Mark,
Mocking was not my intention. I apologize. I was asking forgiveness for not being clear. My thoughts on this problem is you clean up the cylinder walls and go from there. I hope he can return the pistons if 40 over is not needed. That is a large amount of metal. With the scoring I see in the pictures I don't think it would require that much, but you don't know without seeing the cylinders. I hope for his sake it ain't that bad.

 

No, you are correct in that . When you go bigger in bore you go lower in compression. Think at it, you deck a head to lower the volume in the chamber, increases compression. Thicker head gasket increases volume in chamber, decreases compression. So going bigger in bore increases volume in chamber, decreases compression. I'm sure it would not be enough to tell the difference in everyday driving, but racing or heavy towing, you might see it at the throttle. That's another thing these young racers don't think about sometimes. For every action there is a reaction, and you better know what they are before you start down any path.

 

This is the correct answer, although you don't change it radically, more swept volume without increasing the chamber raises the ratio.

I doubt they make the bowl larger in the pistons to accommodate the larger swept volume.

 

I'm sooooooooooooo confused! But that does make sense.

 

Yeah Mark, I always get worried when working on Harleys with adjustable push rods, because some brands use different threads so you're never sure how many turns to back them off.

 

Figure I would chime in on this;

When you bore and hone a cylinder to a greater round you're increasing the compression and displacement of a motor. Although boring in fractions isn't really going to increase power. It will but not entirely. To make things more clear of how it works, a piston goes down the cylinder and draws in AFR (air fuel ratio). When the piston is in this state... this is the displacement of the motor. It can draw in X amount of AFR, when the piston goes up to top dead center this is your compression value. There are more ways to actually increase this which will give you more torque. Shorter rods, smaller journals on the crank, smaller journal bearings and finally smaller piston. Of course you could build the head out as well. The nice thing about running "shorties" is it moves you farther away from valves. Meaning that there is less chance of actually hitting them if ever. If you're honing out the piston by increasing circumference value (going from a 100mm piston to a 130mm piston) it will also change the displacement of the engine as well as the compression ratio. I suppose how you do it is really your choice, they both have pro's and cons.

The nice thing about stock is the tolerances are tight.

 

I talked to Scheid and they reccomend boring .018" over for performance applications. This truck will not be daily driven, beyond breaking it in, and will likely never see more than 30,000 miles if I keep it for 10 more years. I am after reliable power down the 1/4, and loose is fast. So this is not a regular rebuild most people would do, its being purpose built. If I was building it for a daily driver/tow rig I'd go with a smaller overbore and factory tollerances to hopefully last a million miles.

Thanks for all the help guys, I really appreciate it. I am working on the head this week porting, valve job, 60lb springs, seals, and cleaning all the other parts getting ready for reassembly. I HAVE to get this motor back together and engine/trans into the truck before sunday night. Otherwise it may never get done.....

 

This 'thing' about compression ratio is really simple. Just a matter of volume. it is exactly chamber volume plus swept volume divided by chamber volume. Swept volume being the bore area( Pi-R-squared ) times the stroke. Chamber volume is what ever is left over the piston( and in our case in it ).
cheers,
Douglas

 

I don't know if I would go quite that loose. We just got ours together and went with 0.012". Our machinist is a little new to thermo-nuclear diesels and he could not imagine needing that much clearance. Can't imagine what he would have done if we had asked for 0.018"!

But Scheid definitely knows whats going on so... If you go that loose, you should never stick one again.

There are numerous different bowl volumes for these engines.

 

I think not; if you're not changing the clearance then a piston for a +.040 bore will be .040 bigger than the standard bore. Now it also makes a difference where on the piston you're measuring. they're usually narrower, or smaller diameter around the ring lands v. the skirt. I've seen Hardly-Ableson pistons that have a scandalous amount of radial clearance around the ring lands( to the bore ) that close up the gap nicely at the bottom of the skirt.
cheers,
Douglas

 

What me and pwrtripls1 are talking about is having a piston, and a block that has bored. You place piston in the block and can insert a 0.012"-0.018" feeler gauge between the piston and cyl. wall at the tightest point.

Like douglas is saying these pistons are not the same diameter top to bottom or even 100% round.

That is why they need to have the pistons on hand and bore the block to match. The pistons come with a print showing you where to measure on the piston. Hold a straight edge along side even a stock piston and you will see.

Another thing, that was touched on before, is that these pistons are not actually 0.040" over, they are 1mm over. There is a slight difference there.

Not all of the brand new pistons will even measure exactly the same. This is why they are numbered when you get them back from the machine shop. They should be honing each cyl. to match each piston.

 

Just got off the phone with the machine shop and he has already gone .005 over, but can go larger if I want. He recommends not going even close to .018 though and says .005 is on the looser side for factory clearance. And at only 700crank HP this should be fine.

Now I am back on the fence as to go larger or not. A stock 12v short block lasts quite awhile at 600whp, so .005 will be fine? I do NOT want the pistons to swell and cause damage under high EGT though. If this was an LS motor I could build and spec it no problem. The 12v is new territory for me.

 

How do you figure? you make it sound like the OE set up had zero clearance. take what ever the OE bore was and make it n thousandths bigger. If you then make the piston the exact n-thousandths larger, how do you get this zero clearance? Do you need an example of this voodoo you accuse me of?

alright, can't help myself. Take a stock bore of some motor at 4". Say that the piston clearance is .01"...so piston will be 3.99 inches( at what ever point of interest we choose ). So we decide to overbore by .040" and maintain the same piston clearance of .01"...bore will be 4.040" and the piston will be 4.030...or exactly the same amount bigger than the original as the amount we overbored it.

Notice what I compared to what, and what I wrote about clearance in the post you called voodoo, and I hope you see where you took a wrong turn.
cheers,
Douglas

 

Hey now, don't y'all go and get my thread closed. I'd like to keep it open so myself and others can learn more about spec'ing an engine for a build. Many different ways to go as far as loose, tight, stock specs. ect....

But now matter how you want it loose or tight, there is only one correct way to do it. Mic the new piston itself and add your overbore from that point. Never just go on generic numbers. Too many variables. One build may work like that, but another will not work. You have to measure the new piston to determine bore size.

 

I would be tempted to see if you can score pistons that are at the high side of their tolerances to work in your +.005" bores. Since the bore is still within tolerance, fitting two big 'stock bore' pistons might be just the 'thing' for your desire to run slightly looser piston-cylinder clearance.

Now I once had a set of Arias pistons for a CBX that were *EXACTLY* the same size. I measured them on some fine mics at work and could make out their thermal expansion change after holding one in my hands. I was quite impressed...
cheers,
Douglas