and back to the cylinder head, now what

[jimminy]

Got my engine removed a couple weeks ago and took the whole shebang to a well known shop (Garage Company) here in LA thinking (hoping really) I was looking at a simple top end job. Here's the story:

This is a '78 SOHC 750 K8 I've been posting about here for the last few months.

Seems the engine already had the 836cc mod but someone did a bad job of it. One of the rings is broke and there's no cylinder with decent compression. They recommended going back to a standard 750cc or maybe the 836cc somehow, but the upshot is I'm looking at something like $2,500 in parts and labor. And that ain't happening.

So I'm wondering about my options. I could get the thing back and try to do the work myself from here. I think this would mean getting new 836cc sleeves, rings, gasket kit and just do the reassemble, right?. I've already done the work to remove the engine and the shop disassembled the head and did the inspection for me. Everything else seems to be in good shape so nothing else seems to be in bad shape to worry about.

Guess I just wonder how much I'm missing here.

Thanks.

 

[cyclebuster]

I will sell you a running stock top end still on an upper case if you need it.

 

[jimminy]

Give me some details and I'll consider it. Though I would need then to replace pistons, etc if I understand you. What's your guarantee yours is in better shape than the one I have?

 

[dirtdigger]

Couple options. 836 sleeves are just stock sleeves that are bored to 836. A shop should do the sleeve change out if you go that way. If the current cylinder sleeves just need a clean up you can do a Cycle X 850 kit (next size up from the 836). You could have your cylinder re sleeved and bore for 836 and reuse your current piston if they are ok and get new rings. You could buy a cheap stock block off ebay or some where and have it bored to your 836. Are your 836 pistons in good shape? No scuffing or scoring? You can always do a stock cylinder and pistons but if your current pistons are usable I would get new rings and either have new sleeves put in your block or buy a cheap block and have it bored.

 

[cyclebuster]

mine is a stock running 74 K model top end, all standard. I rode the bike, the chain blew the case apart so bad the bike would not go into 5th gear. It ran perfect other wise. The piston kits on ebay from Taiwan I am guessing, cost $500 area. no gaskets, and then $40-$60 per hole for the bore job, maybe $50-$100 for a useable cylinder block, Labor on that top end I am sure is going to be $500 or more. and then the cylinder head is apart. There is an oil passage issue I understand, also, from the early cylinders.

 

[dirtdigger]

Cycle X 850 piston set will cost $250 with head gasket only but without bore job. Wiseco piston set will cost $450 with head gasket and without bore job. Wiseco 836 piston ring set would cost $100. Bore job for 850 is $150.

The oil passage difference is a non issue. The late f2 heads did away with two of the oil drain holes in the head but they are interchangable with the earlier blocks you just have to do a very little bit of oil passage massaging to make the drains merge together, just mild dremel tool work. The swap is done all the time to get the f2's larger valves.

 

[jimminy]

Unless I heard 'em wrong, the shop said the pistons are in good shape, but I'll double check that tomorrow. Seems to me getting a decent block and boring it to 836 is the smartest way to go. Then its just rings. Is the K8 unique or will other CB750 blocks work?

Not following the oil passage issue but it sounds like it can be dealt with.

Man, I don't know where I'd be without this forum, thanks.

 

[dirtdigger]

It would be easiest for you if you stuck with the same year stuff you have now. I just looked and there are several 78 cylinders on ebay right now. 77 or 78 cylinders would be your best bet for fitment.

 

[DADDYG]

If i am reading you right you had a broken ring and cylinder was not damaged?? There were some nice kits on ebay and not too high priced. I would take your pistons to a good shop and have them checked to make sure there was no damage done to them. I use to be a HONDA Dealer in the 70's and 80's and extreame heat on the piston can change the ring slots and a new set of rings will go in but after riding and heat up may give you damage again. Have seen it happen more than once trying to save money. Bore could also be off. Your best bet like double D said is to get a new unbored cylinder and if pistons are good just rebore to a new set of rings and your pistons if they are usable. Just double check what you have before you jump. I had a 1976 motor come into the shop seized up solid and it was a rering job done only 8 months ago but nothing was checked he was told all he needed was a new set of rings and rig broke and seized that sucker up. May have been poor install or heated ring slots too much damage to tell so double check..

