Topic: '78 CB750k conversion to cafe racer (yes, more issues)

[deafcaferacer]

I think they are sold on lulu.com

Yes, I saw that but I also saw that he sells them directly with a portion going to this website, and that was what I wanted to do, because it helps this forum as well as going into his pocket directly.

[deafcaferacer]

Never undervalue the importance of doing T-CLOCS  every time you ride.   

[deafcaferacer]

Well, now there's a oil leak coming out of the final shaft assembly area.. have attached pics (look right below the gear change pedal). Oil will only leak when engine is running or is hot. There is excess oil that's getting on the chain and spraying the whole left side of the tire. It is not a bad leak.. just a slow, steady drip-drip.

What should I look at first?

[Jayhawk]

Hi;
Hopefully it's just the cover gasket.  Other possibilities are the output shaft seal or the engine case is cracked in the front sprocket area; typically caused by a broken chain incident.   

[deafcaferacer]

Hi;
Hopefully it's just the cover gasket.  Other possibilities are the output shaft seal or the engine case is cracked in the front sprocket area; typically caused by a broken chain incident.

Thanks.. I think we can rule out the last scenario as I haven't seen that happen although the chain was new when I bought the bike, in which case it would have been leaking right off the bat. It only started leaking yesterday.

I am seriously thinking of just leaving it the way it is until the fall (when I'm going to completely break down the engine), because at this point it is only a slow drip-drip.

[Elan]

i'd also just check the auto chain oiler. I like to tighten mine in all the way and oil my own chain. It could be just spilling oil out.

[deafcaferacer]

i'd also just check the auto chain oiler. I like to tighten mine in all the way and oil my own chain. It could be just spilling oil out.

Question: wasn't the auto chain oiler removed in '73 models and afterwards? I read that in the SOHC archives.

[Elan]

i'd also just check the auto chain oiler. I like to tighten mine in all the way and oil my own chain. It could be just spilling oil out.

Question: wasn't the auto chain oiler removed in '73 models and afterwards? I read that in the SOHC archives.

Oh I nvere heard that. You could compare the parts fitche...

[Jayhawk]

A '78 would not have an auto chain oiler.  Judging from the photos, my bet would be the cover gasket, which is a cheap and easy fix.

The most common broken chain induced cracked case repairs I've seen involve RTV silicone or JB Weld, both of which can last a long time with proper surface prep, or not very long at all if someone just smeared a big glob of RTV over the wound without preparing the surface.

[deafcaferacer]

Thanks Elan and Jayhawk..  Is this what I'm looking for:

http://4into1.com/ne-brand-transmission-cover-gasket-honda-cb750/

[Stev-o]

Yes, that's it but says out of stock

Here...

http://www.ebay.com/itm/HONDA-1969-1978-CB-750-K-TRANSMISSION-SIDE-COVER-GASKET-HM-506-/360618587015?pt=Motorcycles_Parts_Accessories&hash=item53f68af387&vxp=mtr

[deafcaferacer]

Well, F*** #(!@#%*#$. Worse case scenario has turned out to be true: cracked engine case. 

I drained oil, removed transmission shift cover to examine gasket.. nothing wrong with it, so I had a bad feeling.. removed the final drive shaft cover, and immediately saw that the PO had thrown the chain, so jayhawk's initial diagnosis was right on the nose. There two deep gouges from where the chain hit, and you can see one of them just to the left of the chain (looks like a white streak), and you can see the crack right below the electrical tape. It was easy to trace the oil smear back to the crack.

[Stev-o]

Sorry to hear. Many have made a repair by welding or JB Weld

[deafcaferacer]

Even worse than I thought.. can a crack this deep be repaired with JB weld or will it have to be welded? I assume engine will need to come off frame for this? Will I also need to split the case to repair from inside as well?

[deafcaferacer]

Spoke to someone who thinks JB weld should be ok for short term and suggested I bore a small groove down the  crack so the bond has more grab surface.

And I am going to run with a #530 chain from now on. 

[Jayhawk]

Bummer.  You can make a decent repair with epoxy, without removing the engine.  Do a good job cleaning the area, and use a powerful solvent (automotive brake cleaner works well) to degrease the crack and vicinity.  Use a small stainless wire brush or a scratch pad to rough up the case just enough to create "tooth" for an epoxy to properly adhere, then clean it again.  JB Weld should work, but there are also better epoxies available.  The most important things are to rough up the bond area and remove all oil residue.

