saving a 1980 KZ750 twin

doc_rot

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getting a move on this. Engine is going back together! I sent the whole crank/piston assembly out to be balanced, and i thought they would remove the crank oil gallery plugs and clean them but they did not. I cleaned them as well as i could without removing the plugs, then got a nagging feeling and decided to dremel out the plug screws. Im glad I did because all that black shit was in the crank oil gallerys still. I didnt have a good punch to stake the gallery plugs with so my buddy suggested i tack the screws in with silicon bronze. The bottom end is assembled with new bearings, new cam chain. I honed the small end of the connecting rods with a 320grit brush hone to remove the glaze. they haave a nice cross-hatch now. It specfically says in the manual "DO NOT REUSE CONNECTING ROD BOLTS" but after talking to the dudes at Falicon Cranks they said it would be fine. I'm waiting on a buret so I can measure the piston dome of these custom pistons so I can determine the new compression ratio.
 

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trek97

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Wow so nice and so clean.

That little ball hone is the shit.

I did some searching and wasn't able to come up w one for my 400f about 2".
 

doc_rot

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Flex hone has pretty much any size you could need. i did a quick search and loads of 2" brush hones showed up on amazon
 

doc_rot

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I assembled the new pistons, and did a dry install of the top end. I now have all the figures to calculate compression ratio. I also stuck clay on the pistons to see how much room i have. I have a few questions for the engine gurus on here.

The combustion chambers were not exactly the same size, one was 53.9cc and the other was 53.2cc. stock is supposed to be 50.9-52.1 I had a valve job done so I'm assuming that is why its bigger. Is it worth the trouble of trying to make them the same?

Assuming 53.9cc for the combustion chamber I am at about 8.9/1 compression ratio. I would like to be closer to 10.25/1. There is about 0.090" clearance from the piston to the combustion chamber, so there is ample room to deck the head.

if the combustion chamber were a cylinder I would have to remove about .060" to get the combustion chamber small enough, but its hemi-spherical so even more would need to be removed to hit the mark. That seems like a lot to remove off the head. If I remove .040" it will be pretty much cutting into the cooling fins. At .080" the intake valve would get hit.

My new pistons have a squish band on them I'm wondering if I should instead fill in the combustion chamber and machine it back to give the head a proper squish band, this would not only help me reach my combustion chamber size goal, but a proper squish band will make the engine more efficient, with a "bath-tub" style combustion chamber. Or should I just deck as much off the head as possible? I could also deck the cylinders to bring the piston to zero deck height. That is much easier at this point since the head is assembled.

Thoughts?
 

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teazer

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What I usually do is to get zero deck height, so there's a 0.4mm that can come off the block.

I would rotate the motor slowly around TDC on overlap to see how much valve to piston clearance you have. On old Hondas, the exhaust valve was usually a problem getting enough clearance, but I used to take up to 3mm off a head to get it to where I wanted it with low (ish) dome pistons.

0.090"is a lot to have to take off. That's over 2mm! After you check valve clearance, you will be able to tell how much can come off the head. And if the valves have to be recessed a little or pockets cut into the pistons, that's all part of the fun. Does the squish band angle come close to the same angle as the piston crown ? If it has to be cut, try to get it one degree more than the piston crown to allow for the increase in speed (MSV) as the gas is ejected inwards.
 

doc_rot

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I double checked all my measurements and I realized when I measured the deck height I forgot to torque the cylinders down, so I measured incorrectly. The pistons are actually at zero deck height. Updated numbers are below, now at 9.27/1. This is better, but still makes it a ways to go.

Just to clarify the squish gap is currently .090" I think i would be removing .050-.060" from the head. the stock combustion chamber has no squish band but the new piston does.

The pistons came with some really radical cams from Megacycle with .465 lift. The pistons have deep valve pockets cut into them and I will be using stock cams so I don't think valve contact will be an issue. Unfortunately I don't have the right size shims currently so I can't check to see for certain.

My main concern is that the squish clearance is huge right now (.090"+) I have an extra head to mess around with, so I may try welding up the combustion chambers on that one and see how it goes. I'm not sure the best way to cut the squish band. I have a brand new 4-jaw chuck for the lathe, but I'm not sure if the whole head would fit on there. That seems like the easiest way to cut a taper for the squish band but that much mass off-axis freaks me out a bit. I'm not sure how I would do it on the mill.
 

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trek97

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doc_rot said:
Flex hone has pretty much any size you could need. i did a quick search and loads of 2" brush hones showed up on amazon
Ill be damned. I didn't know it was called a "flex hone". I searched everything but that. lol
 

Nebr_Rex

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doc_rot said:
how much should I remove? .050"?
My calculations are what ever it takes to remove 4.7cc.
372.7cc + 45cc = 417.7cc / 45cc = 9.28/1
372.7cc = 40.3cc = 413 / 40.3 = 10.248/1
What is the diameter of the chamber at the deck surface?
And at .050 below the deck?
You could also fill the chamber with 54.2cc {40.3 + 13.9} and measure the depth.

