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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).

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.

Scrubcadet10 said:

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.

Click to expand...

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?

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

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?

Scrubcadet10 said:

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?

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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 ?.

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!

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 .

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

Scrubcadet10 said:

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

Click to expand...

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

it's normal to hear the knocking noise at too low an rpm.. I have a 1972 opposed twin B&S and a 1973 Kohler opposed twin, if I lower the rpms below 1500 it banks like a bi...h.. keeping up over 1500 and it runs nie and smooth and proper like.. Running so low in rpms will mess with the lubrication, while sounding cool sure everyone likes an idling harley, it will dmge the engine nd is not worth the effect..

run up the rpms to the recomended spec. you'll be fine..

Wondering if it was one of those non OEM mufflers making the ticking sound? They both are different. Or an exhaust leak from the home made plumbing parts for the exhaust??

Take a small hose, one end to your ear and the other end on the suspect part. Listen for a tick. The redneck doctor trick.

slomo

A "chuckle" is normal for these engines indicating that someone liked the sound of the engine idling too slowly as suspected above causing some wear in top main due to lack of lube. Increase idle speed to closer to recommended specks and don't worry about it.



Walt Conner

slomo said:

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

Click to expand...

1750 rs is its factory idle. Torque curve for flathead is pretty low...1800 to 2800. Torque curve for OHV engines is 2400 to 3400+. With fresh oil warmed up I wud think the splash lube does well enuf but ppl can't...check the oil before each run of day or change oil every 50 hrs n clean or new oiled air filter, depending on dust, every 50 hrs. Mi refurbished 18 hp Oppy knocks bad... It was WORN!! I put .010" oversize rings on stock pistons as it had BIG LIP AT TOP OF CYL that had to be dremelled out to get piston out without beating them to death. Honed cyls with 3 shoe hone. Cleaned, oiled it, reassembled it, started, broke in 5 hrs on test stand (plywood on stacked old mower tires) n hv screamed it to 4500...n survived. Flywheel didn't xplode n kill me.. Yet. 6" motor pulley to 4 inch transaxle pulley, that I Gilles transaxle wit STP OIL TREATMENT, N it she go...once I put motor on n figure belt length n guard to keep belt on. 7" pulley too big, clutch pulley ran into 7" motor pulley. With double od it she do 35 mph that is faster than brakes n steering can deal with. Anyway it KNOCKS even with 20 50 oil n 20% STP OIL TREATMENT in it? 12 hrs on test stand so far. Compression.. 60 psi per cylinder. Not 100 psi per cylinder it she have. Oh well it will be down on power some but we'll see what it will do. I put a .015" feeler gauge tween piston n cylinder skirt so it needed .020" over bore for .020" piston I have but machine shop time $$$$ wud cost more than I want to put into motor that was free with the no hood craftsman GT with 6 speed 820 transaxle...now FULL of STP OIL TREATMENT (liquid grease). Got 44 x 25 x 7 mm double lip e bay seals for 1" axles and greased them good with green grease before sliding them on sanded axles so the seals didn't get torn up bmsliding down rusty non greased axle shafts

Knocking Oppy said:

1750 rs is its factory idle. Torque curve for flathead is pretty low...1800 to 2800. Torque curve for OHV engines is 2400 to 3400+. With fresh oil warmed up I wud think the splash lube does well enuf but ppl can't...check the oil before each run of day or change oil every 50 hrs n clean or new oiled air filter, depending on dust, every 50 hrs. Mi refurbished 18 hp Oppy knocks bad... It was WORN!! I put .010" oversize rings on stock pistons as it had BIG LIP AT TOP OF CYL that had to be dremelled out to get piston out without beating them to death. Honed cyls with 3 shoe hone. Cleaned, oiled it, reassembled it, started, broke in 5 hrs on test stand (plywood on stacked old mower tires) n hv screamed it to 4500...n survived. Flywheel didn't xplode n kill me.. Yet. 6" motor pulley to 4 inch transaxle pulley, that I Gilles transaxle wit STP OIL TREATMENT, N it she go...once I put motor on n figure belt length n guard to keep belt on. 7" pulley too big, clutch pulley ran into 7" motor pulley. With double od it she do 35 mph that is faster than brakes n steering can deal with. Anyway it KNOCKS even with 20 50 oil n 20% STP OIL TREATMENT in it? 12 hrs on test stand so far. Compression.. 60 psi per cylinder. Not 100 psi per cylinder it she have. Oh well it will be down on power some but we'll see what it will do. I put a .015" feeler gauge tween piston n cylinder skirt so it needed .020" over bore for .020" piston I have but machine shop time $$$$ wud cost more than I want to put into motor that was free with the no hood craftsman GT with 6 speed 820 transaxle...now FULL of STP OIL TREATMENT (liquid grease). Got 44 x 25 x 7 mm double lip e bay seals for 1" axles and greased them good with green grease before sliding them on sanded axles so the seals didn't get torn up bmsliding down rusty non greased axle shafts

Click to expand...

