke' project.
The only thing is, it doesn't have the extra boss for the primary chain oil sprayer featured way back on page one, is this necessary?

Since it's so easy to remove, I've decided that I'm going to completely dispose of the kick start mechanism.  Whilst I'm getting a plug made for the hole in the clutch cover, do I need to make one for the big hole in the crankcase side too?

[paulages]

Thanks for that info Soos, I'm not sure what year my 750 pistons are from, I'll try and take a picture of them, maybe you can tell me

Anyway, I've got a few more questions after a weekend spent stripping & cleaning my various engines;

I took the engine cases apart on the seized 550, and apart from the seized big end bearings and a sump full of the remnants all seems OK, so I think that will be the one I shall use for my 'long stroke' project.
The only thing is, it doesn't have the extra boss for the primary chain oil sprayer featured way back on page one, is this necessary?

Since it's so easy to remove, I've decided that I'm going to completely dispose of the kick start mechanism.  Whilst I'm getting a plug made for the hole in the clutch cover, do I need to make one for the big hole in the crankcase side too?

the chain oiler is not necessary, it just seems like a good innovation. that chain should see plenty of oil anyway. wasn't that hard to add though.

[paulages]

Finally this topic has a proper home!

I edited a couple new things, most notably a report on the 650 primary ratio. to save those who have already read the article the trouble, i went back to stock springs in the clutch and have had no slippage at all (and a much happier wrist), which seems to support the claim that the 650 ratio was improved over the 550.

[bwaller]

This is a better place for this work Paul.

[paulages]

hold the presses on the clutch report... the 650 primary ratio may be an improvement for a stock engine, but i think i'm getting slip at high RPM. the confusion is that it doesn't slip in the way i'm accustomed with. instead of a slow surge, it just surges for a second or so and then catches. like this (at 8K RPM for example)... rrrrrrrrrrrrrrrrrrrrRRRRrrrrrrrrrrrrrrrrrrRRRRRrrrrrrrrrrrrrrrrrrrrrrrrrrrrrRRRrrrrrrrrrrr... (the capital RRRs represent about half a second).

it definitely increases in RPM, so i do think it's the clutch. i'll report back after i put the HD springs back in.

[Soos]

(sorry if I start a argiment)



What oil are you running?
When did you adjust your clutch last?
Glads to hear it's running well again though!




l8r

[paulages]

(sorry if I start a argiment)



What oil are you running?
When did you adjust your clutch last?
Glads to hear it's running well again though!




l8r

Castrol 20W/50 motorcycle oil. Adjusted the clutch when I put it together last, but I'll check it again..

[Soos]

Well I wouldn't think that oil should be able to cause slippage...




Adjusting it is the only fix I see without the harder springs.



l8r

[Hasenkopf]

Hey guys, you will have to forgive me if this is in the wrong spot,  but it seemed important enough to mention,  does anyone degree in their ignition advance?  seems like there is enough play in these things, what with it being bolted to the crank and all.

[Ecosse]

welcome to the forum gsx69bun! 

[esw07]

Ok so I'm back on schedule with my 650 hybrid build. According to Soos' info on using 750 high dome pistons with the 650 liners, and the lack of availability of the 750 liners (tried looking but can't find right now) should I just go with the stock SOHC 650 liners?

I'm also planning on lightening and balancing my crankshaft to help out. Also seen some people talking about running stock GSXR connecting rods because they were more light weight. Anyone know if they'll be kosher with the 550/650 Big bore hybrid?

[paulages]

Ok so I'm back on schedule with my 650 hybrid build. According to Soos' info on using 750 high dome pistons with the 650 liners, and the lack of availability of the 750 liners (tried looking but can't find right now) should I just go with the stock SOHC 650 liners?

I'm also planning on lightening and balancing my crankshaft to help out. Also seen some people talking about running stock GSXR connecting rods because they were more light weight. Anyone know if they'll be kosher with the 550/650 Big bore hybrid?

find a junked cb750 (soch or dohc, or hell- cb900dohc will work too...) block on ebay or at a junkyard, and just remove the liners.