.
 

doc_rot

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Nebr_Rex said:
My calculations are what ever it takes to remove 4.7cc.
372.7cc + 45cc = 417.7cc / 45cc = 9.28/1
372.7cc = 40.3cc = 413 / 40.3 = 10.248/1
What is the diameter of the chamber at the deck surface?
And at .050 below the deck?
You could also fill the chamber with 54.2cc {40.3 + 13.9} and measure the depth.

.
thanks for that. Ive never done anything like this before so sometimes I need a kick in the brain.

Its tough to estimate because the chamber is not round. Its about 77mm front to back and 80mm side to side. Average that and say the combustion diameter is 78.5. At .050" below deck its about 76mm so using that to calculate the area of a truncated cone should mean removing .050" from the deck height should reduce the combustion chamber about 5.9cc. If i decked it at .040" that would reduce the combustion chamber (in theory) about 4.7cc

That still makes me worried about the squish because there is such a huge gap there. I was reading that ideal squish should be in the .030-.035" range. is that correct? it's currently at about .090" so squish will be a little over .050" after the .040" deck. Does it make sense to do .050" deck and then open up the combustion chamber a bit so the squish is closer to ideal? I don't think I can safely remove more than .050" with out getting too close to the intake valve. As it is decking it will cut into the valve seat.
 

Nebr_Rex

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That intake valve looks awfully close to the edge of the chamber.
If it were me , I would see if the head would bolt down flat without
a gasket. If that checked out I would know clearance anywhere in the
chamber is at least the thickness of the gasket. Then I'd reclay both
pistons on both sides and over the valve reliefs. It's better to measure
twice [or more] and cut once. The thickness of the clay will give a good
visual reference to what can be done.


.
 

teazer

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The issue you are dealing with is that the pistons are designed to work in a head with a squish band set at an angle - much like a 2 stroke or a Honda 350 twin. The head, on the other hand, is designed to work with pistons flat around the outside.

One approach would be to cut the head on a mill to have an angles squish band but you said the combustion chamber is the same diameter or larger, than the bore which makes that a bit ineffective as a solution. Unless you machine say 2mm off the head and machine a narrow squish band. That might require slight pocketing of that intake valve and probably grinding a slight recess in the top of the barrel to unshroud it a touch.

Another option might be to machine say 2mm off the block and then machine a flat section around the outside of the piston so the crown reaches up into the chamber. This would be a thousand times easier if I could draw.

check these out for ideas
 

teazer

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The more I look at the head and piston, the more I like the idea of decking the head and I'd start with 1.5mm and 1mm off the head and cut the piston crowns flat to match. There are some 82mm KZ1100 pistons that might work. Bigger pistons with diameters larger than the head would give you some metal to work with. As it stands, I don't think squish is viable concept in any meaningful way, so focus on getting CR high enough and make teh most of any squish effect you end up with.
 

Nebr_Rex

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teazer said:
The more I look at the head and piston, the more I like the idea of decking the head and I'd start with 1.5mm and 1mm off the head and cut the piston crowns flat to match. There are some 82mm KZ1100 pistons that might work. Bigger pistons with diameters larger than the head would give you some metal to work with. As it stands, I don't think squish is viable concept in any meaningful way, so focus on getting CR high enough and make teh most of any squish effect you end up with.
The KZ750/B has a larger wrist pin than the KZ900/1000/1100, even the Unitrack engine.


.
 

doc_rot

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I double checked with clay. it seems like it will work. the intake valve will be close but I think it will fit. I dropped the head off today to take .050" off. Fingers crossed!

Nebr_rex is right the wrist pin is 19mm on these which makes finding pistons difficult. I found some pistons from a 350 raptor that looked like they might work but i never went deeper sine these pistons fell into my lap.
 

teazer

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Small ends can always be fitted with bronze bushes to let the rods work with smaller pins...... More work and money but that's the nature of the search for power.

What were those high comp pistons designed for and is there a head that matches them somewhere?
 

doc_rot

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They are custom Venolia pistons a dirt track racer had made specifically for the kz 750. I wish I had the head he was using or could at least examine it
 

teazer

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For sure that would be interesting to see. I wonder if he removed the valve seats and welded the combustion chamber and then re-machined it and fitted new seats.

Is there any way to find that OP at this late stage?

I like this one from Spain
 

doc_rot

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Valtoron is the shit. Their bikes inspired me to cast my ignition cover. I wish I had more info on the pistons and cams, all i know i was told from the guy I bought them from, who got them from a guy that owned a Kawi dealership and was flat track racing the twin. Unfortunately the cams were junk but they would have been too radical for the street anyway.
 

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