Topic: SOHC 810/811/812 Piston Kits No More?

[CB_Jeff]

Greetings all from cold, snowy, New England.
While my engine build is a way off, I am starting to plot the build and think about gathering up parts (and funds) for the build.  The goal of the build is a good reliable motor that will offer some noteworthy performance gains... not looking to drag race or take on a Ducati.
I have been doing some preliminary research and what I have read is that an 811 or 836 big bore kit with some mild head work and cam upgrade is the way to go, and after digging a bit deeper it looks like the 810/811/812 is the more reliable option with basically the same horsepower gain.  The catch there is that no one seems to offer the smaller kits anymore... if this true or am I overlooking a vendor?  I checked Cycle X, Dynoman, APE and Weisco.
It sounds like there could be a way to piece this together with CB350 pistons, but that some mods may be neccessary to pull it off?
I had a seized '73 motor with a RC 811 kit, but traded it off... maybe I should have held onto it?
Anyhow if there is a source for these kits out there I would be interested to know... as well as any performance feedback on the installed kit and cam recommendations.
Thanks!
-Jeff

[Stev-o]

Are you familiar with the eBay 836 kit?


http://forums.sohc4.net/index.php?topic=89779.0;nowap

[CB_Jeff]

A bit, they seem to be getting good reviews.

[Powderman]

http://www.ebay.com/itm/69-77-HONDA-CB750K-836cc-65mm-BIGBORE-PISTONS-KIT-W-HEAD-GASKET-CI-CB750KBB-/161167628378?pt=Motorcycles_Parts_Accessories&hash=item258656d85a

[Jerry Rxman Griffin aka MuthaF'er]

Can't say that too many of us would say a 812 is more reliable than a 836. Back in the day 76 I went with 812 just to give me one size larger for the next time I blew it up. I didn't want to go bigger than 836 due to the next step up in expense. After 836 it gets real pricey because you gotta re-do just about everything.

[754]

Not any more they have 850 kits to clean up the 836.. Cheaper boring cost than doing a fresh 836cc

[CB_Jeff]

...and the verdict is; no more 811 / 812 kits unless you find a new old stock one and are up for paying crazy money.
One more question for the group, what is a good back of the envelope number for a mild performance engine build, the reason I ask is that I will soon start squirelling away nuts for a build.
I am not set up for machining, so I am going to sub most of it out.
At a miniumum the build would include:
 - 836 piston kit,
 - mild cam,
 - the full enchilida on the head
 - maybe some carbs and a Carpy Yosh. replica exhaust.
I am thinking this looks alot like 2.5 to 3K nuts, but I would be interested to know what others have experienced.  Again, the goal is a nice, reliable engine that could provide some extra smiles.

[andy750]

Jeff

I put a 810 kit in my CB750K2 and love it! I dont need to run an oil cooler, got the head worked on by MikeR in Ashalnd, MA and he also installed the kit which was a pain as parts were hard to source to make it work. Bike  has 71 bhp at the rear wheel. I am VERY happy with the gains and reliability of this bike and believe I dont ride it slowly. Contact MikeR for pricing details or there is also Anders in Candida, NH who knows a thing or two about machining and CB750s.

See the link in my signature for the 810 K2 build and dyno graphs

Good luck!
Andy

[bjbuchanan]

Most people run the 836, plenty of reliability and like 754 said if you trash it cyclex has the 850 to clean it.

Being in england you are gonna run up a slightly higher price tag because of the location of parts and stuff, gotta ship it all. MRieck does the whole head in different stages/prices, you could contact him. Also APE, cyclex

Depending on your budget and shipping costs maybe buy one of the built cyclex motors unless you are in to the DIY?

[CB_Jeff]

Limited DIY is the plan, Cycle X is no longer doing the built engines.... as I understand it.
Andy, ~70 rear wheel HP is the number that I hope to hit.
Anders has done some work for me, I will consult with him a bit more the next time that I see him.

[andy750]

I have to say its a great feeling when you hit the throttle and feel the bike pull away. I compare this to my stock cb750k4 and the difference is huge!

[bjbuchanan]

Limited DIY is the plan, Cycle X is no longer doing the built engines.... as I understand it.
Andy, ~70 rear wheel HP is the number that I hope to hit.
Anders has done some work for me, I will consult with him a bit more the next time that I see him.

Have Mrieck totally go thru your head, that will really be the big change. My 836 pulls like a freight train compared to my old (broken) 812

[CB_Jeff]

I'll have to connect with Mike R. about the head work and also Anders about the piston kit and cylinders.  All of this relates to the 1978 CB750F that I recently bought from member 73Nancy up in Oxford Maine a couple of weeks back.  Let me tell you, going to pick up a bike in an ice storm is not for the faint of heart! Anyhow, the bike has a 77/78 K motor in it of unknown mileage, so part of the debate is running with that K motor or trying to find one of the F2 motors, which are harder to come by than I would have thought.  It sounds like the F2 tranny and head may be worth holding out for?

[Geeto67]

I used to see a lot of 811/812 kits in roadracers and street racers - the drag guys always went big bore 836cc and up - but the 811/812 are "cheater" kits in that you push the motor toward slightly oversquare (cb750s are under square at 61x63mm)  but not so much that you give up top end gains or revs. Yosh used to sell an 811 kit and a cam (usually know as the road race cam) that went with it.

While the 811/812 kits are NLA - pistons don't go bad unless you burn a hole in them. So I buy used pistons all the time. CB350s also use a 64mm piston and I believe most of the kits use the same rings, so it isnt difficult to get the parts needed to run a set.

[BPellerine]

set on ebay now.bill

[CB_Jeff]

I have pretty much resolved to go with an 836 kit... probably a Weisco kit?.. although the allure of the ebay kit is strong.  If someone was making a new 810/811/812 kit I would go that route, but I would rather go all new instead of trying to piece something together.

[Geeto67]

I have pretty much resolved to go with an 836 kit... probably a Weisco kit?.. although the allure of the ebay kit is strong.  If someone was making a new 810/811/812 kit I would go that route, but I would rather go all new instead of trying to piece something together.

good, those of us with 811/812cc bikes don't want you in our ranks anyway. I don't think you understand the usefulness of this over an 811. 

There is no piecing something together, it's installing a new set of rings on an old set of pistons, something that used to happen all the time back when people drove carb'ed cars. I am pretty sure it is what they teach you on the second day of Mechanics 101 (yes, I know there is no such thing - just making a point). Some people just prefer to be "parts changers" I guess. 

[CB_Jeff]

Thanks for the post, it is very helpful!
If I could find a new kit, I'd go for the 811 kit, or if I still had my old '73 engine with the RC 811 kit it in, I would get some new rings and go that route, but paying a couple of hundred for a some used high-pro pistons and then buying some rings isn't the route that I plan to take.

[Geeto67]

No I get it, you want new parts because you don't really know how your engine will be used so you don't know what combination you want. you just want "more power" and this is the easy and cost "sensible" solution you can just buy off the shelf. You aren't building an engine combination so one size is as good as another to you.

this isn't the internet, I don't live here to be helpful to you with every word.

[BPellerine]

811 kit pros =no cyl bore so less money cons are that rings may be hard to find should something happen.
836 pros more piston choices and ring availability cons more money up front with maybe a little more power .bill