Call that Bore Scoring?

[Robin Walker]

Call that Bore Scoring? This is Bore Scoring on a 356B S90 with NPR pistons and a broken Gudgeon Pin at 70 mph.
I am sceptical of using anything not original in my 356C, I am looking for reliability rather than outright power.

SNV33634.JPG (966.53 KiB) Viewed 1256 times

SNV33640.JPG (939.8 KiB) Viewed 1256 times

[Chuck Allard]

It looks like that engine was assembled without one of the wrist pin circlips. It ran for a very long time to create the groove in the cylinder wall before the pin broke.

[Don Gale]

What does the piston, head, and rod look like? I'm assuming you'll open the case.

[Ron LaDow]

It's amazing your wrench didn't mention a dead cylinder during a tune-up; no way you could tune a carb to that cylinder and the plug had to be dripping oil

[Mike Horton]

It looks like that engine was assembled without one of the wrist pin circlips. It ran for a very long time to create the groove in the cylinder wall before the pin broke.

+1

[John Brooks]

I would think that would have made a different and distinct noise. I had a Circlip brake a couple years ago, but the smoke and oil consumption gave it away.

I don't think that will polish out.

[Mike Horton]

It's amazing your wrench didn't mention a dead cylinder during a tune-up; no way you could tune a carb to that cylinder and the plug had to be dripping oil

+2 !

[Ron LaDow]

I don't think that will polish out.

Might at 90+mm bore...

[Don Gale]

Could the small end of the rod, if bent sideways and is not parallel to the crank, induce enough thrust load on the wrist pin to push the circlip out of the groove and continue to drive the wrist pin into the cylinder wall?

[Martin Benade]

My guess is that the pin broke first (unless the circlip was omitted) but stayed in place for the engine to run for a while, maybe even without any strange noises. I wouldn't say it was noisless but I once drove several miles back home with a broken crankshaft.

[Harlan Halsey]

In the 1960s I lent a VW with a freshly rebuilt 40 horse to a friend to drive from Boston to Maine and back. I must have left one of the wristpin circlips out. The car began to smoke on the return, and was using a quart/50 miles at the end. I think my friend just went to the nearest gas station and put a couple of quarts in when the red light came on. One pf the cylinder walls looked just like the picture above. Only the cylinder was damaged. I replaced the piston and cylinder, and all was well from then on. An embarasing mistake. But if you re wondering how long it takes to do that, it took about 300 miles to go deeper than 1/8".

[Ron LaDow]

Could the small end of the rod, if bent sideways and is not parallel to the crank, induce enough thrust load on the wrist pin to push the circlip out of the groove and continue to drive the wrist pin into the cylinder wall?

616 rods are 'bottom-guided' and the top end is centered in the piston, so there should be no asymmetric load to bend the rod. Regardless of theory, I also had an NPR PC set kick a circlip out and score a cylinder in a similar manner many years ago.
I cannot tell you the cause, but since then I am careful to examine the circlips on installation and make sure the 'sharp' surface (take a look; you'll see it) is on the 'outside' engaging the groove cut in the piston.
Similarly, the results are not proven causative, but I've never had a similar failure since then.

[Mike Horton]

Could the small end of the rod, if bent sideways and is not parallel to the crank, induce enough thrust load on the wrist pin to push the circlip out of the groove and continue to drive the wrist pin into the cylinder wall?

616 rods are 'bottom-guided' and the top end is centered in the piston, so there should be no asymmetric load to bend the rod. Regardless of theory, I also had an NPR PC set kick a circlip out and score a cylinder in a similar manner many years ago.
I cannot tell you the cause, but since then I am careful to examine the circlips on installation and make sure the 'sharp' surface (take a look; you'll see it) is on the 'outside' engaging the groove cut in the piston.
Similarly, the results are not proven causative, but I've never had a similar failure since then.

+1,

The sheared sharp edge, of which Ron wrote, is called out, and the installation index he described, is included in one of the aircraft engine publications I sourced in my FAA aircraft engine work over the years. It is important...!

Thanks, Ron,

[Martin Benade]

So it’s not just a good idea, it’s standard aircraft practice?

[Mike Horton]

So it’s not just a good idea, it’s standard aircraft practice?

Martin, it was in my shop, and I never had a failure using that process. I also still have the rod alignment tool with which I checked the rods for trueness. Older machind shops may have one. I never found a bent rod in those engines, but by comparison, they were massive. I did, however find a couple which were flawed, in the magnaflux process, one of an NOS set for a Ford 427 engine, which an old Friend was going to use in a stroked 428, out to 454, in a '67 Mercury at Bonneville. He later blew it up, in a jet boat...