Topic: CB900 Rods in a CB750

[bert96]

 Thanks Mick!



Bert

[mick7504]

Back to the drawing board with this one.

Here's the result of a cut into the exhaust side relief.

At the breakthrough point, there is/was approx 1mm of material and there's approx 5mm at the peak of the dome. (not including the cast webbing)

Off to plan "B" now.



I've spun the top out of this piston to have a look.

[sohc boy]

mick 7504 , have used 900 rods for over 25 yrs in my drag  bikes,no spacer under barrel , but only with MTC forged pistons yea cast are too thin ,turned off top of piston, made squish band much wider (as pic)so had large dome ,was after high comp 11.5 ,but for street would cut down dome , but was using F2 HEAD(less 60th never had a problem......

[mick7504]

Thanks mate.
That's encouraging to hear that you haven't used a spacer and I'm quite determined that it is possible.

I've started on plan "B" with this one and I'll try and get some photos up tomorrow night.

What I'm doing this time is using CB1100 rods which are the same length as the 900 rods but take a 17mm pin.
I've bushed the small end down to 16mm and put a set of CB360 pistons onto them. (67mm)

It looks like the comp height of these is around about 1.5mm under the 750 piston spec so that should work out good and I'll get some more accurate measurements tomorrow.

Hopefully I can report back tomorrow.

Mick

[sohc boy]

yea your on the right track now , have 1100f rods, an early z1 900 kwak pistons  17 pin in my motor now 72mm mtc ,kawa big pistons were cheap yrs ago as they went to 1348 or 1427 an smaller ones were giving,them away,i have used stock cb 900,11000f pistons in mates bikes yrs ago, minor turn down on crown ,if not confident on turning piston down ,Ridgecrest Eng in Melbourn make solid copper headgaskets ,up to 80" thick to allow for dome,but having a squish band arond top,an a dome , makes better power..... 

[mick7504]

Stage 2 coming together bit by bit.
This is just a template as a start to see how everything will go together.
I've bushed the small end on these CB 1100 rods from 17mm to 16mm.
They are 1.5mm longer like the CB900 rods and have a 17mm pin as against a 15mm pin that the CB900 & CB750 have.



The CB360 pistons (67mm) look like that they should be a good alternative to offset the added rod length because of the lower deck height.



This is the new Hi Tech piston deck height measuring device. 
I've made this to go through the pin centres to compare the heights of the CB750 piston against the CB360 piston.



This photo shows a (edit) CB360 piston against a modified Cyclex 65.5mm piston that I attempted to use on the first round but buggered it by putting a flycutter through the piston relief.



Here's a CB 360 piston on a CB1100 rod and it all looks good so far.
The best thing is that everything is HONDA 



I still have to switch the 1100 rods into the motor yet and if I can weasel out of the gardening this weekend, it will happen.
But I don't like the chances. 

[Triffecpa]

Mick

I already have a set of RC Engineering 1000cc pistons, but I applaude your continued efforts to make this project work.  How do the valve reliefs on the CB360 pistons line up with those on the CB750 pistons?

Tracy

[bear]

Mick,

We have run the 1100 rods on our hy-vo motors.
RC45 pistons are the go,
They are a great piston and will match  your deck height with out the need for a spacer.
They will also give you an acceptable CR for your road bike.
The piston on the left is the RC45.

Cheers,
Brian

BY the by PM me if you want the part #

[sohc boy]

RC Eng stuff is very hard to find here in oz ,those RC45 pistons look like the answer ,what size bore ?

[Triffecpa]

RC45's are four valve motors.  do you recut the valve pockets for the two valve head?

TR

[bear]

They aren't Russ Collins gear.
They are genuine Honda pistons.
We used to order them from our local dealer @ $55 a pop.
Std units are 70mm from memory.
You won't be able to source any over size units though, I've got all that was left.

Cheers,
Brian

[bear]

RC45's are four valve motors.  do you recut the valve pockets for the two valve head?

TR


G'day TR

Yes we cut our own valve reliefs in these pistons and the XR250R units we use some times as well.
The XR pistons give us some pretty radical CR figures and are not really suited to road motors.

Cheers,
Brian

[bear]

Back to the drawing board with this one.

Here's the result of a cut into the exhaust side relief.

At the breakthrough point, there is/was approx 1mm of material and there's approx 5mm at the peak of the dome. (not including the cast webbing)

Off to plan "B" now.



I've spun the top out of this piston to have a look.

Mick,

If it makes you feel better your not Robinson Caruso.
The way I look at it, you can't make a decent omelet without breaking a few eggs.

[sohc boy]

sorry Bear ,i worded that wrong, RC Eng stuff was for Triffecpa 

[mick7504]

Mick

I already have a set of RC Engineering 1000cc pistons, but I applaude your continued efforts to make this project work.  How do the valve reliefs on the CB360 pistons line up with those on the CB750 pistons?

Tracy

Tracy

The CB360 pistons don't have the offset relief although there looks to be a reasonably substantial amount of material underneath.
Here is picture of a vertically gas ported RC Piston next to the CB360 Piston.

