Topic: Do I have 12:1 or 10:1 Pistons?

[rogue6]

Hey everyone.  I recently picked up a ’74 CB750 that has a Wiseco 836cc kit in it.  What I’m not sure about is if it’s the 10:1 or 12:1 kit.  I realize this isn’t an exact method and can be affected by many different things such as cams and even the volume of air in the tester, but does anyone have any idea what approximate numbers I’d see on a compression test with stock, 10:1, and 12:1?  I’d like to get a better idea of what I’m working on as I start to order parts for it and put it back together.  I’d prefer to avoid pulling the head if at all possible to determine what pistons are in it.

 

Thanks much, any information/tips/tricks would be greatly appreciated.

[754]

Get a small inspection lite, pop a plug out and have a look.
pretty soon there will be a pic posted and the difference between the 2 is easy to spot

[eurban]

How about just running it about on pump gas like 93 octane.  If it runs well then you can be pretty sure that it is 10:1.  12:1 in these old beasts pretty much requires race fuel.

[Big Jay]

I don't believe Wiseco ever made a 12:1 836. They used to make an F2 piston that we would put in the K motors for more compression.

Jay

[rogue6]

I looked inside with a borrowed boroscope and did some image searching.  This is, by far, the closest matching image that I found:



Big Jay, this is listed as an F2 piston on the dynoman site that I linked the image from.  What do you say experts?  Name that piston.

[billybobobrain]

I don't believe Wiseco ever made a 12:1 836. They used to make an F2 piston that we would put in the K motors for more compression.

Jay

Wiseco made them, but they stopped for some reason, I am pretty sure I have seen a few 12:1 kits available, But yes you need racing fuel if you are running them.

The pictures look like 10:1

A link to JE 12:1 836 pistions: http://www.dynoman.net/engine/pistons/j-and-e.html

[rogue6]

A little more information via the search feature, my pistons do not have the double cut valve relief on one side like the picture i just posted.  They look identical to the pistons that JLeather had with the symmetrical valve relief.  I cannot see any markings on them however with the boroscope.



http://forums.sohc4.net/index.php?topic=45633.msg479165#msg479165

Billybobobrain, they do not look anything like what you posted.

[billybobobrain]

They look like 10:1 with a deeper valve relief for a bigger cam. the 12:1 pistons have a taller valve relief, almost full circle. I would bet they are 10:1. any takers?

[rogue6]

The PO did say there was a aftermarket cam as well. 

I hope you're right.  I purchased this bike to put some longer trips on.  If these are 12:1 pistons, I'll most likely just resell it and find something else.

[Soos]

I would bet money if they look like that there is no way they are 12:1.

Now the buggers I got laying around... those look like at least 11:1.


The piston in the center is a stocker cb750 piston, the 2 on the side are (??) 11:1 or so.
Approx. 6.5mm taller than the stockers.


l8r



Edited 'cause I have a in-ability to communicate mind to hand when typing....

[billybobobrain]

I believe if they are wiseco then the are either 10, or 12. Unless they are a 12 that are machined to 11 but they look like the 10.

Here is a pic of a 12.5:1 836 piston.



This is my 10:1 850 kit form cyclex:

[Soos]

AHA!

I ran into the PO of that cb750 my pistons came out of.
He remembers them being a 836 10.5:1 kit for the later '77/78 F heads.

So in a K head, I would guess they make 11:1 or so?



l8r

[JLeather]

Then ones I posted ended up being an early Wiseco 10.5:1 kit.  It's unlikely that you've got 12:1 pistons.