The Cessna 120-140 Association Forum

I found a broken cylinder mount stud on my preflight this morning. It's the #2 cylinder, forward top location which looks to be one of the two through-bolts for the center crankshaft bearing. Being Sunday, I won't be able to talk to my A&P until tomorrow, but had a couple questions to throw out there.

- Any guesses on why the bolt failed? Per the logs, the cylinder was last removed 7 years and 500 hours ago. Maybe I'm just an unfortunate victim of metal fatigue.
- Looks like cylinder #2 has to come off to get the bolt out. Any detrimental effect to cylinder #3 by loosening those 2 through bolts?
- Since this bolt holds the center crank bearing together, what are the chances of having resultant damage there? I'm pretty confident the engine ran for maybe 1 - 2 hours with this bolt failed. I normally check all visible cylinder bolts for security and leaks on the first preflight of the day and didn't notice it last flight.
- Given the criticality of torques for reinstallation, I'm assuming I should get all new replacement studs and nuts for cylinder #2 in addition to the two through-bolts?
- I see that Texas Air Salvage is offering 2 used C85 center through bolts for sale. Aren't these bolts supposed to be single-use only? I assumed that I'd need new.

Long story short, it was a really disappointing trip to the airport today. But it's a great reminder of how important a good preflight is!

Unless you have a new case (relative to the plane) there are no through bolts. There are studs that originate in the left side of the case, go through the center bearing and anchor the aft edge of Cylinder #3. On cylinder #2 there are short studs that anchor in the left side of the case.

What is the nature of the failure? Do you have a nut with a short threaded bit in it that fell out? Or, is it a short stud that pulled out of the case? I think it should be a 7/16 x 1-23/32 stud.

Unless you have a new case (relative to the plane) there are no through bolts.

Interesting. I was referencing the Continental illustrated parts catalog on the references page here and it shows through-bolts in Figure 2 (Item 15, P/N 627275, Bolt, Through, 7/16 x 10-39/64). It claims those are valid for all C-series engines, but it also doesn't give serial number ranges, so maybe it's only valid for later engines?
My engine isn't original to the airframe (was installed in 1997) and has the STC O200 crank, rods, and pistons. Serial number is 27566-7-12, if you know where I might find the serial number change-over point?
As to the failure, I found the nut with about a half inch of threaded section behind it wedged partially out of the hole in the cylinder base. I don't see debris in the threads and it looks like a pretty clean flat failure surface. If it is a stud, it doesn't look like it pulled material out of the crankcase. I've got pictures, but have to read up on how to post them.

As far as I know, all C85 engines used in the 120/140 came from the factory with studs, not bolts. The through bolt case seems to have been introduced about the same time as the O200. The area where the studs screw into the left half of the case is a weak point and the added stress of the larger displacement, increased compression ratio and higher RPM may have been the impetus for the change. (Photo attached). The studs are a carryover from the A-series cases.

If you look the parts catalog 1977 edition page 2-5 at the notes at the bottom you'll see the part numbers for the studs on the "obsolete case".

If you have a through bolt, the opposite end will be loose. Put a wrench on it and give a little clockwise torque and see if it turns.

If you have through bolts, they can be reused (as can the studs).

How to post photos.

For an 0-200A perspective-

https://www.fresnoairpartsco.com/produc ... -3-4-long/

My #2 cyliinder I have marked for the thru bolt, 0-200A, part number 627275, bolt, thru, 7/16 x 10-39/64 inch long. I saw your post on the Facebook site. You'll need to pull the cylinder to replace, but I'd be more interested in any damage you might (hopefully not) find inside. Good catch on the pre-flight.
Here's an internal shot at the left half case of my 0-200A showing where the 627275 thru-bolts go and the bumps in the case for the cylinder base studs. You can see what I'll call 'thru studs' on the left for attaching the cases together. Similar 'thru studs' from the right case up by the thrust bearing.
What John was referencing on the older cases, this example from an A65 Easy check, go to your #3 cylinder and see if the nut at the 10:30 position turns easily. If so you have the 627275 bolt, thru that failed.

John and David, thank you so much for the responses. Pictures and knowledge like that are what make this association and forum so incredibly valuable.
Unfortunately the A&P I've been working with is in Alaska this week, so won't get to start pulling things apart for a little while. I'll go see if that corresponding #3 cylinder nut is loose, obviously being real careful about it. Assuming the broken one is a stud, it helps put my mind at ease about the center crank bearing even if it's going to suck trying to extract the broken end from the casting. I'll post back here with what we find.

If you have studs instead of through bolts then the stud on that side is only long enough to reach through the wall of the case under the cylinder flange. Once the cylinder is off it should be pretty easy to replace. The only concern is why it broke in the first place, and whether there is any other collateral damage.

Did you measure the length of the piece that came out? Could it be that the stud just backed out of the case?

I didn't measure it, but it looks far too short to not be broken. Again assuming it is a stud, would that side have just used the same 7/16 x 1-1/2 inch stud used in the other 7/16 locations on that cylinder? Figure 1, Item #3, P/N 402040? If so, the loose part is definitely shorter than 1.5 inches. Pictures I've found of 402040 look like there's a different thread pitch in the casting from what protrudes through the cylinder base. Might have failed at that transition between threads.

I think it's 639418-4. It should be the same as the two next to it on #4. The threads that go in the case are coarse and the ones the nut goes on are fine thread.

Get someone who knows what they're doing to help. the threads in the case are interference fit. I don't think there is an oversize stud (if I have the part number right), so you don't want to mess up the threads in the case.

[Edit] My parts catalog doesn't have a 402040

I had a cylinder stud break off on a C-85 engine a few weeks ago. It broke off at the case so the only way to remove it was to purchase a 1/4-20 left hand tap and drill and tap the stud. I was careful to make sure that the hole drilled was in the center of the stud. I also purchased a left 1/4-20 bolt to use. I did not have to use the bolt since the stud came out during the taping process. I did not use an ez out tap since they seem not to work for me, Maybe this will help with your removal.
Earl Evans