Topic: 836 Clearance? Thicker head or base gasket???

[CB750F2]

OK, I will try and post an Excel file which has the results of what I have done. Thanks, Pat

[CB750F2]

Someone may like to comment.

[ekpent]

So your conclusion then would confirm what Terry and others have thought that using the cheap flat top big bore 836 pistons in the F2-3 engine will yield very poor compression numbers copared to stock,correct ?

[Retro Rocket]

So your conclusion then would confirm what Terry and others have thought that using the cheap flat top big bore 836 pistons in the F2-3 engine will yield very poor compression numbers compared to stock,correct ?

That should be relatively easy to work out just by looking at the difference between the piston tops, having numbers to confirm it is even better.....

[CB750F2]

Expent, yes my results show that if the flat topped pistons are used on the F2 & F3 motors without any modifications to the cylinder head compression would fall to 7.65 to 1 - far too low. However, I would like others to look at my results, check my calculations and maybe repeat my tests to confirm or otherwise. The Excel spreadsheet attached to my previous reply shows the calculation formulae.  I did repeat the tests at least three times but I am no expert in this area.
I have read through a number of posts on this subject and figures of 26 to 27cc are mentioned. Maybe those heads had been milled previously. Is it possible the Australian bikes are different? Pat

[ekpent]

I do not think that the AU bikes would be different in that aspect.Most differences between countries is more cosmetic like lighting etc probably to meet certain specific regional standards.

[Terry in Australia]

Someone may like to comment.

I told you so. Just call me "Hondaman" from now on Pat............ (Thanks Eric)

[CB750F2]

Thanks, Terry. There are a lot of people saying that the F2 chambers are only 18% larger than the Ks. My measure suggests 46% larger. This is why I would like someone else to repeat the tests. Yesterday I I dropped 23cc into one of the chambers and measured the distance from the surface to the face of the head. The value I got was 2.35mm!! This is what I would have to mill off just to achieve 9.0 to 1 - standard CR. I ahve just noticed that one of my worksheets does not have the formulae so I have attached the corrected file to this message. Pat

[Terry in Australia]

Ha ha, I think I might have mentioned that the combustion chambers in an F2 are cavernous compared to a K, but the experts were assuring the faithfull that they'd still get a CR of 9:1 with those flat tops in an F2, so who was gonna listen to me?

Anyway, I've gotta run Pat, I'm working on chapter 2 of my new book, "CB750"s for Dummies"

[lucky]

You will need a thicker cylinder base gasket.
Call Cycle-X.

[Retro Rocket]

You will need a thicker cylinder base gasket.
Call Cycle-X.

Why would he want that.? That would lower compression even further.....

[MessnerMoto]

I found this old thread.....

I am checking if my head was decked before....  Does anybody knows what is the thickness of OEM head ? Presumably f2 head height

[Garage_guy_chris]

im not gonna get into the CR arguements, the only thing i have to add in that regard is are you guys accounting for the increase in bore size? just increasing the bore will increase compression shown below very crudely.
736cc/4=184cc into 23cc 184/23=8.36CR
836cc/4=209cc into 23cc 209/23=9.08CR

As for the cruzin_image deck height issue i had tall pistons and stacked tolerance problem too.

Well over the last few days I have been building my engine. Its been going together good but I have had a few surprises.

Like a number of people have encountered, My pistons came out 0.011" above the cylinder deck. this isnt a huge issue as the head gaskets are thick at about 0.060" so they provide sufficient space between the head and the piston face. (In my case i measured 0.048" with the solder method)

Yesterday I was working on setting up a Kenny Harmon D grind and checking valve to piston clearance and I came upon some issues. I degree'd the cam in within about 2 deg i put the rockers into the towers for cyl 2 then adjusted the valve lash to 0.005" and rotated it through a few times. Upon dis-assembly I measured my clearances again and got 0.047" on the intake and 0.035" on the exhaust.

My research indicates 0.065" intake and 0.080" exhaust are about as tight as you would want to run.

If the pistons had the proper deck height at 0.016" - 0.020" below the deck or in the hole the clearances probably would have been borderline or OK as this change would have provided an extra  0.027" - 0.031" of clearance increasing valve to piston clearance to  0.047 + 0.027 = 0.074" intake and 0.035 + 0.027 = 0.062" in my case.

For me i think the Kenny Harmon D grind is gonna be a no go, im gonna dial it back to an F2 cam i was initially gonna use.

So the moral here is if your adding a cam to these pistons know your deck height and carefully check valve to piston clearance with clay or plasticine!

If anyone has successfully checked clearance and successfully ran a performance cam please share the details
-Im still considering something milder, like a webcam #41 , #41a or a Megacycle 125-00

[Terry in Australia]

I think you better read Pat's post again mate:

Thanks, Terry. There are a lot of people saying that the F2 chambers are only 18% larger than the Ks. My measure suggests 46% larger. This is why I would like someone else to repeat the tests. Yesterday I I dropped 23cc into one of the chambers and measured the distance from the surface to the face of the head. The value I got was 2.35mm!! This is what I would have to mill off just to achieve 9.0 to 1 - standard CR. I ahve just noticed that one of my worksheets does not have the formulae so I have attached the corrected file to this message. Pat

[CB750F2]

Chris, increasing the bore will increase compression but not enough to compensate for the extra volume above the piston brought about by using almost flat topped pistons instead of the dome shaped ones that are standard on the F2/F3 models. You would be correct if the engine was from a K bike where the standard pistons are flat or almost flat. Pat

[HondaMan]

When I worked this out some time ago, presuming the 26cc head chamber for the F2/3, it comes out to 9.04 CR with a stock F2/3 cam (0 BTC inlet opening) and flat-topped 65mm pistons, using 1.0mm for the compressed head gasket. So, I deck the heads 1mm to bump it up to about 9.2:1.

[CB750F2]

Mark, something is not right here. I measured 28.4/28.5cc as the volume of the combustion chambers of my F2 head. Add the volume of the head gasket, the volume above the top ring and minus the volume of the small dome of the almost flat topped pistons and I came up with a total volume above the piston at TDC of 31.4cc giving a CR of 7.67:1. See my attached excel file about a half a dozen posts back. I was not prepared to machine in excess of 1.00mm from my F2 head so I fitted a K head with a mild port and polish and I am very happy with the result. If you or anyone else can see mistakes in my calculations please feel free to give me a heads up. It boils down to the measured volume of the combustion chambers. I measured mine with a burette and flat piece of Perspex and I did all cylinders at least three times. Mark, I think I will take my F2 head to a local automotive engine builder and ask him to measure the chambers. I shudder to think what he will charge me but at least it will confirm or otherwise my measurements. Pat

[CB750F2]

Mark and others. The measurement obtained by the local engine builder was 26.0cc - my figure was 28.4cc. So my method of measurement or equipment used is flawed. So please accept my apologies. However, when you add the volume for the head gasket etc the total volume above the piston at TDC is approx. 29cc giving a static CR of 8.22. I look forward to comment. Now I need to find the cause of my high measurement. Pat