I've also read Briggs stating low idle speed is 1750rpm. Still Briggs says to run at full revs for max durability. I'm not into idling a mower like a car engine. Give her the beans.... Top of the engine will appreciate it.

slomo

Mine KNOCKS after rebuild...cyl WORN BIG TIME so put .010 rings on good piston n new top rod n put back together

Knocking Oppy said:

Mine KNOCKS after rebuild...cyl WORN BIG TIME so put .010 rings on good piston n new top rod n put back together

Click to expand...

Was it bored and honed to 0.010" over? Did the bore have a ridge? Did you do the boring or a machine shop? Was the piston 0.010" over?

Cylinder worn big time might need a more aggressive boring to clean it up proper.

Did you check ring gaps with the piston in the bore?

slomo

It bored itself...over the years with few oil changes, run low low on oil. Just dremelled the big ridge put of top of cyls to get piston put without destroying them when rings ran into that BIG LIP AT top of cyl. Just honed it smooth top to bottom n put .010" oversize rings on stock piston that seemed fine. New top rod that spun freely on rod journal where old rod wud hang up on one spot on crank...egg shaped rod big end? File fit new .010 rings to get .016" ring clearance...

Later reading u tube Oppy rebuild by guy that really was meticulous abt the rebuild surface sanding block n heads on flat piece of... Tabletop granite? With sandpaper to make sure head n block were flat. He said .010 is enuf ring gap.

Took that wonky gov mechanism out of motor n that wonky gov linkage off front that flutters up the motor n can fail n lbs junk in the motor when it goes.

E bay digital tach n hour meter works fine for $10 to keep eye on revs. Put 15-40 n 20-50 oil in it with 20% STP OIL TREATMENT to THICKEN OIL BIG TIME...still knocks bad like the almost worst of the ones on posted videos??? Piston slap or rod knock? ???? I don't know, 14 hrs on test stand running banging away. Seems to run fine. No choke needed to start so it must run rich at this altitude 3666'.

Supposed to go to smaller jet over 3000 and lose 3.5% for each 1000' of higher elevation cuz less air pressure. 13 psi here... Not 14.7 psi at sea level. Less air, no matter how much gas dump in it, = less power.

Oh well they r torque down low engines...1800 to 2800RPMs. OHV engines torque is from 2400-3600rpms. Better flow thru OHV head.

656 cc briggs intek v twin is 20 hp...the 656cc briggs Oppy 16 hp is 656cc also, so down 4 hp from same size engine. ANYWAY I will keep it FULL OF OIL, run it, be ez on it with yard donkey /street cruiser occasionally n it might last...???

I shut off fuel every time n let carb run out of gas so fuel won't go bad in carb. Stabil gas treatment in all tanks of gas, MMOIL in gas? To lubricate the rings n valve's? Snake oil but for $4 bottle. Not that much to try to make motor last...50 hrs? 14 hrs so far per e bay digital tach/hour meter.

Open up square access port on back n check RODS/torque? Retorque RODS? Plastigauge them? Nothing I can do if crank journal is worn, but looked fine to me. No imperfections on it??..

Read somewhere "briggs flathead like more rod clearance than the factory states (.002-.003"). More like .005" rod clearance...for oil to get to rod journals on splash lubricated engines???

Did the oil mod to the crankshaft n ground off the big lips over the top of rod journals. The big overhang acts like eaves of house, to keep rain off side of house, n out of Windows. Great idea for house, not so good for splash lubricated engine that needs oil falling into the groove tween the rod big end n crank journal...on the crankshaft throws that stick out in the oil rain

Most all of them sound like that never seen one blow up though. Have heard that it could be carbon build up on top of piston but you cleaned up good when you built it so probably not that. I would run it and not worry about it if it is too bad you could buy some ear muffs. LOL