Inliners International Forums General Discussion Engines boring out a 250

Yup, this is a balancing act.
The most universal, generic, fits-many-engines will be the cheapest to produce (since volume is high, and some machining operations will not be performed) but will require the most professional level of final modification, which places it out of reach of some D-I-Y ("I don't have a good+cheap machinist within 300 miles, it's just me and my drill press"), raises the total cost-to-use, shortens the time frame from order to production but lengthens the time from receipt to installation.
The other side, a dedicated piston with specific valve reliefs (centerline, diameters, separation distance, depth, stem axis inclination) really narrows the number of buyers, but they'll be getting a finished products that can be simply installed (rather than merely Step 1 of a series). The 307 uses 4 reliefs (as do many Pontiacs) because the pin is offset, and this permits a single piston to be used in both banks. Not to insult those who understand this, but the positions of the intake vs. exhaust valve may be left or right so they're made the same (largest, intake) size - which is also true of the 250 (exception: many hemis). Since the pin offset is completely independent of which valve goes where (always on the upstream side of rotation), so the offsets on opposite banks of a V8 face in the same direction.
If it's going to fit the 307 you'll need 4 reliefs, or a centered pin. The most efficient 250 piston (if it has valve reliefs) will be made in handed pairs (left intake, right intake) with the intake relief larger to keep chamber volume low for compression. Note: many engines have extra reliefs for different reasons, the Chrysler "poly" V8 has 2 intake reliefs so a single piston can be used on both banks (no exhaust relief is needed).

I have not done a check for piston to valve clearance lately.

But Last I checked w/a cam of around 230 degrees duration @ .050 & a valve lift of .550" lift,,,I had a mile of clearance.

So basically speaking,,,unless you are running a big camshaft,,,I think there is no reason to have valve reliefs.

The angle of the cylinder head is less than a standard SBC of 23 degrees IIRC.

Two cents thrown


MBHD

I agree, 230° is pretty hot for most street builds, and I think most builders would be happy with that as a limit - especially if it speeds up the piston process and reduces the price.
I have a nagging thought that may just be my bad memory playing tricks - do hypereutectic pistons have a "skin" of higher Rockwell at the external surfaces? This phenomenon is, IMHO, not yet well understood as a physical property (how extra surface area improves the strength of some objects), but my concern is if it's safe to machine reliefs afterward if needed?
Since the alloy is a bit brittle (vs. the usual casting and forging material) any cut at all should be done slowly, proper lubricant (I'd ask John Erb, even though he's retired from KB he's still around somewhere), have a generous radius, and blended in to the remaining casting for safety.

If this is a "yes" KB may be happier to accept this commission because it gives them 2 steps forward on 307 and Sprint (which I think has reliefs) pistons - always nice to use OPM to develop your own products!

New member here, Mike from Rock Island IL USA. I've been reading here for a while and this thread got me to register. Haven't joined yet but I will.

I'll buy a set of these pistons if it becomes reality. Suggestions: Specify the compression height at 0 or .010 below stock deck height instead of the common .030 or more in the hole for stock replacement pistons. I would prefer blank tops for the most flexibility, also thick enough that a shallow dish could be machined in for those running boost. I also think that if going to the trouble of a special run, why not do it with a forging instead of cast? JE can make whatever you want for around $600. My guess is a larger quantity might drop that to $400 range which probably isn't much different than what a special run of cast pistons might cost.

Hypereutectic pistons (16 - 18% silicon) allow 1) running tighter piston to wall clearances and 2) harder aluminum alloy for better wear life. Piston ring life is also extended due to less rocking in the ring grooves. Hypereutectic pistons are mainly for normally aspirated engines (non-boosted).

The SRP line of JE pistons forged pistons does use a 4032 aluminum alloy with a higher silicon content of 11%. The SRP forged pistons run 0.025" to 0.035" clearance compared to the hypereutectic clearances of 0.0015" to 0.002". The standard low silicon forged pistons run clearances of 0.0035" or greater.

http://www.jepistons.com/dept/tech/dl/piston_instrc4032.pdf

Agreed on all counts.

I'll buy a set, hypereutectic or forged, just let's get the compression height up close to zero deck relative to factory deck height. I have one block with virgin deck (mid-'70s 250) I can measure for deck height. Just one data point, it would be good to have a couple more or perhaps someone has the blueprint dimension.

If it was my choice I would choose lightweight modern forgings. I'll take the strength and light weight in exchange for potentially shorter service life and cold start noise. Also should consider taking another .300 off the compression height to utilize the available Crower 6.0 rod length. But I understand if that's deemed cost prohibitive.

Let's keep this going, I'm willing to help with whatever I can.

$400 range which probably isn't much different than what a special run of cast pistons

I would guess more than twice as much.

I've only ordered one set of custom pistons and it was from JE back in 2000 so limited experience. That set was for a stroker inline (4.120 stroke, 6.0 rod, 250 block) with a small "D" dish like a stock 250. Price was $100 exactly per piston for piston, pin, pin fit, and locks. I have to confess though that the pistons are still sitting in the box.

Surely JE or another manufacturer has forging or casting dies to produce 283 v8 flat top forgings. Move the pin bore up and that's it.

I'm willing to contact JE and get a quote. I'll work up some specs and post here for comment first. Any interest? Or if someone has a direct contact with JE or experience with another manufacturer that might be an alternative.

Not sure if any of these calculators will help but they are interesting to play with. http://kb-silvolite.com/calc.php

I don't have a block with virgin deck. The three I have measure 9.142, 9.125, 9.110, all have been decked. I'm guessing the factory deck height may be 9.150. Anyone have a bare block with virgin deck they would measure for me? I used an 8" caliper to measure between the deck and the front main bore, then added 1.250.

measure between the deck and the front main bore, then added 1.250

???

Main bore size is 2.5 /2 is 1.25.

Thanks, didn't recognize the size, I was looking for the ID of the bearing itself, but you're right.

Is there a way the moderators can make the last couple of pages of this discussion a separate thread, such as "250 Pistons"?

I will have a contact in Brazil in a month, late June. I would like to have more information on the Chevrolet 250 (4.1L)cast pistons available in Brazil. Are the OEM and OEM replacement cast aluminum pistons a standard cast alloy (low silicon), eutectic alloy, or hypereutectic alloy for the pre '92 (5.7" connecting rod) and the '95 to '98 (6.0" connecting rod) engines? Could someone from Brazil (Edy, Douglas Carbonera, etc.) help out on this information?