Topic: 66mm pistons !!

[Soos]

I have a set of GPZ750 66mm(!!! !!) pistons that i'm seriously thinking of using as an overbore.... in a cb650!


The 836 pistons I have are roughly 6.5mm taller than stocker cb750 pistons(and yes they were in a '74 cb750 with a K head, not one of the later '77/78 F heads!)
These 66mm pistons are the same height(from the wrist pins to the top of the dome), as those 836 pistons are!!
Deck height is another story, but from what I can feel, there is plenty of meat to modify them to work.


Good thing they are only 5.5mm taller than the cb650 pistons, 1mm less to hassle with.

Valve pockets might need modified as well, not too sure yet, but I would bet they do.
More for position/angle than depth to match the cb650 valve angle/positions.



Definately going to need machining to work, either in a 650 or a 750, as deck heights are different.(taller on these GPZ pistons)



The gasket problem has been sorted(so far), either a cometic, or a copper head gasket.
Either should work..... probably going to have a leak problem, so external oil lines are going to be a must I'm thinking.

This site is AMAZING!!!  Paulages 64mm overbore in his 550/650 hybrid thread gave me quite a few insights as to the problems inherent in big overbores, the oil lines being one of them.






More sharing the enthusiam I have at the moment than anything else I guess....

I'm downloading pics right now, I will post a few later....
I need to check out photobucket for hosting them....




l8r

[speedracer741]

I say go for it! I have some KZ750 pistons as well. You will need custom sleeves rather than stock sleeves from another engine though. When I did a 65mm overbore in the 650 I used custom straight sleeves with no step at the bottom. I bored the block the maximum it would take to put the sleeves in and the sleeves are quite thin. Had they had a step at the bottom they would have been paper thin. To run the 66mm piston you may encounter breaking through into air when the block is bored. I started to see pin holes of light boring for the sleeves for the 65mm pistons and ran .005" press fit. If I had known about the KZ750 piston when I started mine I would have tried using them instead. External top end oil lines would also be a good idea. You can also machine the coumbustion chamber to better match the pistons and will unshroud the valves at the same time giving you better flow. Can't wait to see how it turns out!!!

[Soos]

I say go for it! I have some KZ750 pistons as well. You will need custom sleeves rather than stock sleeves from another engine though. When I did a 65mm overbore in the 650 I used custom straight sleeves with no step at the bottom.

WHA?!!!!

I must have missed that one!! I had no idea someone has gone 65mm before in a 650!
Yeah, I'm kinda expecting to have to do some welding to keep the integrity of the block going this big.

Was your 65mm 650 a for fun bike, or did you race it?


I think I have sourced some sleeves I can modify.
A previous employer answered a E-mail today.
I was inquiring about some sleeve material that would fit the dimensions I am looking for, and he said he might have something that will work for me.
Monday or Tuesday I find out exactly what he has, and if it will work...


Another option I have looked into is CB900 liners.

The bore is already 64.5mm, and i'm hoping I could get 1.5mm out of it, and be able to machine the OD to my specs still.

I am hoping to keep the wall thickness around 0.080 at a minimum, more in the neighborhood of 0.125 if the block will accept that without much fuss(ie welding)....

Good thing I have 3 few blocks to test this out on!!
Speaking of that..... I have machine time the next 2 weekends at work......
I will be bringing a cb650 block in and bore it to the upper limit of the sleeve OD I would like to run with just to see where daylight starts showing(if it does).
I'd be tickled pink if it is able to go 2.848(without welding), but if it can only go 2.758 without showing light I will be happy as well.

