Topic: Need some high compression pistons

[mlinder]

So, for my project (cb750), I'm looking for some pistons in the 10:1 or 10.5:1 CR range.

I've looked at Buzzs' 12.5 to 1 CR pistons, but they are just too crazy for my project.
I'm looking for 2 over (or thereabouts) in the compression range mentioned earlier.
Need to be high quality. Forged.

Thanks

[Jim F]

The obvious would be Wisco or Arias pistons
you might get lucky with cosworth but I doubt it

good luck

[754]

Get the hi comps and simlpy cut them down..
lathe or mill..

[mlinder]

I didn't know wiseco made high comp pistons that werent 836. I'll check it out.

754, cutting these down would be more work than would be worth it.
The edges in the valve reliefs are too sharp for everyday riding.

[JLeather]

Why don't you wanna go 836?  It's not that big, and Wiseco makes a real nice 836 set at 10.5:1, forged and all.  Dynoman's got 'em.  He also does cutom orders.  Not sure how expensive, but ask him.  www.dynoman.net

[754]

Please explain what,
"the edges of the valve cutouts are too sharp for everyday riding "..means..

[mlinder]

Those edges are knife-like.
At 12:5 to 1, or even 11:5 to 1, those edges will heat up to white hot after a bit. As they do, they'll cause preignition, especially with non-race gas.

As far as going 836, I don't want to. It's not what I want for this build. Going 836 changes the engine to an oversquare design. Changes the engine characteristics.
I want to stay with the undersquare design.
I need high compression pistons.
Not an 836 kit, and as I said, those beautiful pistons I posted above are just not for street or long distance riding. There's a reason they sell those to people building race-bikes.

[754]

Well i am not following your logic there. You can always radius the cut out edges, but I dont see any problem with them.

Sounds like  you are looking for a piston that does not exist, a 10 1/2 to one will have to have valve reliefs, which you dont seem to like the edges on..


Go to an engine shop, tell them your concerns and perhaps they can put your mind at ease..

[mlinder]

Well i am not following your logic there. You can always radius the cut out edges, but I dont see any problem with them.

Sounds like  you are looking for a piston that does not exist, a 10 1/2 to one will have to have valve reliefs, which you dont seem to like the edges on..


Go to an engine shop, tell them your concerns and perhaps they can put your mind at ease..

Can you cut these down for me? Make sure they are all exactly the same? For a reasonable price? Cause if ya can, I'll buy em and send em to you.

[MRieck]

 I always blend the sharp edges with flaps and by hand. I then put the domes to a buffing wheel. I bead blast the dome, skirt and underside after that with low pressure, fine grit bead to peen the surface.

[754]

I could do them but the border and shipping hassles take a long time..like 2weeks or more each way even with airmail.Plus customs hassles.  If there is no other way I could.

So I will walk you through a couple ways to do it, may make it easier for you, it can be done on a cheap mill/drill.

milling,

Stick a bar about wristpin length x2 and slightly smaller id through the piston.

Use that to clamp piston to table, providing piston has 2 flat spots on shirt that will support it.

The top should be flat (can check with an indicator), then touch off with cutter, set 0 and mill off desired amount. Next piston, check 0 and repeat process (re Zero if need be)

lathe,
Skirt has taper so a good way is to build a fixture. Either a flanged plate with a center hole, then you can put a eyebolt looking thing where your rod small end fits on wristpin and draw it tight thru the center hole.

Or , You can machine a big bushing that fits your ring groove fairly close and bore so skirt fits inside but not touching. Should be a bit longer than skirt, so you can also use for milling valve pockets. Leave a lip on the od so you can push this back against chuck jaws to line it up. when finished, split this with a saw.  The you can assemble both 1/2s and put piston inside and push against chuck to insure straight running.. a good way if you do these repeatedly but a bit tricky to build.

Easy, eh..

[mlinder]

I'm not a machinest, nor do I have the equipment.
The compressionw ould have to be brought down a bit. That piston design is for the stock head shape.
If I used them, I couldn't really go hemi design, which would help reduce compression ratio, if I could.
It's more than just smoothing the edges that needs to be done.

[754]

The reason I ran you through it is in case you know someone with a lathe or mill you can use.. You could do it yourself if you have a machine to use.

Usually a lathe is easiest to find.. maybe I should whip up a fixture that pulls it tight by pulling on the wrist pin, that we could use, like pass it on to who needs it type of thing..


Most engine shops can do it or have someone nearby that they use.

