Topic: CB900 Rods in a CB750

[mick7504]

I'm building a motor with CB900 rods, and the last one I put together was with a .060"spacer under the block to compensate for the added rod length.
Here are some some photos of a "dry"assembly without the spacer or gaskets.

(Spacer on top for detail)



The outer edge of the piston protrudes approx .025" above the block deck (as it is) and I'm thinking that with a base gasket and taking a .040" cut from the edge of the piston may just be enough.

Any thoughts on this brain storm?





Top veiw of spacer


Side View


 

[kmb69]

If you do not space the cylinder from the bottom, you sure better check the valve to piston clearance. It could be an issue depending on your camshaft lift. Trimming piston edges should bump the compression ratio up some if you have proper valve clearance.

[bwaller]

I bought Viton coated steel base gaskets from Cometic for my 570 and they offered different thicknesses. Playing with both head & base gasket thicknesses may save you from machining the pistons.

As mentioned you won't know until you measure valve/piston clearances to be certain. But I don't think I need to tell you this.....

Looking good Mick.   

[turboguzzi]

how well does the piston  crown match the combustion chamber? would do a plasticine "squash" to get a better idea. who knows, you migh tbe able to create some squish around the edge that will greatly increase your resistance to detonation. would spend some time on the subject.

[Jon]

What is your target compression ratio?

As TG said doing the work to add some squish areas would be worth the effort IMHO.


Cheers
jon

[mick7504]

Thanks for all of the input men.
I'll clay the pistons to get an idea of how it all marries together and then post some more photos.
Mick

[mec]

these rods are longer than the stock rods: is the stock cam chain long enough?
otherwise you will run into troubles.

mec

[mick7504]

these rods are longer than the stock rods: is the stock cam chain long enough?
otherwise you will run into troubles.

mec

Hi Mec
Yes the stock chain will go on with the .060"spacer under the block.
It is a neat fit but it will go.
Mick

[Sam Green Racing]

Mick, what's the reason for wanting to use the longer rods ?

Sam.

[Retro Rocket]

Mick, what's the reason for wanting to use the longer rods ?

Sam.

Phallic symbol maybe.....

[mick7504]

Mick, what's the reason for wanting to use the longer rods ?

Sam.

Sam

The CB900 rods are a bit more substantial than the stock 750 rods and have 8mm cap bolts as against 7mm.
The small end and big end diameters are identical but are 1.5mm (.060") longer.
Weight wise, they're up about 2 grams or thereabouts.
They are a relatively cheap upgrade and everything works fine with the block spacer to offset the added length.

Here are some photos for comparison.
Mick

[lordmoonpie]

You could just do what I did - 1.5mm longer rods is what is required when you use a 750 DOHC crank in an SOHC engine as the DOHC engine has a shorter stroke. You also get the benefit of hy-vo cam and primary chains if you see that as an advantage? no messing with deck heights then...

[mec]

Mick, what's the reason for wanting to use the longer rods ?

Sam.

Sam

The CB900 rods are a bit more substantial than the stock 750 rods and have 8mm cap bolts as against 7mm.
The small end and big end diameters are identical but are 1.5mm (.060") longer.
Weight wise, they're up about 2 grams or thereabouts.
They are a relatively cheap upgrade and everything works fine with the block spacer to offset the added length.

Here are some photos for comparison.
Mick

longer rods are good to reduce side forces on the piston walls, friction reduction. for best results this should go with an 2mm offset (to the exhaust side) of the cylinders

mec

[MRieck]

Mick, what's the reason for wanting to use the longer rods ?

Sam.

Sam

The CB900 rods are a bit more substantial than the stock 750 rods and have 8mm cap bolts as against 7mm.
The small end and big end diameters are identical but are 1.5mm (.060") longer.
Weight wise, they're up about 2 grams or thereabouts.
They are a relatively cheap upgrade and everything works fine with the block spacer to offset the added length.

Here are some photos for comparison.
Mick

longer rods are good to reduce side forces on the piston walls, friction reduction. for best results this should go with an 2mm offset (to the exhaust side) of the cylinders

mec

Both very good points mec. Also...many modern bikes are running cylinder offset now.

[MRieck]

What kind of pistons are those Mick? The reliefs look huge.

[Sam Green Racing]

Mick, what's the reason for wanting to use the longer rods ?

Sam.

