Topic: RC Engineering Big Bore

[CB_Jeff]

Greetings!
I have been hiding out in the weeds for some time getting up to speed on the SOHC engine and following builds, great site... but now it is time to get started on my own build.
I have a 1973 CB750 K which I bought in 2007 and have been doing little things here and there to keep the project moving forward, but now it is time to get down to it.
A previous owner (two owners before me) put some serious time and money into the engine... including a RC Engineering big bore kit (also, RC Engineering 4 into 1 exhaust, Dyna Ignition, ported head, carb. pods, etc...)
After running a compression test and getting back some discouraging results, I pulled the engine and the top end; the good news is that the pistons (64mm dia.) look good (will need a good cleaning) and the cylinders look good (will likely need to be honed), the bad news is that the rings are toast.
While I am somewhat new to the SOHC community, I know enough to know that Russ Collins is no longer doing big bore kits.
Following some initial searches I have found some options:
 - A custom set from Total Seal, and
 - A .40 over CB350 ring set.
The prospect of a custom fab'ed set concerns me from a monetary vantage and following some initial availability searches I don't know how easy a 40 over 350 ring set will be to find.
Is there any interchange-ability with the Wiesco rings?
.... and what to do about a gasket set? ... I know that each kit came with its own gasket set.
The overall build is a cafe racer, the overall engine build goal is something that will not strand me on some back woods New England road (reliability) and something that will pull hard in all five gears.
I would love to keep the RC parts... but I don't want to do this twice.
Help pointing me in the right direction would be greatly appreciated.

[andy750]

Hi Jeff

I see you are in New England and so instantly you have an advantage in finding what you need - contact forum member, CB750 engine guru Mike Rieck (MRieck forum name) who is located in Ashland, MA. He will surely be able to help and if he cant he will certainly know someone who can.

Check out the CB750 K2 link in my signature - for old 810 cc pistons and the effort it took Mike in finding the right rings...

Good luck
Andy (in Boston)

[MCRider]

64mm is an 810 (or 811) kit. Don't think RC was big on those. Could be though.

They focused on 836 and larger. Might be Wiseco pistons anyway. Or Yoshimura, they were big on 811 kits.

Do your pistons have "RC" on them anywhere, like on the inside? In any event, MRieck or others can help. I doubt that rings are more than a few phone calls away.

Just curious, does your pipe have an "RCEngineering" logo on the megaphone?

All the gasket sets are the same except for the head gasket. Single head gaskets are readily available in that size from many sources. Wiseco, APE, Cometic, many more.  Buy an OEM gasket set and a head gasket.

[MCRider]

A few revisions to my post. First, don't mean to be a buzzkill. Having some RC Parts would be a treat. But to get the rings, you'll need to nail down for sure what pistons you have.

An RC Pipe in good shape is a find as well, hope that's what you have.

A 1973 engine may be the one that takes the extra ORings between the cylinder and head, that the earlier engines don't have. Could have been 1974 I don't know for sure. Usually gasket sets come with everything and you just discard what you don't need if you have an earlier engine.

[754]

Mesure the rings or ring goove widths..
 RC did make a 64MM 820 KIT..

[Jerry Rxman Griffin aka MuthaF'er]

Frank,

820 would be 64+ as the 64 is 810/811/812 I believe.

My 812 64mm Yoshimura kit uses stock size 1968+ CB350 twin cylinder rings. The pistons have PopY on the side I think

When you're talking .40 over is that 1mm over? If so that would put you in 836 size rings or 65mm pistons

There had been a guy selling aftermarket 64mm sets on EBay. Search for 175 or 350 or 810 rings as they are all the same.

[flopshot]

can't help you on the gaskets but i'll say some good things about Total Seal rings.  i used them on a small block chevy and after making a pass with the water pump off i pulled into the pits puking water everywhere with a pegged temp gauge.  after cooling down it made another pass on the numbers and showed a four percent leakdown reading.   no doubt i'd use them again.

[MCRider]

Mesure the rings or ring goove widths..
 RC did make a 64MM 820 KIT..

I was only saying that less than 836 was not RCs focus. I would defer to those in the know (754) as to its actual existence.

While on the other hand, Yosh concentrated on that market. CB750 Jeff may have RC parts. We (he) just needs a confirm.

As Jerry says, very possible they are stock CB350 Twin rings.

[754]

I at first had forgot RC had the smaller kits.. you can call the size what you want..ie  Honda Motorco and their 750!

