Topic: Cracked Case Half Main Oil Gallery Seal

[LiveeviL81]

Hey All,
Opened up my engine cases after 12 minutes of engine running due to an issue with the 1st gear not working on my Hondamatic. While the cases were split I noticed the main oil gallery feed between the case halves has split the cast aluminum. I was not able to find the piece. The oil pressure on the main gallery maintained 60+psi at cold idle for the 12 minutes it was running, but I worry about it in the future. Anyone have ideas that don't require full engine disassembly?

Pics attached.

[calj737]

It’s quite viable to have that area repaired by a welder. They will simply build up the chipped area, then grind it flat and even with the mating surface. Aside from cleaning up the gasket material, you should be able to have them weld it with the remainder of the motor in tact.

Others may recommend using JB Weld to build it up. If you do go that route, press the JB into the creases little by little with a cold punch or similar small tipped tool to ensure you get it fully adhered to any irregular surface.

Pretty simple repair actually either way you go.

[LiveeviL81]

I agree that a welder could fix up the area, but my main concern is when he has to grind it, It would be very difficult to protect all of the other assembled engine components from the grinding dust.

I do like your idea of JB weld, as my worry was also adherence to the cast aluminum.

[willbird]

I agree that a welder could fix up the area, but my main concern is when he has to grind it, It would be very difficult to protect all of the other assembled engine components from the grinding dust.

I do like your idea of JB weld, as my worry was also adherence to the cast aluminum.

I'm not sure if any responsible welder would be thrilled about welding that on an assembled engine.

Bill

[scottly]

What kind of material is the O-ring, which appears to be too large, and possibly responsible for the damage in the first place?

[LiveeviL81]

What kind of material is the O-ring, which appears to be too large, and possibly responsible for the damage in the first place?

I believe this is a Viton o-ring, I measured the ID to be 18mm and the Ring Width to be 3mm, which matches the part number for this bike on Partzilla. (91311-044-000) (#13) https://www.partzilla.com/catalog/honda/motorcycle/1978/cb750a-a-750-hondamatic/crankcase, though it does look like it swelled to me as well. Since I wasn't able to find the piece in the oil pan or oil filter, I believe it broke when I was torquing the case halves together. So I would be surprised if swelling caused the break because it didn't see oil until after the case halves were together.

I believe I got an oring kit from CycleX or Vintage CB750, Each bag was labeled, and I can't remember the exact wording used, I am pretty sure this is the correct one.

[scottly]

Did it fit snugly over the steel spigot when originally installed? Did it return a round shape after it was removed from the case? Please post a pic of the damage on the case with the seal removed. It looks like maybe only a very thin wall?

[LiveeviL81]

I honestly do not remember if it was snug, I do know it didn't have to be stretched over. After removing it from the case it has not yet become fully round, but is not as bent as it was.

My current thoughts are to try to build the area up with a bit of weld, and then RTV the steel spigot in to make it the primary sealing piece, replace the area with a new oring, and hope for the best.

[HondaMan]

My 2 cents: Disassemble everything in the case and have it welded up and reshaped. The Oring deformed from the 60+ PSI oil pressure pushing on it. That will let much oil escape, reducing the volume to the smaller passages that feed oil to everything.

[scottly]

Are you absolutely sure that is under high pressure? Usually high pressure passages are much smaller.

[M 750K6]

I wouldn't leave the shrapnel in there. Could become very expensive.

[madmtnmotors]

NO RTV! RTV silicone tends to find its way to the oil restrictor jets in the head ultimately starving the cam bearings. Not pretty.

[rotortiller]

Maybe a short piece of thin walled metal tubing and adjust the o-ring size as needed

[LiveeviL81]

Maybe a short piece of thin walled metal tubing and adjust the o-ring size as needed

Are you saying a short piece of thin wall that is narrower than the ID of the casting lip that broke off, and then a smaller o-ring to match?

[scottly]

 I believe it broke when I was torquing the case halves together.

I do too. The oversized O ring was squeezed into the recess until it broke out the thin piece in the case. There is no way that 60 PSI would have done that. I'm not even convinced the broken piece has any affect on the sealing. I would fit the proper O ring and bolt it back up.

[rotortiller]

Yes, a steel thin ring as close to the ID as you can find with an O-ring that gives the correct squish for sealing.  If the ring is ruffly 50% of the o-ring thickness and does not interfere with case closure it should stay put, seal, and contain the rubber in place under oil pressure. I would look at thin-walled SS tubing cut to correct height with a dremel or other method. JB weld may work and I have seen it used to plug chain damage on crankcases, welding and machining is ideal but likely not the best choice for some. Many ways to skin the kat, you do want to skin it once only though.