If I gotta weld it to make this work.... so be it.



l8r

[speedracer741]

Well if you have the ability to machine the sleeve OD then it sounds like the CB900 liners will do the job. You will essentially end up with sleeves like mine with no step but you will have saved yourself the money of having them custom made. Mine were done by a local place called Northwest Sleeve. I did all of the work on mine back in 1997-98. Shortly after I was displaced from the nice shop I had so most of my projects went into storage for years. I am just now making progress on many of them so my 650/741 engine has still never fired after a decade! It was my first post on this forum and can be found here: http://forums.sohc4.net/index.php?topic=18521.0

If I were to start the project over again now having another 10 years experience then when I was 20 I would do some things differently and I may just do that still. It is a few projects back in the list of priorities and my work schedule is making things slow but one of these days it will be tearing up the streets. It was originally going to be for vintage roadracing in the 750 class and with the CB500 sidecovers still in place it earned the nickname "secret weapon".

I think the 66mm pistons would be my first choice now. What you could do is ban saw one of you cylinder blocks to get a cross sectional view. You will then know what you can get away with in terms of sleeve dimensions. You can then apply weld if needed to the block you are actually going to use. The KZ pistons seem like they will work.

[754]

LA sleeves can fix you up but probably around 200 or more

Now I got a welding question,
 are you planning on boring to 20 thou under, then welding from the INSIDE ?

Cuz, i just cant see welding them easily from the outside, even with a spoolamatic (messy)

I am thinking that with all that heat, and any voids negating the cooling effect, you are just going to have all kinds of trouble.(warpage, leaking)

 Now there is one instance where it is preferreable to have a stock block.. if you are building a sleeper.

 Otherwise as has been proven over and over, custom blocks (cast or carved) really are the way to go..IMO..

[Soos]

Are you planning on boring to 20 thou under, then welding from the INSIDE ?

 Now there is one instance where it is preferreable to have a stock block.. if you are building a sleeper.

As far as welding from the inside, yeah, it will pretty much be the only option I have access to for aluminum welding.

Custom block...
If I had the chunk 'o aluminum.... yeah, i'd have one. In fact I would make it.
I have enough skills(i think anyway) to do the work, and limited $$.
So sleeper is about the only option there.
Anyways I always liked sleepers.


The external oil lines however necessary will probably be the only telltale sign.
If I could avoid them I would, but I see no real viable alternative unfortunately...

And as far as $200 for sleeves?
No.

For the material used.... I'll spend my money on a used cb900 block, press out the sleeves, and do the work myself.
NO matter what I am either going to have to flat the sides of the sleeves(not something I want to do), or bore the CB at the top for the next sleeve, "notching" with a radii each already installed sleeve with the next ones needed counterbore at the top of the block.
It's a PITA, but not the first motor I have had to do it to.
Not the most stable either for a block assembly this small, But I think no matter what I'm going to be seriously pushing the limits anyway.



After the rough boring(within 0.020 or so) and the welding....
Bore 'em all within 0.010(or so), do all the counterbores within 0.010, then do the fourth bore to final size.
Mod the sleeve (if needed) to fit the ID dimensions of the counterbore and bore in the block, press in.
Go on to next hole, finish counterbore(making a radii in the previously installed sleeve), and bore, mod sleeve(if needed) press in... repeat until done...
I'm almost guarantee that I will have to do this and interlock the cylinders.
Once done, surfaced the block, THEN the boring of the cylinders can begin.

Like I said a PITA, but at least I can do all the work there with time and not $$.


Chasing down a set of better rods.
I may have gotten lucky, dunno yet.
I find out tomorrow.




l8r

[754]

Pretty sure a 900 block will be cheapest.

I did find a sleeve set for 970cc Honda(cb 750) last year for about 130 shipped. I will have to do up another cyl as the ODs were .025 smaller than my other 970 cyl. They were LA SLEEVE and will need shortening. I was happy to get them..

 Re the 900 sleeves, Honda tends to overbuild, and you have little room, so prepare to reduce the OD.

 Find the sleeve OD then use you bore centres to figure if this will be happening, even then you may have to flatten the side.

 I took a really good look at 1080s stock cylinder that was taken out for 1080 sleeves, and it is a miserable leaking piece of alloy... now running with a R.C. big block.

 the worst part about making one is, it is so much wider just at the cam chain hole that it makes it 50 % more material and machining.... at least 200 bux of aluminum..