Sam

The CB900 rods are a bit more substantial than the stock 750 rods and have 8mm cap bolts as against 7mm.
The small end and big end diameters are identical but are 1.5mm (.060") longer.
Weight wise, they're up about 2 grams or thereabouts.
They are a relatively cheap upgrade and everything works fine with the block spacer to offset the added length.

Here are some photos for comparison.
Mick

longer rods are good to reduce side forces on the piston walls, friction reduction. for best results this should go with an 2mm offset (to the exhaust side) of the cylinders

mec

Both very good points mec. Also...many modern bikes are running cylinder offset now.

All my Benly twins from 58 onwards have about 2mm offset but the race kit pistons for them have hardly any.

Sam.

[mick7504]

What kind of pistons are those Mick? The reliefs look huge.

Mike

These are 65.5mm Cyclex pistons.
The large valve reliefs is what got my head going and then doing away with a spacer under the block.
There was a bit of "jiggin" around with the the cam chain and the spacer last time although it all went together in the end eventually.
I'm thinking that there is a better way of doing this, and the big valve pockets in these pistons is a good starting point.
Mick

[mick7504]

Without the base gasket, the piston contact to the head is around the outer edge which was to be expected.
The piston edge is close enough to .010" above the deck of the block at TDC.



To get the head to to sit flat onto the block (without a head gasket) the pistons need to drop back into the bore 1mm (.040") with the outer edge of the valve relief flush with the top of the block.



If the top of the piston is machined in square to the edge of the valve reliefs (like the Wisecos) and down .020", that should be more than enough with the head and base gaskets installed.
Once that's done, I can check the valve to piston clearances and also the piston dome in relation to the combustion chamber and the block spacer shouldn't be a requirement.

[Jon]

I would be tempted to clay it up with the shim installed & take the thickness of the shim of the clay measurements.

It will give you a pretty good idea of how much & where you are going to have to machine the piston.

Valve clearances are going to be tricky as you need to allow for the shim thickness in line with the bore not the valve travel.


Cheers
jon

[turboguzzi]

mick, i think your best option is actually turning machining a flat area in the pistons crown to achieve best fit with block and head,

have a look at the "gentleman's express" article, its the same idea

that's what i did in my builds, gets you some squish area too.

TG

[mick7504]

Thanks Jon & TG.

I'm going to cut the edge of the pistons back in square and down .5mm close to the valve reliefs to eliminate interferance with the head.
The compression height will be reduced and I don't know how much the c/r will increase.
Guessing that it maybe by a point or a bit less.

I'll solder crush the piston crowns and post some photos and see what you think.
It'll be next week sometime.

If valve to piston clearances and the dome to combustion chamber profiles are acceptable, I'm feeling reasonably satisfied to run this.

Mick

[mick7504]

Well so far so good without any problems.
The pistons have been cut back in 2.2mm and squared in.
Compression height is now at 22.8mm.

Here is a before and after.







The photo doesn't do the machining job any justice.
(well that's my story & I'm sticking to it  )

I installed the head with a .050"copper gasket and a .020" base gasket.
A piece of solder was taped to the top of the piston and then the motor was fully rotated.

Here is the result of that.





This last shot is of the piston @ TDC.



The valve to piston clearances still need to be checked and then it should all be good.
I'm still agonising over whether to trim the piston domes or leave them as they are now that the C/R will be slightly up with the longer rods.

 

[bert96]

I was wondering if he install the base and the head gasket from cyclex (MLS)if he would still need a spacer?

Would like to know....!


Bert

[mick7504]

Hi Bert

Because these rods are 1.5mm (.060") longer, the two areas of the pistons that I expected would need some modifying are the piston edge and also the valve reliefs.

Initially, I assembled the top without a base or head gasket just to get a benchmark to work from.
For the head to sit flat onto the block, the pistons had to drop .040" into the bore.
To get the piston dome to fit into the combustion chamber, the edge of the pistons had to be cut in square for clearance. (See earlier pics)

I've just finished checking the valve to piston clearances and the valve reliefs will need to be opened up a bit more.
The exhaust valve is contacting the piston and the intake has about .050" clearance.
This was checked with a .020"base gasket, a .050" head gasket and zero lash.

Here are some photos.
Also one of the camshaft that I've been checking with which has a fairly aggressive grind.

Intake Side


Exhaust Side


Ivan Tighe Camshaft
Specs:
Lift - .370
Intake - 36/59 deg @ .040"
Exhaust - 66/36 deg @ .040"

[Retro Rocket]

Thats a nice looking bump stick there Mick.....