Topic: CI 836 & v/v cut-outs

[01Thomas]

Hi
Has anyone here deepened the valve cut-outs in the CruizinImage 836 pistons? If so, how much deeper can one safely increase these cutouts?

I tried a search through the forums but I couldn't find anything.

[HondaMan]

My first thought is, "why?".
There is already more than 2mm clearance in a normal rebuild with these pistons, as your head gasket is 1mm thick (or more), unless you have decked the head by more than 0.020" and the cylinders, too.
Also: if you don't already have some, PM me for a pair of thicker O-rings for the oil feed ports through those cylinders, because your new head gasket is thicker than Honda's was, and those ports WILL leak oil after your rebuild. That's why I send them out.

[01Thomas]

Hi Mark
Why is because I have two (maybe three) more SOHC 750's to build in the near future and at least one of them will have to be lightly breathed upon. I have four non-stock cams to choose from, one of which has a full 10mm lift at the cam.     I want to draw up a shopping list so that I can buy the pistons etc in bulk and save both in terms of purchase price and shipping costs. To that end I need to decide which pistons will be most suitable, and what potential they have for modifying, if the need arises.

And thanks for the offer of the o-rings but I've had an aluminium headgasket made of 1mm thick ally (to fit the cruizinImage 65mm piston bores) and right-size o-rings aren't a problem. And I'm in South Africa so shipping will be more of a headache than it's worth.

[01Thomas]

Any one else with experience in deepening the valve cut-outs on these C-I pistons?

[MRieck]

I haven't flycut those pistons but in general I'd say minimum dome thickness on a flat surface is .125 and about .060 at a point like the bottom of a valve pocket. Seeing how those are cast I'd add .020 to each of those numbers.

[MRieck]

 Will you be running OS intake valves? Personally I'd wouldn't go over .375 lift with those pistons seeing how you will need to mill the head to get any decent compression with them.

[PeWe]

Compression is most likely better with these I found now. Have not seen before.

I guess valve pockets to be done deeper for non F2-F3 heads. If domes are thick enough.
https://www.cruzinimage.net/2024/10/15/77-78-honda-cb750f2-super-sport-836cc-bigbore-pistons-kit-65mm-pistons-10-h410bb/

[MRieck]

Compression is most likely better with these I found now. Have not seen before.

I guess valve pockets to be done deeper for non F2-F3 heads. If domes are thick enough.
https://www.cruzinimage.net/2024/10/15/77-78-honda-cb750f2-super-sport-836cc-bigbore-pistons-kit-65mm-pistons-10-h410bb/

I agree...a better solution for getting the compression up.

[01Thomas]

Thanks Mike and PeWe!
Stuff to think about.

[PeWe]

If the pockets can not be cut much deeper, use a less lifting cam.

I used Action Fours SS1 cam in the 80's with ported head, 34 mm in valves. 836cc
The lift is 0.360".  @ zero lash.
If you can find one.  It works fine.

I have found the hard way that higher lifting cams and K valves with a tall waist (groove) for keepers need sunken guides. If not, it will drink oil when in valve's keeper waist dip inside the guide seal.

The head I used in the 80's did not have need of sunken guides since it used F2 34 mm valves with high sitting groove for keepers.
F2 original valves have very soft valve stems, mushrooming very quickly.

If head is not ported, the valves need to be open longer.
Maybe Kenny Harmon D cam?
I remember all CB750's with D cam back in the 80's had no chance against my bike with the SS1 cam. Maybe not ported as mine.

A little bit more lift and less overlap keeping the dynamic compression.

So it is not strange that most Webcam cams have relatively low lift. To fit a head with not sunken guides.

I have heard from a guy that used F2 pistons with K head that something hit. Maybe valves and pistons only
Investigation before order can be good.

[MRieck]

 You're correct Per....    .360 lift is about it with OEM guides. If you notice all the Web cams for the SOHC 750 are .360 max lift....they just increased the duration. I'm no fan of the "B" and "C" grinds.....too much duration for good street performance.😉