Did you ask the supplier if they can cut them down or get them that way in next batch?

[cafebob]

why not use the 12:1's with an F head, and a big bottom gasket?  F head would get you 11 or so, and a big bottom gasket would get you down to 10:5.

bob

[JLeather]

I've got a set that is 2 or 3 over stock from way back when.  I don't know who made 'em or how long ago, but it was somewhere around the 80's.  They're an 823 (?) size set.  Dude who built the engine the first time cracked an oil ring during installation (while spiraling it, not hard to do) and assembled the engine anyway.  It ran for maybe an hour and the oil ring gouged one cylinder a bit and dropped compression on that cylinder (#4).  So, the set needs one oil ring and the 4th cylinder needs to be resleeved and rebored.  That's how I bought 'em.  The pistons are fine, just need cleaning, and three of the cylinders are great (can still see the hatches).  If you're interested, $200.  I think they're forged, but if someone has a way to check I'm all ears.  They've got very small valve reliefs, and a K stamped into the top of each piston.  Oil rings can be had from any of the custom piston makers (Arias, J&E, etc).  Just send them the piston or some very accurate measurements.

[mlinder]

I've got a set that is 2 or 3 over stock from way back when.  I don't know who made 'em or how long ago, but it was somewhere around the 80's.  They're an 823 (?) size set.  Dude who built the engine the first time cracked an oil ring during installation (while spiraling it, not hard to do) and assembled the engine anyway.  It ran for maybe an hour and the oil ring gouged one cylinder a bit and dropped compression on that cylinder (#4).  So, the set needs one oil ring and the 4th cylinder needs to be resleeved and rebored.  That's how I bought 'em.  The pistons are fine, just need cleaning, and three of the cylinders are great (can still see the hatches).  If you're interested, $200.  I think they're forged, but if someone has a way to check I'm all ears.  They've got very small valve reliefs, and a K stamped into the top of each piston.  Oil rings can be had from any of the custom piston makers (Arias, J&E, etc).  Just send them the piston or some very accurate measurements.

Do you know what their CR is?

why not use the 12:1's

Which 12:1's?

Dunno about using an F head. Different size valves, no? And, different combustion chamber shape? and problematic valves?

[JLeather]

I believe, comparing them to my Wisecos and ARTs, that they're around 10.5.  Maybe a little less.  I can snap a pic tonight if you're interested.

As for the F head, the '76 F head is fine.  The 77/78, I've heard, has a tendencey toward valve problems.  They're +5 cc combustion chambers which lowers compression a smidge.  You'd go farther by doubling base gaskets.

[754]

Sam with the CR750 should know where to get exactly what you need for a stock bore hi-po piston..

[mlinder]

Sam with the CR750 should know where to get exactly what you need for a stock bore hi-po piston..

.... I thought he was running an 836......

[USN20]

I'm not a machinest, nor do I have the equipment.
The compressionw ould have to be brought down a bit. That piston design is for the stock head shape.
If I used them, I couldn't really go hemi design, which would help reduce compression ratio, if I could.
It's more than just smoothing the edges that needs to be done.

Don't know if this even applies to SOHC CB750 engines, but here goes ...

Old Harleys and other engines that used the aftermarket higher compression pop-up piston design would sometimes have "flame transfer slots" machined into the domes. This was done by some engine builders to enhance flame travel. Also, a flame slot will lower your static compression ratio with minimal disruption to the configuration of your piston dome design. I had it done once on my '73 Harley XLCH (30 years ago) because it was popular performance modification. Might be an option worth looking into?

[JLeather]

I've seen people have great success putting those "flame transfer" troughs into the head itself.  It's especially helpful on an engine that knocks badly because of squelch area problems.  I was gonna do it on my old YZ490 (13:1 CR) but I blew it up before I got around to it (detonated a piston from the knocking  )

[mlinder]

Buzz contacted me and said he could do something in the CR I wanted, with a bit 'milder' design.
Will see what he comes up with, and see if I can afford it. 

[754]

What I meant the other day was anyone working on CR 750,s probably knows where hi-compression stock size pistons are..

[mlinder]

What I meant the other day was anyone working on CR 750,s probably knows where hi-compression stock size pistons are..

I see.
Well, we will see what Buzz says. If not, I'll have to try something else.

[scondon]

    M3 racing offers 62mm pistons(Wiseco) for some coin. Not sure of the comp ratio, but judging from the pics on their site I would guess they are high comp racing pistons. If M3 sells'em they can usually be found cheaper elsewhere