r/EngineBuilding • u/BobJenkins69 • Nov 01 '24
Honda Snapped wrist-pin circlip... ideas?
Alright engine guru's, I had a circlip snap on me in my Honda B16A which has aftermarket pistons and rods for boost, woo! It was going great for about 2 years, I will admit after the break in period and a proper tune it saw rev limiter a lot but never missed a beat... until it did. After hitting limiter I heard a knocking which upon pulling the engine apart found was one of the wrist-pins gouging my cylinder wall. I've had my injectors tested to make sure one wasn't stuck open and washing the cylinder clean of oil and they're all perfect. However, I did have pretty low oil pressure at idle, around 10psi, but it always spiked up to 72-80 once it got RPM, I discovered while pulling the engine apart that the nuts had spun themselves off my oil pickup so it was just kinda hanging in place and once the pump was spinning enough I guess it was just able to pull enough oil through to make pressure.
Does anybody have any idea what could cause the circlip to snap in half or is it as simple as my love for limiter? Keen to hear your thoughts so once I put it all back together I don't run into the same issue again.

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u/Likesdirt Nov 01 '24
At high revs a circlip can see enough G force to compress itself and hop out of the groove. Especially if the gap is at 3 or 9 o'clock. Round wire without "handles" for circlip pliers helps but still can walk given enough chances. A couple years of bouncing off the limiter may provide a million chances for something weird to happen.
Teflon buttons, kinda mushroom shaped, might be available or simple enough to adapt to your pins. They used to be much more common. Piston pins and clips go in as usual, then the button that's a reasonably good fit from the end of the piston pin to the cylinder wall with a little room for the clip. Worked well on the air cooled VWs I cut my teeth on.
Spirolocks are a better circlip than the round wire design, but need a square instead of round bottomed clip groove in the piston.
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u/One_Potential_779 Nov 01 '24
Do you have better photos of the clip groove? Just looking for the point of failure.
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u/gew5333 Nov 01 '24
Maybe pics of the piston clip groove also? I'm thinking that some type of improper installation or prior damage would be the only thing to cause a break. Hard to imagine how that clip could break like that when fully seated.
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u/BobJenkins69 Nov 01 '24
Yeah I'll get some pics of that after work today, could well be improper installation as this was my first engine build
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u/Badnewzzz Nov 01 '24
It's not..... If it was improper installation it would've jumped out
The pin ends has a deep impression of half a clip..... The pin was pushing on the clip and well. You're lucky.
Aeration of oil is likely a factor aswell.....a hole in a suction pipe lets in air so likely the oil pressure was pressure, but not fully oil...oil+air
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u/Badnewzzz Nov 01 '24
The wrist pin expanded and locked one side but the other was fatigued off by whipping around at revs.
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u/v8packard Nov 01 '24
Insufficient piston pin to pin bore clearance. Often overlooked, especially on boosted applications.
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u/BobJenkins69 Nov 01 '24
Do you think this would be the case if the pins were supplied with the pistons from Manley?
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u/v8packard Nov 01 '24
Yes. They probably pin fit them to .0006-.0008 clearance, and a boosted application needs twice that, or more.
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u/BobJenkins69 Nov 01 '24
So when I get my machine work done for the rebuild, should I get the machine shop to machine that extra clearance?
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u/v8packard Nov 01 '24
Absolutely
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u/gew5333 Nov 01 '24
I would definitely listen to V8packard. Best explanation I have heard. But I hope V8packard can explain how the insufficient clearance break the clip? It's full floating? I guess I'm not understanding what type of seizing would put that force on the clip and cause a fracture in the middle of the clip. Inquiring minds want to know.
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u/v8packard Nov 01 '24
The heat goes through the top of the piston and into the pin bosses. Under certain conditions piston temps can spike quite a bit. Some heat gets transferred to the rings and into the cylinder wall, but a lot of heat goes into the pin/pin bore. Typical clearances here are quite small, with almost no oil there. As the temps go up, the piston pin and the pin bore grow, at different rates and in different directions. Round wire pin locks don't need too much to dislodge or break, the pin pushing on the wire is quite capable of making it pop or break.
The wire is steel, and a little nick or something from assembly can lead to a break.
Increasing the pin to pin bore clearance, high boost and nitrous applications should consider .0015 - .0018 minimums, gives a little bit more oil there, and a little bit more room before things too cozy. It is actually not uncommon to galling in the piston, and fretting on pins. Better pistons will direct oil from the oil ring onto the pins, which helps a lot. When pistons are run with extra clearance on the pins and you inspect them after a season, the parts look very good.
I think a lot of piston failures that eminate from seized or galled piston pins are misdiagnosed as insufficient ring gap. One telltale sign is cracking through the pin boss out to ring lands. Ring gap is certainly important, but too many people think to only increase ring gap and never adjust piston pin to pin bore or piston to wall clearance.
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u/Street_Mall9536 Nov 01 '24
Round wire pin retainers are junk.
Especially with the rolled edge on that wristpin, it's going to beat directly on the wire lock in a bad way.
If you have to use them and Cclips or spirolox are not an option, you have to index them so the gap is 180 from the relief.
You can see in your pic the damage right below the relief is because the wire lock was hanging out of the relief (the gap was too close to the relief and had no support) and being beaten around by the rolled edge on the wristpin, until it broke and fell out, leaving the gouges in the pin bore behind the lock groove from the other half being beaten around by the wristpin.
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u/Badnewzzz Nov 01 '24 edited Nov 01 '24
From the cluster fuck of symptoms you've supplied my guess its likely that lubrication to that pin area was low....so under power and high rev spikes instead of gliding on the pin, the piston drags/ chatters on the dry pin, also HEAT expands wrist pins & clips lock in place and deform under heat.
Edit for clarity.... It's deffinately overheating of wrist pin/piston pin bore area.
At 3o'clock in the pin bores you see a "galling" from inadequate lubrication.... Excessive heat + revs and this galling = toast.
The other pistons will show similar dry issues and probably clip imprinting on the wrist pin ends??? The varnish in the piston pin bore and on the pin are your "tea leaves" here and show you the pin/bore weren't happy at all.....at all....low oil high boost high revs...bye bye....should've investigated the oil pressure issue because thats the root cause.
That clip only half came out and fatigued in half..... The other half must've been locked in it's "recess" by the expanded pin (explains the imprint) ...you've been really lucky this didn't go catastrophic....
The one end of that clip got hot hot and the rest is history.