You can see where the valve has been making contact.
These were in an engine that I pulled down years ago and were running under this camshaft which has some fairly nasty lumps ground into it.

This is the same shaft that I installed on the first attempt but I'm thinking that I should consider something a little less aggressive.

RC on the left & CB360 on the right




This is an Ivan Tighe camshaft.
They are an Aust Company who have been around for many years.

Mick,

We have run the 1100 rods on our hy-vo motors.
RC45 pistons are the go,
They are a great piston and will match  your deck height with out the need for a spacer.
They will also give you an acceptable CR for your road bike.
The piston on the left is the RC45.

Cheers,
Brian

BY the by PM me if you want the part #

Thanks Brian
I'll PM you about the RC45's

Back to the drawing board with this one.

Here's the result of a cut into the exhaust side relief.

At the breakthrough point, there is/was approx 1mm of material and there's approx 5mm at the peak of the dome. (not including the cast webbing)

Off to plan "B" now.



I've spun the top out of this piston to have a look.

Mick,

If it makes you feel better your not Robinson Caruso.
The way I look at it, you can't make a decent omelet without breaking a few eggs.

Mate
I felt your pain when that air gap appeared. 

[mick7504]

Here are some photos with the CB360 Pistons installed on the CB1100 rods.

They are contacting the head with the top and bottom gaskets installed but that can easily be overcome.

The reliefs will definately need to be deepened and because these are "retired" pistons, it gives me a bit of room to play around with to get it right.

Here is a familiar sight.



CB1100 rods fitted.



CB360 pistons fitted



Pistons In



It's a bit hard to see the head lifted but here are a couple of photos to have a look at.
Time for a cold beer now and I'll attack it again through the week. 

[sohc boy]

Mick are you using this on the street or ? why not use your russ collins pistons an bush 1100 rod to 15mm ?

[mick7504]

Mick are you using this on the street or ? why not use your russ collins pistons an bush 1100 rod to 15mm ?

The plan is for it to be a street motor mate.

My thinking is that the Russ Collins pistons would be more suited for the drag strip with the top (vertical) gas porting.

There might be some different ideas about this (which would be good to hear about) but I've always thought that laterally ported pistons are best for street use.

I had some "big ideas" for the RC Pistons a  while ago but got cold feet on that one and shelved it for the time being.

In the background of the last picture is the next project with Mike Rieck's billet block and that one is right up on top of the menu.

There's quite a bit of mechanical porn here. 
http://forums.sohc4.net/index.php?topic=74086.0

[754]

Before you cut a piston, make a rod and dial indicator setup to measure thickness.. no sense in cutting if already too thin..besides you will want to know your crown thickness..

[mick7504]

Before you cut a piston, make a rod and dial indicator setup to measure thickness.. no sense in cutting if already too thin..besides you will want to know your crown thickness..

Thanks Frank

I should have done that the first time instead of going like a bull at a gate about it.

I know better and paid the price for not following the old golden rule - Measure twice, cut once.

Because these CB360 pistons have gone past their "use by date" I've got an opportunity to drill some small holes through the tops this time and have a close look and measure.

I'll put some more photos up here as I go along.

Mick

[voxonda]

Superjob, Mick.

Lost some pistons too along the way.

PM-ed you.

Rob

[mick7504]

Superjob, Mick.

Lost some pistons too along the way.
PM-ed you.

Rob

Thanks Rob

I've got these locked into a 4 jaw chuck just incase they try to escape and still have to check under the reliefs for material thickness but a prelim check indicates that there is plenty there.

Took a .020"swipe from this piston and it still needs another .015" removed just to make clearance.
I may even dress the combustion chamber a bit where the piston is contacting.





I came across this homemade relief cutter video that I thought was quite clever.

http://youtu.be/c4W2p_8aXxI

[mick7504]

Here's where I'm at so far.

I've drilled through the sacrificial piston to get some idea of the material thickness.
It measures approx .250" through the crown and also the relief pocket.

 

In the meantime, I've cut a total of .060" from the piston crown which is enough to allow free rotation through TDC.



Crown clearance to the head measures .0355" with a solder crush.





I've roughed out a piston relief to a depth of .100" and another .040" should do the trick (On Paper  ) but there has to be a physical point to work from.



When everything is reassembled and the camshaft is dialled, final measurements can be calculated accurately.
It looks promising after all of this "brain surgery" 



 

[mick7504]

I'm going to cut these reliefs with the pistons installed in the motor which is a far more accurate way of doing it.

Using the valve guide (as the guide) will postion the cut exactly where it should be.

Here are some photos of the "new/old" idea so far.

750 valve with spacer attached.





I'll glue the abrasive disc to the spacer.

The next photo has been hijacked from a Ford Muscle forum to have a look at.

With an adjustable sleeve attached to the valve stem to determine the cut depth.



Mick

[bear]

That's the way we do it Mick.
Works a treat as long as you don't have to many cleansing ales before you do your math.
WE use masking tape around the pistons to cut down on chatter.
Just check the depth of your cutter our you may run into trouble getting your piston to TDC.

Cheers,
Brian