Back when I was fiddling around with high performance Jet Ski 2-strokes, I had good results with Devcon brand metal-filled epoxies for porting and repairing cases:

http://www.devcon.com/products/products.cfm?brand=Devcon&cat=Metal%20Repair

Devcon is a lot more expensive than JB Weld, but this is one of those situations where you get what you pay for:

http://www.mcmaster.com/#devcon-adhesives/=mxcr2t

I suggest trying JB Weld first, and if it doesn't last, step up to Devcon.

[deafcaferacer]

Bummer.  You can make a decent repair with epoxy, without removing the engine.  Do a good job cleaning the area, and use a powerful solvent (automotive brake cleaner works well) to degrease the crack and vicinity.  Use a small stainless wire brush or a scratch pad to rough up the case just enough to create "tooth" for an epoxy to properly adhere, then clean it again.  JB Weld should work, but there are also better epoxies available.  The most important things are to rough up the bond area and remove all oil residue.

Back when I was fiddling around with high performance Jet Ski 2-strokes, I had good results with Devcon brand metal-filled epoxies for porting and repairing cases:

http://www.devcon.com/products/products.cfm?brand=Devcon&cat=Metal%20Repair

Devcon is a lot more expensive than JB Weld, but this is one of those situations where you get what you pay for:

http://www.mcmaster.com/#devcon-adhesives/=mxcr2t

I suggest trying JB Weld first, and if it doesn't last, step up to Devcon.

Thanks for the detailed SBS. Going to get all the stuff tomorrow when the store opens.

[deafcaferacer]

Been reading up in the archives on conversions to 530 chains. A lot of those threads just stop abruptly. 

Going to go with 17/48, and a 110 link 530 to be safe.

Looking at Sunstar 50717 17T front sprocket which is compatible with '78 CB570k. Thoughts?

Will get corresponding 48 rear.

[Jayhawk]

From: http://forums.sohc4.net/index.php?topic=74547.msg1383741#msg1383741

Thanks.. sorry, I don't mean to hijack your thread.
Last bunch of questions I'll ask- I checked out that front 18t 530 sprocket on amazon and they say it is not compatible with 1978 cb750k which is what I have?

I assume that since the "correct" OEM chain is a 630,  Amazon's parts fitment database is unable to cross reference the sprocket with a '78 CB750K.

I am also going to go for 17t because I prefer low-end acceleration increase over top end speed.

For reasons I have previously mentioned, I suggest using JT sprockets:

http://www.amazon.com/JT-Sprockets-JTF288-17-Steel-Sprocket/dp/B001AVTZPO/

I took another look at this photo.

I think you might have an earlier engine; that is not 77-78 750K front sprocket attaching hardware.  What is your engine number?
That also doesn't look like a 630 chain; you might want to measure the pitch, a 630 will be 3/4" between link pins, while a 530 is 5/8".

It's my understanding that 1976 was the last year of the 750 front sprocket with the two bolt flange:

http://www.cmsnl.com/honda-cb750k-750-four-k-1976-usa_model494/partslist/E++16.html#results

The '77-78 CB750K front sprockets are retained by a single center bolt:

http://www.cmsnl.com/honda-cb750k8-four-usa_model14397/partslist/E16.html#results

[deafcaferacer]

Thanks for clearing this up. A lot of things are starting to make sense. So I have a '76 engine on a '78 frame, and the PO is wrong?

[Jayhawk]

Thanks for clearing this up. A lot of things are starting to make sense. So I have a '76 engine on a '78 frame, and the PO is wrong?

I think it might be older than that; what is the engine serial number?
Look at the sprocket, gaze a few inches to your left and you should see the S/N on the case.

[Jayhawk]

Once you find the engine number, go here:

http://www.sohc4.net/cb750k-serial-numbers/

If you have a '78 engine, it would be in the 3000001 – 3062980 range.

[deafcaferacer]

Thanks for clearing this up. A lot of things are starting to make sense. So I have a '76 engine on a '78 frame, and the PO is wrong?

I think it might be older than that; what is the engine serial number?
Look at the sprocket, gaze a few inches to your left and you should see the S/N on the case.

2244944 which makes it a '73 according to that chart.

[Retro Rocket]

If you are still using the 78 rear sprocket carrier then your chain line is also 10mm out , the K8's had a 10mm wider sprocket carrier on the rear and 10mm wider out put shaft from the engine, if you have a 73 engine and a K8 rear end then you have a problem with alignment........

[deafcaferacer]

Surface prep and application of JB Weld. I think I put on too much as you can see some of it glob up at the bottom and start to roll down. Maybe in another 4-6 hours, I could put on another coat of JB?