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Discussion Starter · #1 ·
Welp... never thought the day would come so quick lol, trying to figure out what really happened and what caused this, drained the oil and it was mixed with my coolant. Also was my first time out on bike after build
Here is the modification

787cc makes 85-90hp on 110. Falicon 6mm crank, 105.5mm big bore, cases are bored for 108, Web 290 cam, +2 APC Racing ported head, 54mm billet TB, Hydrographic plastics, 14 paddle extremes, +10 arched Swingarm, +6 forward a-arms, custom ostrich seat cover, pipes are round can dual Barker’s
139981
139982
 

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Wow...that’s not gonna buff out. At. All....
 

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I don’t have anything constructive to say so I will leave you with this. Matt has to use billet cases because his engines always fail catastrophically. He even eluded to the necessity in a Facebook post because he kept breaking cases in 80ish hp dune builds.
Assembly error or bad tune.
 

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I would be suspicious of the cases bored 108, and using a 105.5 cylinder. Leaving no support for the bottom of the cylinder.

Did the crank break?
Are the valves intact?
Did a counter balancer hit the crank?
Check the head gasket for leakage.
You didn't mention compression?

I don't have any experience in that combination, but I would be sure that;
Had a correct map.
The crank was trued and welded
Has a crank bearing support plate.
Use Cuervo oil mod.
DLC pin?

Is this one of Matt's builds? He's been using billet cases for years.
 

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Discussion Starter · #6 ·
I haven’t tore it apart yet, engine is sitting in my work bench but something went wrong for it to blow cases, I’ve never seen it in my life, I’m just wondering if I should try again making the same power on a 700 motor but I want it to at least last a few years, I only ride ounce or twice a year down in the dunes.. not trying to bash on anyone’s name I just had very poor luck with a tired motor. What are your guys thoughts on reliable 80+hp motors is that even possible? Sorry I am new to 700s I’ve only played with yfz450s before
 

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I haven’t tore it apart yet, engine is sitting in my work bench but something went wrong for it to blow cases, I’ve never seen it in my life, I’m just wondering if I should try again making the same power on a 700 motor but I want it to at least last a few years, I only ride ounce or twice a year down in the dunes.. not trying to bash on anyone’s name I just had very poor luck with a tired motor. What are your guys thoughts on reliable 80+hp motors is that even possible? Sorry I am new to 700s I’ve only played with yfz450s before
787 should be a very reliable motor at around 90hp. I’m running a 795 at 93hp without any issues.
If your head is okay, you should be able to use it to make 80ish on a 734.

I’ve seen that bike around for years. I cannot attest to that particular build or the condition of the parts when it was put together.
I wouldn’t hesitate to run a 787, 795, 804 engine in a dune build.
 

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I may be getting older, but I refuse to grow up!
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You can look thru the case and see if the crank came apart.

Pull the head and see if a valve came apart.

If you need, I have some trans internal parts, side covers and stroker pistons for sale.
 
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I’ve been thinking about this failure and think I have a sound theory on the cause.
You have a 105.5 cylinder which means the walls are thinner than factory.
You have a 6mm short rod crank meaning piston is getting lower in the cylinder skirt area.
You have bored cases, meaning this cylinders skirt is un-supported.
I think the cylinder broke, then the crank did its thing and made your hole.
 
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