Briggs & Stratton Opposed Twin Knocking?

[TylerFrankel1]

Hey everyone! Got questions about my old opposed twin Briggs again. Long story short, I work at a shop, last year someone dropped off a half-disassembled opposed for scrap. I took it home, tore it all the way apart, cleaned up insides, honed, re-ringed, re-sealed, checked valve clearances (were miraculously all good), inspected rod/crank by eye and saw no wear scars, reassembled. Didn't plasti guage crank unforunately. But test fitting rod to crank was tight, didn't feel slop. Anyway, I finished my minor rebuild and installed it on my Troy Tractor. Got it running pretty nice. Did a break in oil change after 5 hours. Currently running super tech SAE30 oil. Probably cut ~20 yards with it for like 1 hour each time average since the build. So I'd guess 20-25 hours running. After sorting out some troubles with the ancient carburetor it's been doing just fine.

However, at work the other day, a mower with an opposed twin came in (they're getting more rare as they age). I fired it up to move it and noticed it was a lot quieter than mine. Wrote it off as a fluke. Another one happened to come in. It also is a lot quieter. Not exhaust, but the engine noise itself. I brought mine in to work, had the boss listen. He thinks bottom end/rod knock. Not what I wanted to hear. But it's always sounded this way since I built it, I didn't think anything of it because I didn't have anything to compare to at the time. Just assumed they were loud engines. It hasn't gotten worse or anything in the 25 or so hours its been running. I got curious so I searched around youtube for videos of these engines running. Most of the ones I heard running on youtube had the same characteristic knock-like noise. The two at work almost seem like outliers being so quiet. Is there a known phenomenon with these "twin-II" engines where they end up sounding knocky/clacky? I read somewhere something about timing gear key getting worn could be a cause but I didn't explicitly notice that when I was building the engine. I have videos of my engine running, the two engines from my work running, and various clips from around youtube for reference below. Just looking for anyone with thoughts, advice, opinions, etc. Thanks!

My Opposed Twin:

Opposed Twin from Work #1:

Opposed Twin from Work #2:

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Opposed Twin from Youtube #1 (Sounds Knocky):

Opposed Twin from Youtube #2 (This Guy Has The Same Question):

Opposed Twin from Youtube #3: (This Definitely Sounds Like Rod Knock, Has Different "Snappier" Sound, Makes Me Think My Sound is Something Else):

Opposed Twin from Youtube #4: (Again, Definitely Rod Knock, And Sounds Different from My "Knock"):

Opposed Twin from Youtube #5: (Sounds Knocky):

Opposted Twin from Youtube #6: (Sounds Knocky):

Opposed Twin from Youtube #7: (Sounds Knocky):

Opposed Twin from Youtube #8: (Sounds Especially Knocky):

Opposed Twin from Youtube #9: (Sounds Knocky):

Opposed Twin from Youtube #10: (Sounds Knocky):

Opposed Twin from Youtube #10: (Less Knocky):

(Yes I know I'm crazy for finding and posting this many videos. Trying to provide evidence for my claim, that most sound like mine from youtube, LOL).

 

[Scrubcadet10]

is it pressure lubed or splash lubed?
IIRC, the governed idle rpm spec is 1,750 RPMs... if yours is the splash lube version and you don't have the proper RPM set , the top rod journal and crankshaft bushing/bearing journal has lubrication issues if idled too low and will chirp, and eventually start to wear.

 

[TylerFrankel1]

is it pressure lubed or splash lubed?
IIRC, the governed idle rpm spec is 1,750 RPMs... if yours is the splash lube version and you don't have the proper RPM set , the top rod journal and crankshaft bushing/bearing journal has lubrication issues if idled too low and will chirp, and eventually start to wear.

It’s splash. I thought it could possibly be upper bushing actually, the noise seems to resonate from the top. I should probably up the idle, oh, 1000 rpms. The Harley loping idle when you set it low is intoxicating though lol. It still has this sound at a medium rpm like 1750, though less noticeable because the engine is spinning faster.
If you listened to the video of mine running (I don’t anticipate you watched the other like 12 lol) did it sound normal to you for an opposed twin?

 

[TylerFrankel1]

This is very unscientific but I tried rocking the flywheel back and forth by hand (by grabbing the debris screen on top) and it doesn’t budge from what I can tell. So upper bushing doesn’t seem worn bad but who knows it could be

 

[Scrubcadet10]

sounded normal to me, other than low on the RPM's and the knock.
have you also check the engine mounting screws, and anything else that might could cause a repetitive knock?

 

[TylerFrankel1]

sounded normal to me, other than low on the RPM's and the knock.
have you also check the engine mounting screws, and anything else that might could cause a repetitive knock?

Yeah i checked it’s not loose. I ran it without any of the cooling tins (briefly of course so as not to overheat) when I first built it and it sounded the same. So I don’t think any exterior parts are doing it.
In your opinion, doesn’t that sound a bit different than rod knock? It’s definitely a knock, but engines I’ve heard with rod knock almost sound more “sharp” and “severe” like this

This is a mystery to me. I don’t want to tear the engine down to solve it but curiosity might kill me ?.

 

[TylerFrankel1]

Yesterday I pulled one of the opposed twins at work in the shop to do an estimate. It's only running on 1 cylinder, blades won't engage, etc. But while I was doing that I noticed it says "quiet package" on the blower cover. I checked the other twin and it also had "quiet package." Now, I don't know if that "quiet package" really makes a difference or if it was marketing noise, but it's a potential variable in my search, lol!

 

[bertsmobile1]

Usually it is a different grind on the cam lobes
From a mechanical point of view the "perfect cam has a saw tooth profile with a rapid rise the nothing stoping the valve slambing closed very quickly . Lots of high performance cams look very much like this.
At the opposite end you get the pimple shaped cams with a slow rise and a gradual close.
These are generally called quietening ramps and were popular on a lot of commuter motorcycles .
The down side of them is lots of heavy contact between the cam & the cam follower thus lots of wear .

 

[slomo]

Sounds like lack of lube from low revs. Not recommended to idle down a splash lube engine. Keep her at 3200-3600 revs at all times, if possible.

slomo

 

[S854]

... anything else that might could cause a repetitive knock?

This is a long shot but... Julie's old Craftsman began "knocking" after one of the drive belts snapped and wrapped itself around an engine pulley... I replaced the belt and fired up the tractor which had developed a loud knock...

Turns out it was the "locating" block on the electric clutch that had been moved ever so slightly by the impact of the broken belt...

I loosened the two mounting bolts and adjust the block to where the clutch wouldn't chatter against it... no more knock