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Discussion Starter #1
Thought you guys might like to see the inside of my CB450 engine after I tore it down.

The short version
- New motor, 50 miles or so on it, but compression was up
- 85mph run on the freeway
- started pinging
- bang, one cylinder dead
- locked up pretty soon after

Left Plug - looks good.


Right Plug - uh oh.


At least it still had oil in it - I guess it didn't all blow out.


First hints that it's worse than just a hole.



Where's the piston?


Ouch!


Collateral damage(that lump was the piston!)





My motor :(
 

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Discussion Starter #2
So final tally of broken parts:

- right piston, rings, pin
- cylinder
- crank
- right intake valve
- intake cam
- right intake cam follower
- head gasket

The cases, oil pump, transmission and cam drive parts survived, no marks at all, thankfully. I'll have a real time getting all of the little bits of aluminum out of the cases, but that's all. I may have them checked for cracks by my machinist just in case.

I don't know what happened, and that's the worst part. I have some theories, but I just don't know. Maybe head-piston collision from my ultra-tight .028" squish clearance? Maybe one carb went lean? Maybe timing slipped?

Theories and speculation are welcome!
 

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Tough to tell now....but my guess would be you hung a valve and beat the piston up. Usually holing a piston doesn't do a lot of cam/valve damage...unless a chunk gets caught in the exhaust valve. But usually it's the exhaust valve that hangs....
Course the pinging you heard first does sound lean that lead to a holed piston that might have shattered and spread enough pieces around to cause the rest of the damge.
Put it all back together....and don't run that kind of squish....jeez how about something reasonable like .040" ....or more.

Nice piece of work though :)
JohnnyB
 

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That's kind of amazing.... I've never seen a piston so impressively destroyed.

You should sell it on e-bay... "lightly used".

I'm going with the lean carb theory - holed the piston and set off a chain reaction. You haven't converted the head to springs, have you? Even with fairly worn torsion bars I'd be surprised if it hung at 85.

A
 

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Discussion Starter #6
All 4 torsion bars were NOS scored off ebay.

So were the cam followers, which really kills me.

Also, the valves don't appear to be bent, even the intake one that wiped the cam lobe/follower.

Could be lean.

I'm sending the piston pieces to my friend the piston engineer. He claims that he'll be able to tell what happened to it. We'll see!
 

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Yeah you can usually tell if something melted as there will be "globs" of balled up alloy at the edges of some pieces. Sometimes you gotta look close cause really beat up aluminum also looks kinda melted. I've lunched my engine at 12,000 rpm before from a broken valve...and there were areas that looked melted unless you examined them very closely.
JohnnyB
 

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i toasted a couple pistons a few years back due to a lean situation but it took a while ( 5+ racing laps). i know phil t. almost burnt through a piston a couple daytonas ago (it went from convex to concave without completely holing before races end). that took some time too. a couple pings then bang sounds to me like a mechanical/interference scenario. i know i'm talking two strokes and this was a diesel motor but i bet the same principles of toast apply. plus you (diesel) guys have all those valve things which usually end up hitting pistons eventually.
-parks



Edited by - parks61 on Jan 13 2008 8:02:09 PM
 

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Were the pistons quality items or Taiwan E-bay parts? Any sign of seizure in the bores or piston skirts? Assuming there were pieces of the piston large enough to inspect. Just wondering if piston swell was a factor. Pistons tend to swell more during the first few heat cycles after a fresh rebuild. A lean condition would accentuate this condition.

Cheers, Bret @ Glass from the Past

Edited by - bret on Jan 13 2008 8:07:43 PM
 

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How phat is that engine? (Overbore).

Four Strokes are usually pretty freakin hard to run so lean you cook them....assuming you have both carbs setup the same....it would mean some dirt in a jet or something....but still, usually just drop a cylinder rather than melt one. Course 85mph on the freeway is kinda rough for a new engine....and probably a hot one.
Problem with twins is that good cylinder will keep driving the lean cylinder until it's too late.

With a squish like that....and I'm guessing pretty high compression you could have run into some detonation....sharp edge on that new piston...piece of hot carbon lodged somewhere. Pump gas? Probably 10:1+ compression? :)

JohnnyB
 

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Discussion Starter #12
I don't have my notes with me, but yeah, it's roughly 10.5:1 compression.

Bore is 72.5mm vs. stock 70mm. The liners were pretty thin but I know the vintage race guys bore them to 74mm.

CA 91-octane gas.

Oh, and the left piston isn't quite perfect - it's sandblasted around the squish area like it was detonating.
 

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That's probably you answer right there. You'd get some real high pressures in the squish area. Good compression, probably running a decent amount of ignition advance, fill up with some marginal gas and it could be a recipe for disaster.

If your cold squish was .028 you could also lose a few thou as the piston heats up. I run about .035 minimum...but it can depend on the width of the band...as it gets wider sometimes you have to give it a little more room or funny things happen within the band. Basically my deck height is even with the top of the cylinder so I set the squish with the head gasket thickness.

I know on real tight squishes you have to warm the engine up good first...get the cylinder liner to elongate some before you hit any high revs.

I bet if you build the same engine and add about .015 to the gasket it would run forever...but you'd of course lose a bit of power/compression.
JohnnyB

PS. Here's the pistons I designed for the race bike....I've since gone out another mm, but the squish band is still pretty narrow compared to some big bore kits I've seen.





Edited by - jbranson on Jan 14 2008 03:41:34 AM
 

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jbs right. most good tuners tune comp and squish with the gasket. the reason youll see 5 or 6 different gaskets in a racers box. im not that serious though.


jc
 

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Discussion Starter #15
Thanks for the info. That's the way I have them set up - 0 deck height with a stock paper base gasket. I milled off the .010 "step" in the combustion chamber so that the gasket thickness = the squish clearance.

I used a vintage "big-bore" composition head gasket I scored off ebay that was the same thickness as the stock head gasket - .030 uncompressed or about .028 compressed.

For the next motor I will use a .040 copper gasket since my chances of finding another composition gasket are about 0.

That should give me .040 squish. The cylinders I had last time did not require any decking to be parallel, but if the new cylinders I use do, I may have to double up on base gaskets to set the proper squish.

I know you can get steel shim base gaskets from Cappellini in Italy, but I don't think I need to be that precise.
 

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Discussion Starter #16
Here you can see the piston. It does have a pretty big squish area - but I've always been taught that you want lots of squish and not much chamber.

 

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Discussion Starter #17
and my other decision now is whether or not to build a new head. The chamber on the jug that blew up is coated with melted aluminum and pitted in a few spots from rust (which I found out after I paid to have all the machine work done on that head).
 

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Jeez, your lucky...my 12,000 rpm explosion left a exhaust valve head turned upside down with the stem stuck into the chamber.
Here's some pics.
JohnnyB





And here's a lean failure of one of my pistons in Aaron's bike done in at Daytona.

 

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Are we starting an engine carnage thread? I've got photos around here somewhere of my Triumph 500 motor after it tried (and mostly succeeded) to cut itself in two. The left piston broke around the oil ring land while turning 9500 RPM down Portland's front straight. (Don't over lighten your pistons!) Through the windscreen I saw shrapnel being thrown ahead of me as parts began exiting. Half of the left connecting rod with the wrist pin still in place was later handed to me by a corner worker. Fun stuff.

Cheers, Bret @ Glass from the Past
 

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aaron does nice work. if you can be hit in the face with your own piston pin, or rod parts, thats pretty good.


jc
 
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