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Discussion Starter · #1 ·
Hello, I bought someone's frankensled off craigslist unfortunately, and now I am trying to straighten it out. When I purchased it I was told it was a 1998 Summit 670 standard (not X / HO).

A few hours into riding it the engine shat itself. I found one of the pistons had apparently melted enough that the ring departed and smashed around in the cylinder.

There was a little exhaust port damage. I had a machine shop bore the jugs 1mm over. I installed the pistons pictured below. I just put it all back together; haven't fired it up yet.

I am just now reading some threads on Dootalk, and I realized that this engine may be a 670 X / HO. It has the 2 temp sensors in the head. It has the 44mm throated carbs. No "X" stamp, but apparently lots of summits don't have it.

I also had to stack up all 3 gaskets from my kit to get the .070" squish.

So apparently I have installed standard 670 pistons in a HO / X head. What's gonna happen?

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Discussion Starter · #3 ·
Just looking at spec differences between the 670 std and 670 X/HO..... I notice the X/HO requires 91 octane minimum. I was definitely running 87 when she blew..... hmmm. I'm surprised I didn't hear detonation if that's what happened.
 

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Dunno this engine itself, but if like many/most 'HO' engines, the static compression ratio of the HO is higher than the standard. The octane difference indicates that. Whether the pistons make the CR higher or the heads, IDK. If the pistons are different, then you'll just have lower static CR and less 'juice'.

Hard to hear detonation of you have a helmet on....not any surprise.
 

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Just compare part numbers here:


 

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STOP!

Yes that is 100% the HO head (though it's basically destroyed).

With the dome pistons you will run way higher compression, and may actually interfere at higher RPM.

Maybe you'll get lucky but I would seriously not recommended running that combo. Either swap the head or, much better, out in the proper flat tops.

I've owned, wrenched, etc both variants if the 670...the HO is worth it, and that's an HO head. Don't run the dome top pistons or you'll meas up another top end real fast.
 

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HO pistons are flat top and standard are dome top. My worry is that I will raise the compression even higher than HO due to the dome top piston.
OK understood. Sounds like the dome on the standard pistons partially makes up for compression loss with a more open head chamber. Really hard to know what you actually have without compression heights and dome volumes from the 2 pistons and the head chamber volumes, but like DenonO and you have said, you may end up with high CR's.
 

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Discussion Starter · #12 ·
Thanks for talking some sense into me. Yall are right and I will get the correct pistons and tear it apart again.

Maybe the starter pawl sheared off because of the higher compression... It felt harder than usual to pull over.
 

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You did not reuse the head you show here did you?
 

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Discussion Starter · #16 ·
You did not reuse the head you show here did you?
I did. I asked the machine shop that bored my jugs what they thought, and they said "We have sent them to the start of the Iron Dog looking worse than that, it'll be okay." Which was good enough for me.

To be fair, it looked a lot better than that picture after I cleaned it up.
 

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LOL.... I never would have thought about asking about re-using it. How do you 'clean up' the 'chewing' that those bits if ring did on the head surface? The raised edges and peaks that will be come hot and can promote detonation. Did they promise how long 'It'll be OK.'?
 

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Discussion Starter · #18 ·
LOL.... I never would have thought about asking about re-using it. How do you 'clean up' the 'chewing' that those bits if ring did on the head surface? The raised edges and peaks that will be come hot and can promote detonation. Did they promise how long 'It'll be OK.'?
Yeah I don't have the experience to have any idea what's okay and what's not... Cleaned up sharp indentations with a die grinder. So I have no idea, just went with what they said.
 

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I have no idea about the shop there.... but such a statement on that head condition is not confidence inspiring to me. I am sure that some folks have thrown such parts back on and run it and maybe it lasted, maybe it didn't. But poor parts usually results in poor durability.

Hard to say what 'cleaning up' did to volume in the head and changing/lowering compression ratio. I can understand needing to be economical to some extent, but I'd be making it right, just to get the jagged/rough surfaces out if nothing else. There is not as much margin to failure in these engines compared to automotive engines.
 
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