saving a 1980 KZ750 twin

teazer

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Perversely enough, low crown pistons are better for flame propagation. I like the flattest chamber and crown combo I can get away with and that allows timing to be backed down a long way in some instances.

There's a fair amount of carbon for not a lot of miles that could be cleaned out with a wire brush. The side electrodes look like they have run a little hot but are not showing much sign of distress. Try colder plugs and a touch less advance and see how she responds.

You are right that on a 4 stroke we measure static compression and valve timing and pipe design can change that in a running motor.

On a long stroke motor with big old fashioned pistons, actual squish on the exhaust stroke (ie when nothing is being squished) actual running clearance at high revs will be getting close to zero. Unlike a 2 smoke that never has an uncompressed stroke, a 4 stroke has a lazy stroke where the rod can grow as it's not cushioned by compressing gas.
 

der_nanno

Faster!
Pretty much spot on what Teazer said: You want an as flat piston you can get away with. Squish is ideally a band (ring) all round the combustion chamber and then some room to escape towards the plug. (Modern combustion chamber design favours roof-shaped combustion chambers mainly because they are cheap and you can adjust compression ratio as you need it, without changing tooling too much.) I run an all out 5mm squishband on a 95mm combustion chamber on my TR1. IIRC yours is about 80-85mm bore so you can quite easily aim for the same overall specs, but don't go wider than 5mm or you might run into issues of creating hotspots in the squishband, because it can't be evacuated fast enough at mid rpms.

If you want to see a piston that does exactly this to perfection, look at a standard XT600 piston (plenty of others out there as well, but these I know personally.
 

doc_rot

Oh the usual... I bowl, I drive around...
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sandmanred said:
Congratulations on September BOTM! It's a well deserved win. Really great job on the build.
Thanks man. I appreciate it. I dig your bike as well.
 

doc_rot

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getting a move on this. I decided to bite the bullet and get some custom JE pistons in 86mm bore. this will put the bike at 906cc. If you would like to get in on a limited run of pistons and sleeves let me know soon as I will be pulling the trigger on this in the near future.
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doc_rot

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I have dismantled the engine, I cleaned everything and carefully inspected the combustion chambers, pistons, and spark plug sand have found zero evidence of detonation. However I did find the cause of the ringing/ticking sound... no.2 rod bearing is completely shot. more scrutiny is required.

Also... New JE pistons! dropping them off at LA sleeve tomorrow to bore the cylinders. The increase in bore has done two positive things. One, it has created a squishband, and two it has allowed for a very flat piston design, while maintaining a 10/1 compression ratio. now that i have the pistons in hand the rest of the build can proceed.
 

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irk miller

You've been mostly-dead all day.
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I feel ya, bro. I have a 383 stroker build I need to dismantle because of the same thing. Wiped out a cam, which took the bearings with it. Fun times.
 

doc_rot

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oh man! shits getting exciting! Got the block back with the custom sleeves. I am a little worried about the ultra thin gasket area by one of the studs. I guess if it leaks a little there that is the penance I pay for the big bore, this is the same surface area the stock gasket has but I have lost the o-ring around the sleeves. I wonder about the efficacy of the o-ring on the sleeves anyway. time will tell.

Last pieces of the puzzle are getting the new crank balanced, trans undercut, and getting the cylinder head back from having some custom goodness implanted in it.
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doc_rot

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I bored the cases to accept the larger sleeves. It only took 2mm to make it fit. There is one area by a stud that will be sealing on a hope and a prayer. I might try to fit an o-ring in the counterbored portion. One thing I noticed when dry fitting the block is the case side of the cylinder block has about .006” of taper. I called LA sleeve about this and they said it would be fine, and that they don’t want to press out the sleeves to cut this surface at this point as it will distort the sleeves. Kind bummed they didn’t catch that but not a lot I can do now except cross my fingers.
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doc_rot

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Just got a sneak peak from Chris at Schumann Motor Works. He is making his top of the line custom valve train, complete with titanium valves (1mm bigger on intake) custom bronze valve guides (reduces the valve stem diameter from 8mm to 7 mm), beehive springs, and titanium retainers. He is also doing some porting to blend out the bigger intake valve. This was way more than I initially reached out to him for, but he called me up saying hes stuck in his house with the "Shelter in place" measures, so for a small increase in cost he is giving the cylinder head the full treatment! This program just got kicked up a notch. This may be the only KZ750 twin in the world with titanium valves.

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