Topic: 82 CB650SC Head Gasket Replacement and Other Engine Maintenance Questions

[janich4644]

My 82 CB650SC has developed quite the head gasket leak. So unfortunately, it needs to come apart. This project will be my first time taking engine components apart, and I admit that I'm quite timid. However, while I have the engine apart, I would like to take care of as many items as I can.

There are 16k on the engine.

My first question is do I have to take the engine out of the frame in order to take the head off? If so, does anyone have advice for doing this? I’ve watched some videos and it seems like people struggle taking them out. I like the idea of placing the engine on a bench vs having to bend over the motorcycle for hours while working it, however I don’t know if the “juice is worth the squeeze”.

Besides replacing the head gasket, I plan to do a compression test before I dig into the engine. My understanding is that this will tell me the condition of the valves and piston seals.
I found this rebuild kit: https://4into1.com/engine-rebuild-kit-honda-cb650-1979-1982/
Does anyone have experience using this kit, or does it at least seem to include everything at I will need? If I'm also replacing the piston seals, I know they will have to be ordered separately. I'm worried about going down the rabbit hole that can be engine rebuilding, however I want to make sure I do it once and do it right.
Let me know your thoughts and suggestions for this job.
Thanks

[Nicklopic]

All of what I'm saying comes.from rebuilding my 80' 650C:
The top end can be rebuilt completely in the frame. Getting it in and out is a bit of a pain but compared to other models it's relatively easy because of the removable frame section. If you want to leave it in the frame, get some universal paddock stands or whatever they're called. They raise the bike up a couple inches so you're less hunched over... Or get a low stool.
There are 2 different head gaskets used on sohc 650s. No one that I found has said what is used with what so DO NOT order a gasket set until you pull the head.
I generally do not suggest ordering anything until you take the motor apart. For example, I ordered standard rings only to discover cylinder pitting and then going 1mm over sized. If you need standard rings, I have a brand new set of cruzinimage rings.
Download the FSM and buy a Chilton. The Chilton has more detailed step by steps and better pictures. Just resealing the motor and touching nothing else is honestly not that difficult and straight forward.
If you have valve issues, the valves themselves like to deteriorate and you can't just lap them.
If new valves are needed like on mine, then NOS only or old bike barn carriers some Japanese aftermarket version that's decent. (What I went with)
Valve springs are still available from Honda.
If pistons and overbore are needed, cruzinimage has STD, .5mm over, and 1mm over. Dynoman carries "674" pistons that are like 2mm over or something.
NE and Athena are probably your only options for gaskets, again based on what you pull out unless you got some fancy ones from dynoman.
According to hondaman, aftermarket head gaskets are .2mm thicker than factory (on 750s.atleast). Without decking the jugs you may get oil leaks still. Honda does have a .1mm thicker oring for this and it's on partzilla. Check the description. There's 2 listed.
When reinstalling the rocker box, back the tappet screws out and use rubber bands going from the into to exhaust rockers to hold them open so you do not break a valve stem during installation.

Mrieck on here is the cylinder head guru and I also used him to bore my cylinders.

[DaveBarbier]

I agree with what’s said above. But I would recommend NOT getting any rebuild kit. I would recommend getting all seals and gaskets you need OEM. If you click the link of my build thread in my signature it’ll take you to my table of contents. Down the list there is a link for close to 100% of all the seals, gaskets, o-rings and chains needed to build a CB650 engine (1982 CB650 specifically).

If I were you I wouldn’t replace the cam chain or tensioner unless you see damage or cracking. The chain is expensive and requires you to split the cases and the tensioner is hard to find and also expensive.

I’d say replace the following:
•Valve cover gasket
12391-460-770
•Head gasket o-ring-4.9x2.5 (2)
91301-426-010
•Cylinder sleeve o-ring-61.8x2 (4)
91304-286-000
•Base gasket o-ring-11x2.5 (2)
91310-426-000
•Base gasket
12191-426-306
•Rubber sealing pucks (6)
91318-300-013
•Head gasket (this will change if you need to do an overbore)
12251-426-010
•Copper crush washers under the valve cover screws:
90425-300-000 (16)

I’d replace the head bolts only if yours look bad:
•Short head bolts (10)
90023-MW3-600
•Long head bolts (2)
90036-426-004

I’m unaware of the different head gasket thing. Dynoman sells only one head gasket for the 674 kit. Not exactly the same as what was there for me but still worked just fine.

To find the above parts I type “Honda” followed by the part number into eBay. Some might be hard to find.

Do NOT use a torque wrench on the small valve cover screws. Just nip them down by hand with a 1/4” ratchet. They can easily strip out then you’re f’d.

[janich4644]

 Thank you both for the great advice.

Dave, that's a really good point about a kit vs OEM.  I think that will be value added to keep problems from happening in the future. And thank you for pointing out the resource you built around your build. That's some amazing work and documentation.

Should I assume that most if not all parts for a 82 CB650 engine are the same for the SC model? From most of what I've read, it would seem to be the true.

[DaveBarbier]

The motors are the same, but there are probably different tanks or side covers or something.

[janich4644]

The motors are the same, but there are probably different tanks or side covers or something.

I think you're right.

[Nicklopic]

The front of the gasket where the guide drops into the jugs has a different shape. I really don't know if it actually matters or not because they look like they'll both seal. if you look at every aftermarket listing they mention the differences so I just waited and ordered whatever matched.
dynoman not having a difference is probably a good sign it doesn't matter but no one on the interwebs could confirm.

[janich4644]

Hey Dave,

Thanks again for the parts list. I think I've found most of what I need.

I'm going to be taking a hard look at my valves. I'll will probably want to take the valves out, and clean everything up really well and lap them.

Do you know if any of the valve pieces cannot or should not be reused?

[DaveBarbier]

All that can be reused. Keep the valves mated to where they came out of. Like don’t mix them all up. Same with the rockers, but you’ll probably keep those in the valve cover. No need to remove them.

I recommend this video by Jafro:

And no problem! Glad to help!

[janich4644]

Okay, cool.

The video was very helpful, thanks again.

I'm getting excited about taking this thing apart. Going to try to get it compression tested soon, and then as parts start trickling in I'll start taking the head apart.

[DaveBarbier]

Cool, yeah go nuts. I have some more info in my build thread if you need. Just have a look at the table of contents.

[janich4644]

Cool, yeah go nuts. I have some more info in my build thread if you need. Just have a look at the table of contents.

I've skimmed the table of contents a few times. There's a lot of great information; I think it will be a great resource for me.

[DaveBarbier]

Cool, yeah go nuts. I have some more info in my build thread if you need. Just have a look at the table of contents.

I've skimmed the table of contents a few times. There's a lot of great information; I think it will be a great resource for me.

Hope so. Glad to help.