 I have 4th over 350 rings here fir an 836, they are one piece.. nowadays there is a preference for 3pce.
 This outa the RC catalog;
 820cc A.M.D.R.A. Class D limit bolt in kit using stock cylinder liners.
10.5 CR, and 12.5 CR  part K820-F, and K820-H

[Jerry Rxman Griffin aka MuthaF'er]

I see what you are saying now. They do say "820 class D limit" but don't mention actual size. If it were an actual 820 vs 812 would that be 350 first or second oversize?

Gaskets - individual Honda or an aftermarket kit. Check APE and see if Jay lists a kit that covers the range of 64mm to 65mm. Dynoman also. Most head gaskets are sold for that size differential ie 890-915. If not you can always get a standard size kit then a separate head gasket. Cometic would be my choice for the head gasket.   

[MCRider]

I see what you are saying now. They do say "820 class D limit" but don't mention actual size. If it were an actual 820 vs 812 would that be 350 first or second oversize?

Gaskets - individual Honda or an aftermarket kit. Check APE and see if Jay lists a kit that covers the range of 64mm to 65mm. Dynoman also. Most head gaskets are sold for that size differential ie 890-915. If not you can always get a standard size kit then a separate head gasket. Cometic would be my choice for the head gasket.

64.25mm (1st over (.25mm) CB350 pistons) is an 817cc. 64.50 is 823.7cc+ so over the Class D limit.
http://www.nightrider.com/biketech/calc_displacement.htm

[CB_Jeff]

All, thanks for the responses... I hadn't considered a smaller displacement kit... I figured the options were stock, 836-ish and then up to 900+ cc, I am 34.... so please forgive my ignorance;)
The bottom of the piston reads:"RC 64H ENG"
I mic'ed the bores and confirmed that they are 65 mm diameter.
The 4 into 1 is definately a RC product... it has the plate on the pipe "R.C. Products.... or Performance"... I forget which, it is not 100% but it is solid.
Also, I am in southern New Hampshire... so very close to Boston.

[MCRider]

All, thanks for the responses... I hadn't considered a smaller displacement kit... I figured the options were stock, 836-ish and then up to 900+ cc, I am 34.... so please forgive my ignorance;)
The bottom of the piston reads:"RC 64H ENG"
I mic'ed the bores and confirmed that they are 65 mm diameter.
The 4 into 1 is definately a RC product... it has the plate on the pipe "R.C. Products.... or Performance"... I forget which, it is not 100% but it is solid.
Also, I am in southern New Hampshire... so very close to Boston.

Sounds like you got the real deal. Congrats!
with "64" on the piston, I'd think that was its size. If the bore is really 65mm then its completely worn out and you'll either need a good used block to bore out to 64 for those pistons, or you'll need to step up the displacement to 836+ with new pistons.  Or I could be wrong assuming the "64" means 64mm.

I find it amazing that RC probbly sold more pipes than all others combined, yet there are so few surviving. Good one there too!

[CB_Jeff]

My measurements were off a bit. The diameter of the pistons is a true 64 mm.
The cylinder bore is slightly greater than 64 mm.
I am running a low budget caliper....which is shameful.

[MCRider]

My measurements were off a bit. The diameter of the pistons is a true 64 mm.
The cylinder bore is slightly greater than 64 mm.
I am running a low budget caliper....which is shameful.

Well it all makes sense. The "64" on the piston is 64mm. The bore may be reusable. You just need rings, head gasket, whatever you're going to do to freshen up the head, like lap the valves and new valve stem seals. Gonna be a sweet motor.   

PS: no shame in low budget. Just gotta be careful. Let a machinist check the bores for clearance relative to the pistons and possible taper.

[Jerry Rxman Griffin aka MuthaF'er]

let me know the thickness of the rings/lands. I can measure my 812 pistons and let you know if they are the same. If so just buy stock Honda CB350 twin rings and you're good to go. Send me a PM so I don't lose track.

[754]

I think the 811...820cc 64 mm, is Std CB 350 rings.. ie , 836 is 4th over...1mm oversize

[HondaMan]

I think the 811...820cc 64 mm, is Std CB 350 rings.. ie , 836 is 4th over...1mm oversize

754:
I have a set of ring-less 836 pistons (long and embarrassing story...). And, I actually PREFER the 1-piece oil rings.
Do you have a set that you could sell me?

[754]

 I only have the one set, I bought long ago.. and was going to use them on a nice set of forged pistons. So I want to keep them.
 My guess is you may get more(or for longer) compression with 3pce oils, but at the expense of faster bore wear. The 1 pce sure gave long life in some stockers..
 

[HondaMan]

The 1 pce sure gave long life in some stockers..

They sure do!

[Sbc63trls250]

Hey jeff you contacted me about a chain guard. Your inbox is full. Shoot me an email at sbc63trls250 at yahoo dot com or instant message me again

[Sam Green Racing]