 

[jimminy]

I got most of my ducks in little rows with this engine rebuild, except the cylinder block. The shop wants a lot for this part and worse for the machine work, so I'm back to finding one and get the conditioning done myself.

There are a few on ebay, but I'm not sure what to look for. Is it a matter of getting accurate bore measurements from the seller making sure it's smaller than the 65mm for the 836 pistons? If the one I buy is stock, then do I need to re-sleeve?

 

[dirtdigger]

No resleeving for the 836. They just bore the stock sleeves for the 836.

 

[jimminy]

Wow, fast! I was coming back to post an item and you were already back to me, thanks.

How about this one -> http://www.ebay.com/itm/1978-CB750-...Parts_Accessories&hash=item35bda86384&vxp=mtr
or
http://www.ebay.com/itm/1978-HONDAM...Parts_Accessories&hash=item43b7d96cb1&vxp=mtr

 

[dirtdigger]

I checked the part numbers between the 750k and the 750a and they had the same parts numbers so either one should work for you. The second one seems like a pretty cheap buy it now price for some reason. Either one should work though. I would have the head gasket surface checked for warp, they do twist a little after a lot of miles and heat cycles.

 

[jimminy]

I guess I do the cheap one if there's no reason not to. Thanks for taking a look.

The warping you mentioned, will resurfacing take care of that? I gueess i will be doing that anyway. Is that done on the cylinder and head surfaces or just the block?

 

[dirtdigger]

Yes the resurfacing will take care of any warp as long as it is minimal. The cylinder heads are also prone to warping. The main thing is to make sure they only take the least amount possible off to get the parts flat. If you take too much off then you will start running into issues like the cam timing will start to become more and more out of time as you machine more material off in which case you can fix that with an adjustable cam sprocket. The type of finish you have done on your cylinder and head surface will be determined by the type of head gasket you are using. If you are using a conventional graphite or OEM style head gasket then nothing really special needs to be done. If you are going to use a newer MLS (multi-layer steel) head gasket then you need to specify that you are using that gasket because the surface finish is many times smoother then the type finish.

 

[jimminy]

OK. I'm going back and forth with the guy selling the cheap one and he's sending me the cylinder measurements. This resurfacing thing makes me nervous to buy something like this online. Is there a measurement I can get from the seller that will tell me if too much has been taken off already?

 

[dirtdigger]

I wouldn't worry about it. Odds are in your favor it hasn't been machined. Un officially honda actually changed the height of the cylinder along the way to reduce the compression to help meet later emission standards. The only thing he could tell you is if the surface is warped, in which case he would need an accurate straight edge and feeler guages. For the price I dont think you can go wrong and should be ok with it. In any case it is easy to get the cam timing set with an adjustable sprocket. Stock cylinder bore is 61mm with 4 honda oversizes going up to 62mm. 836 is a 65mm bore.

 

[cyclebuster]

digger is right, these bikes run so long and so well I would bet 90 percent of the engines are still on standard bore assembled by Mother Honda herself. the odds of a used cylinder block being machined is mega slim

 

[jimminy]

Ok! That's good news, then as long as the cylinders check out its a go. Thanks for looking.

 

[jimminy]

I'm back to it! I wasn't really off of it, but it took such a long time to gather the parts and get a new cylinder block bought and re-bored I almost forgot where I left off.

So now I have everything to get the top end squared away, new 836cc piston set with rings and head gasket from Cycle-X, and a rebuild kit for the top. There's a lot of parts in that kit and I guess I'm going to have to figure out where they all go. At the moment I'm just getting things cleaned up.

Someone at the shop mentioned using brake cleaner to get the residual gasket material off. Don't know why I wouldn't use gasket remover, but I'll give it a try since I already have the brake cleaner. Is there anything I should use to scrape with or anything I should avoid other than a hammer and chisel? The material left on there now is very thin, I can barely feel it with my finger. Is there any amount I can leave on there, or does it need to be bare metal?

Thanks all, glad to be back!