[Soos]

Pretty sure a 900 block will be cheapest.

 Re the 900 sleeves, Honda tends to overbuild, and you have little room, so prepare to reduce the OD.

 Find the sleeve OD then use you bore centres to figure if this will be happening, even then you may have to flatten the side.

 I took a really good look at 1080s stock cylinder that was taken out for 1080 sleeves, and it is a miserable leaking piece of alloy... now running with a R.C. big block.

Yeah, I am glad honda over engineered the sleeves they use at least.
With having to bore the block so big, there are trade off's going with a thinner or thicker wall as I see it.

A thicker wall sleeve will give a more stable cylinder, allowing for less leaks internally, and less likely hood of the cylinder of warping under use.
But a thicker wall takes away from the block. Take too much, and the block will distort easily making sealing the head to the block difficult to say the least.

Thinner walls... well there is the distortion problem that can cause the cylinders to go out of round easier, causing blow by, and loss of compression.
But the head will be able to seal better as the block will have more structural integrity to deal with all the stresses I will be throwing at it through use.
As well, the thinner the sleeve walls the more likely to be able to burst/crack/warp much easier.




I am going to have to get the 900 block and sleeves out to determine what I need to do with the liners, and see what the max the block can accept is.
I'm glad I will probably need to machine the OD of the sleeves in fact.
From my experience in the engine repair shop I worked at(I repaired the blocks) this is a better method, as it is easier to change the OD of the sleeve than the ID of the bore.
You can match taper(if any),get a more exact press fit etc....
Glad work have liquid nitrogen and a heating oven. Beats trying to press things together!!

One thing I am curious about though.. is press fit clearances(interference)...

On most cast iron blocks, I gave about 0.005 to 0.001 interference foot per 1" of diameter of the sleeve.
What is the "norm" for installing sleeves into a aluminum block?
I would assume more on the 0.001-0.0015 per inch side?
Really not to familiar with aluminum blocks....




And as for the hunk o aluminum... it's the $$ that keeps me from doing it is all.
Too bad I can't find a slightly larger block I could re-bore the sleeve bores to the positions I need to accommodate what I am trying to do.
Last I looked into it, I got quoted $240.
Then there is the TONS of hours machining it...
If I had gotten a mill this year, then it would be another story.
At work, all I can get is 2 hours here, 4 hours there, lots of setup and teardowns.
adds to the overall effort needed to finish a long project

But to find one I would have to have access to WAY more info that I have been able to get my hands on as of now.
Or access to original blueprints to a variety of bikes over the years.




l8r

[Soos]

Well, I'll re-post this info here as well.....




http://www.motorcyclespecs.co.za/model/kawasaki/Kawasaki_Z750N%20Spectre.htm

At that website there is a TRULY needed info tidbit when getting your rings for your 66mm pistons...
make sure that if you have GPZ pistons you get GPZ rings.
And same for KZ pistons.... get KZ rings.

Spectre/GPz pistons have higher domes (also part of the gain in c.r.) To reduce friction the pistons have thinner rings, 1.0mm steel for the top ring, 1.2mm cast iron for the center ring, while the standard KZ uses 1.5mm cast iron rings for both.

HAHHAHAAhhhahahaha
Not funny.

I thought that since the GPZ 750 pistons were 66mm with 2 valves per cylinder(even though DOHC, go figgure!) and the KZ 750 pistons are 66mm with 2 valves per cylinder the ring sets would be the same...

HAHHAHA.
NO.


I really don't wanna buck up the $$ for the GPZ rings, best $$ deal I found is $45/set!
So about 180+ for a whole set of rings... not gonna happen.


Already have 4 sets of KZ750 rings,and paid under $60(with shipping) for all 4.
Not bad considering they seem pretty rare on E-bay.
I might want to snag a set of KZ750 pistons as not only will I not have to mod the piston grooves, but I will end up with a slightly(guessing 1-2mm) shorter piston.

Dunno......


Anyone got a set of KZ750 pistons they wanna trade for a set of GPZ 750 pistons?






l8r