Took top end apart tonight

[hogtyed]

It's been awhile since I've taken a engine apart. I'm pretty sure that this is not good. I was really hoping I wouldn't have to split the cases, but now I must. Any thoughts as to what would cause this? I picked this up as a basket case awhile back. Also, where should I pick up new piston from?

 

[TwoManyXS1Bs]

Hoo, boy. You've got twins.

In a shop environment, that engine would warrant a complete teardown and inspection.

The "gray area" method would be to pull the cylinders and pistons, flush the case, and try to determine crank/rod conditions in situ.

As to "cause", could be a few things, like timing, lean fuel, extended WOT on untuned engine, ...etc. Just consider the "cause" as irrelevant, since you don't have the rest of the bike and its history. Think in terms of "return to new or servicable", instead of "fix what's wrong"...

 

[xjwmx]

Right. What's wrong is now history, and when you're finished it will be a completely different what's wrong

cruisinimage on ebay has lots of good parts cheap. Not sure of the spelling at the moment. Best bet would be ebay, or gggGary on here, or anybody else who chimed in with pistons. Put a want ad in the classifieds here; it is pretty brisk. I have a feeling it was run on one cylinder for awhile and the other cylinder might be worse, like gas-washed cylinder walls. Everything is repairable though.

 

[kshansen]

Have a feeling compression was a bit low near the end!

 

[lakeview]

With damage as pictured, where would the missing material had gone? Would it have been vaporized and blown out the exhaust, or liquified and run into the crank area, or a combo of each. I am guessing best practice would be to split the cases and thoroughly clean, but since we are not looking at a broken ring rattling around or the piston touching a valve and creating chunks of metal, how likely is it that bits of the piston crown went down in instead of up and out?

 

[TwoManyXS1Bs]

Most "holed piston" engines I've run across have an aluminum spray on the conrod top, and things down below...

 

[hogtyed]

I’m definitely going to split the cases to clean out the bottom end. I have been finding debris, which I believe is from the cam chain tensioner. It’s going to be more in depth than what Ihad figured, but it is a 42 yr old engine with unknown mileage. The cylinders look good, no scratches. Although I have yet to put a micrometer on them. Wishful thinking is that a worst case is a 1 over set of pistons.

 

[TimeMachine]

hogtyed, Is that blown engine compatible with your Special II seen on your profile pic ? That is a very nice looking Special II.
So what year is the "basket case" engine ?
You definately should disassemble the engine looking for further damage. Realize, anybody who'd have run it to that condition was not caring of components and the rest likely may be neglected as well.
So the year/model components may be of higher value than the engine itself ? -RT

 

[hogtyed]

hogtyed, Is that blown engine compatible with your Special II seen on your profile pic ? That is a very nice looking Special II.
So what year is the "basket case" engine ?
You definately should disassemble the engine looking for further damage. Realize, anybody who'd have run it to that condition was not caring of components and the rest likely may be neglected as well.
So the year/model components may be of higher value than the engine itself ? -RT

The ‘79 I, I picked up last year to make my bobber out of, but it was in too good of shape to cut up. So I I picked up a bastardized ‘75 and I’ve already installed a brat kit onto the frame. I just started the engine teardown.

 

[TimeMachine]

Glad to hear that, all quite appropriate. Will be interesting to see whats inside that basket case engine. Cheers

 

[grizld1]

The single most common cause of holed pisons in these engines is the advance governor (AKA "automatic timing unit"). If the cogged nut that secures the unit to the camshaft loosens, ignition timing advances severely, and if the operator tries to "ride through" the resulting misfires, holed piston(s) can result. Also wear on the bobweights in the unit can spread the advance curve, so that setting timing correctly at idle can put advanced timing in the piston holing range. I've seen only one XS650 motor with a holed piston that couldn't be traced to a defective advance governor.

 

[hogtyed]

Did more dissassembly today. Previous owners are @$$holes! One of the clutchpack screws was stripped. Couldn’t get visegrips on it, so I had to drill it out. Got the clutchpacks out and called it a day. I’ll get back to it on Friday.

 

[motormike]

Stock carbs.. or aftermarket sliders..?... too lean.. ?... Years ago a couple friends rode their 750 kitted XS units, with aftermarket slider carbs.... from down south. After their first gas stop in Iowa... 20-25 miles later.. both engines holed a piston.. they blamed the corn gas.

 

[grizld1]

Yeah, Mike, I know who you're talking about, heard the tale of woe before, and here's my guess. In those days most of the 750 pistons available were domed for high compression, and I'd bet dollars to donuts that our acquaintances didn't cc. those motors out to measure static compression ratio and do the needful to lower compression into a streetable range. I'd also bet that they timed their ignitions at the factory advance mark of 40* BTDC instead of retarding a touch to make preignition less likely. Put in a tank of 87 octane fuel, add ear plugs so you can't hear the engine pinging, and you have the makings of the perfect storm. Moral of story: educate yourself before you even think about modifying an engine, and if you had anybody else do anything on the job don't assume their work is right, and don't take their word on anything. Inspect for yourself, measure until results repeat, and do the needful. And don't try to "ride through" misfires!