Topic: 811 Pistons - '75F

[RSchaefer]

I am in the process of bringing a '75F back to life.  The motor has around 14K miles and was never opened since the factory.  I did not intend on getting this far into the rebuild when I started but probably everyone thought that.  The farther I got into disassembling and with the enormous help of the forum I continue to dig deeper and deeper.  I got Hondaman's book at the beginning and am attempting to incorporate most of his low cost upgrades into the project i.e. bought an older swingarm and sent it off to Hondaman for the rebuild, picked up a Hondaman ignition, etc.

I finally made the decision to pull the head and cylinders.  I polished up the valves and after sending pictures of my seats off to Hondaman decided to get a full valve job.  Seems the chambers saw allot of moisture over time even though it sat in a Phoenix backyard on its side for probably 15-years.  Also the tops of the pistons are somewhat pitted so I was sanding them down and going to polish them this last weekend, plus the ring seats are really full of carbon.

From the evidence it seems the PO did not change the oil often but I do not see any serious damage.  I was going to hone the cylinders to remove the top ring marks per Hondaman and was assuming I would stay stock.  I found a set of 811 pistons and was thinking maybe I should just replace the pistons rather then clean and polish the originals.  So what are 811 pistons?  Is it stock diameter but domed to increase compression?

Anyone's thought or opinions would be greatly appreciated!!

[754]

 The diameter is a few mm bigger.. which is 4 -10 times more material removed than a regular overbore.. which means higher boring cost..
you will probably like the result..

[MRieck]

If the seats are that corroded I have to assume the valves faces are shot as well.

[Jerry Rxman Griffin aka MuthaF'er]

The 811/810/812 pistons are +3mm oversize ie larger diameter than stock. Depending on which manufacturer, you may be able to use the old CB350 twin piston rings from Honda. My Yoshimura kit uses these rings. Otherwise rings may be difficult.

Nothing wrong with using those pistons. Of course you have to fit rings and bore the cylinders. The compression will likely be in the neighborhood of 10.25:1 vs your 9.2:1 which will help with power along with the bore increase. Can you ID those pistons? Post a picture?

Now, on the other hand, if you're dead set on increasing the bore and compression you may want to consider the cheap 836 kits on EBay that the guys have been talking about. May be cheaper than finding rings alone! And you gotta bore anyway.

Another good/better option for a stock or mild performance engine is to have the head ported while you're doing that valve job to increase the airflow resulting in higher HP/better performance. 

[Sam Green Racing]

I wonder where this is leading. ...................................



................................................................MURPHY!!!!!!!!!!

[RSchaefer]

I did do the port, but may have gone to far.  I have been sending sideshows of the progress over to Hondaman.  You can see my pictures by going to my web site at (www.AccuProINSPECT.com), hitting the button "View My Report" entering the Clinet Name: MParis, and the Password: cb750.

[RSchaefer]

How much of a bore can the stock head gasket take?

[RSchaefer]

Anybody look at the pictures?

[Sam Green Racing]

It's very difficult to tell from the pics Rich.
With porting, It's important to take metal away from the right places to get the required tune for the type of use that the bike will be put through/used for.
One of the more important aspects of porting is to make sure all the work that you do is matched in all the other ports.
The stock gasket is good for a +1.00mm rebore, no to sure after that.
Good luck.

Sam.

[Jerry Rxman Griffin aka MuthaF'er]

Looks like there may have been a lot of nicks inflicted on the valve seats.

[RSchaefer]

Yes, lots of nicks!!  Do you think a valve job would be able to take these out?

[Sam Green Racing]

Wow, I thought the nicks were in the valve throat not the seat, after looking again they look like toast, and well done toast at that.

Sam.

[Bill/BentON Racing]

Time for Mike Rieck magic! Good Luck!

[RSchaefer]

I took it over to the local AZ version of Hondaman (Charlie, http://www.motorcyclerestorationpros.com/Home.html) and he took a close look at the seats and said "no problem" the CNC valve job I was planning on him doing in any case would cleanup the seats, no problem.  Man what a relieve, I was afraid I had screwed the head for sure.

I had suggested milling .005 inch from the cylinders and .010 from the head to get her flat and add a little combustion per Hondaman.  He was fine with the .010 on the head but recommended against milling the cylinders.  He though my cylinders looked fine.  Suggested using a sanding block on the cylinders to get it smooth.  Said milling the cylinders would effect the cam and the F0 already has a little hotter cam.

I had already honed the cylinders and he measured with a .0018 feeler gauge and said the cylinders and pistons looked great, no reason to bore and get new pistons.

So on to the next issues, thanks for everyone's comments!!

[Sam Green Racing]

Rich, does your machinest know what he is talking about.
Machining head or block will afect the cam, with either you are reducing the distance between crank and cam which must alter your cam timing, give it to someone who knows.

Sam.

[RSchaefer]

Well thanks Sam, lots get lost in translation as they say.  This guy specifically rebuilds CB750's.  I have seen some of his bikes and the work appears to be the highest quality.  Wrenches tend to have ego's especially over time.  They also tend to view their way of doing things as the only "right" way to do it.  Honestly I think it comes with the territory and you need to get references and see their work.  This guy is in his '60's, travels to Japan to do seminars on valve seats, flow, etc. so he seems very competent and confident.  I do trust his work!

I may have misunderstood what he was saying as it relates to the milling of my head relative to the effect on the cam.  Your right, obviously any reduction of the distance between the crank and the cam is going to have an effect, whether its the head or the cylinders.  I felt like he was viewing me as the newbie, as I would probably do if I were in his shoes.  I've only physically met him twice, but talked to him on the phone probably 6 times.  He is obviously very busy and in high demand from what I have seen.  I had to wait a couple of weeks to talk to him last year as he was doing a seminar in Asia.  I am assuming he viewed my cylinders as not needing any attention in terms of milling and dismissed it with some comment regarding the effect on the cam.

Anyway hopefully I will not be posting any sad refrains as it relates to my head job!!

[Bill/BentON Racing]

I was going to echo Sam,but if guy does what you said,he'll know.Bill

[Sam Green Racing]

Well thanks Sam, lots get lost in translation as they say.  This guy specifically rebuilds CB750's.  I have seen some of his bikes and the work appears to be the highest quality.  Wrenches tend to have ego's especially over time.  They also tend to view their way of doing things as the only "right" way to do it.  Honestly I think it comes with the territory and you need to get references and see their work.  This guy is in his '60's, travels to Japan to do seminars on valve seats, flow, etc. so he seems very competent and confident.  I do trust his work!

I may have misunderstood what he was saying as it relates to the milling of my head relative to the effect on the cam.  Your right, obviously any reduction of the distance between the crank and the cam is going to have an effect, whether its the head or the cylinders.  I felt like he was viewing me as the newbie, as I would probably do if I were in his shoes.  I've only physically met him twice, but talked to him on the phone probably 6 times.  He is obviously very busy and in high demand from what I have seen.  I had to wait a couple of weeks to talk to him last year as he was doing a seminar in Asia.  I am assuming he viewed my cylinders as not needing any attention in terms of milling and dismissed it with some comment regarding the effect on the cam.

Anyway hopefully I will not be posting any sad refrains as it relates to my head job!!

He sounds the Bees knees Rich, hope it all works out for you.
Does he use a Serdi mahine for cutting seats ?

Sam.