Inliners International Forums General Discussion Hi Performance Doubts about chevy 250.

Hello everybody... Im posting here because I have a couple of doubts and I would like some help.
Frist of them is about forged pistons that I could find exactly what I need, so knowing that my engine are derived from a Small Block I would like to know if I could search for pistons of SB and most important which kind because I could notice angles of dish from 17º to 23º.

And second how I choose correctly a Damper? I tried find I some sites but didnt find precise answers.

Thanks,

Augusto,

For the Brazilian Chevrolet 250 with carburetors and 3 siamesed intake ports, the pistons for the Chevrolet small block 307 V8 work. They have the proper bore 3.875" and compression height. Forged pistons for the Chevrolet 307 usually are not stocked by the major piston manufacturers anymore. They can be special ordered or found in smaller companies individual inventories. For example, here's a link:

http://www.12bolt.com/250292_inline_products/pistons


The later '92 to '95 fuel injected Brazilian 250's use 6" connecting rods versus the earlier engine's 5.7" connecting rods. So the piston compression height is shorter on the later engines. There is not a stock USA piston available for the later 250 series Brazilian engine.

The 17º and 23º you mentioned refers to the valve stem inclination angle, related to the wedge combustion chamber used on small block V8 engines. The Chevrolet 250 cylinder head combustion chamber is more of a bath tub shape with much less valve inclination than a wedge cylinder head used on small block V8 engines.

If you do need pistons with valve reliefs they will need to be application specific for the 6 cylinder because they have a unique valve angle of 9 degrees +/- 1/2 degree.

Just a guess: the dome probably isn't hollow under the valve reliefs, which means that they can be re-cut to the correct shallower angle. There may also be a significant difference in the stem locations.
This isn't really safe going the other way (more angle needed) because the deep end will reduce the dome thickness.

You loose too much effective dome volume and compression by using a piston with different relief angles already cut into them and trying to correct them. But it can work if you aren't concerned with the loses. Quite a few piston companies have forgings readily available for these engines, so having to settle for the "wrong" piston really isn't necessary.

Winter mine is 5.7'' but anyway I was planning overbore the engine to 4'' so using pistons/rods of 350 that I think it possible because secondly my research on web the diference was only the stroke by .05'' so Is it right?

CNC thanks for this precious information because in fact I plan run with 13~13.5 of compression ratio (remembering that here in Brazil sells pure ethanol as combustive) so probably must use domes, but with information I think I can ask the manufaturer put this angle.

Augusto,

Several comments:

1. Recommend purchasing the new 2nd Edition of "Chevrolet Inline Six-Cylinder Power Manual' by Leo Santucci.

2. Boring to 4" cylinders can result in thin cylinder walls. This is performed some, though generally for specific racing engines. Do Brazilian cylinders blocks typically run thick or thin? Check some of the previous threads on this forum board. Search using "Mexican" helps as the Mexican 250's had thicker cylinder walls and are frequently mentioned in the discussions on boring and cylinder wall thickness.

"boring out a 250" started by Al on 05/15/09 #49941
"Max Overbore for 250" started by JeffF on 06/25/10 #58727

If you elect to use the 4" bore then Chevrolet 327 pistons will work. A small selection of stocked pistons will be available both cast and forged. 327 pistons are available for use with both 5.7" and 6" connecting rods. Forged pistons are available in both the 2618 Aluminum alloy (typical forged aftermarket choice) frequently use for high horsepower applications, and 4032 Aluminum alloy (typically in OEM forged performance applications) which runs tighter clearances.

3. You can use either the 5.7" or 6" rods. Small block V8 rods are narrower on the big end (crankshaft end)and will not work. They must be for the Chevrolet inline 250. As mentioned before, OEM stock 5.7" and 6" rods are used in Brazil. In the USA 5.7" and 6" aftermarket connecting rods for the 250 are available from SCAT and Crower.

Small block V8 rods are narrower on the big end (crankshaft end)and will not work

Why?

You can install a small block V8 connecting rod in a Chevrolet 250 and the engine will run. The excessive big end rod end play will allow additional oil spray, lowering oil pressure slightly.

The connecting rod big end width is 1.029" for the Chevrolet 250, and 0.941" for the small block V8. That's a difference of 0.088" smaller for the V8 connecting rod. The recommended big end rod end play for the Chevrolet 250 is 0.006" to 0.017". General engine application recommendations go up to about 0.020". So with the SB V8 connecting rod, the connecting rod big end side clearance starts at approximately 0.094" (3/32") and goes up.

It is better to run a flat top piston than to use a domed piston.

I had used 307 forged flat top pistons w/4 valve reliefs & had 12.0 compression.

If you need a domed piston, run as small of a dome as possible thats needed to get your compression.
A domed piston hurts the flame travel.

Keep your quench tight & as big as possible..

If you are running the Brazillian 12 port head the combustion chambers are a lot smaller to begin with as compared to the siamessed cyl head. IIRC.

Where does the extra oil come from?

Panic:

I recently had K1-Technologies build a set of 6.0" billet H-beam connecting rods for a future 250ci build. I provided them with an OEM 250 rod for all critical measurements and spoke to Tom Molnar about the use of "small journal" V8 rods in the 250 and he pointed out several potential problems with that approach.

Winter is spot on regarding the difference in "big end" width between the SBC V8 rods and the 250 L6 that would allow excessive oil out around the bearings due to the larger side clearance. Don't know if that will "starve" the rod bearings and cause a failure or not but it would sure be a concern.

But there are two other differences between V8 and L6 rods to consider:

1. The V8 rod has the beam slightly off-center when compared to the big and small ends when looking at the rod from the side. The L6 rod is centered. Whether this would create a problem when using V8 rods in a 250 (perhaps a side load?), I don't know.

2. The 250 rods have a bevel on both sides of the big end to provide clearance for the crank throw fillets. The V8 rod has the bevel on one side only since two rods share each throw. The V8 rods have no fillet on the side of the big ends that face each other. I would guess that lack of bevels on both sides of the V8 rod might damage the 250 crank.

Anyway, something to think about ...

Russ King,
just curious how much a set of billet rods cost?

I talked w/old timer roundy round track guys & they told me they did use SBC V-8 rods in the inline 250 6 cylinders.

I did not get all the details, but it did work & it did splash alot of extra oil around.

MBHD

Hank:

The rods were custom billet 6.0" full floating H-beam rods with ARP-2000 fasteners. As I recall, they ran about $620 (give or take) plus a small shipping charge. There were actually two sets made so I picked up the second set as well for $400. The pricing is very good when compared to Crower and Oliver. The quality is outstanding but the down side was the extended time it took for manufacturing.

Russ

Guys:

Sorry for the typo! My H-Beam billet rods are 6.200" c/c; not 6.000". The necessary compression height for the [custom] pistons will need to be something like 1.185" to get to a zero deck.

Russ

The stock side play distance is more than the radial clearance, no extra oil leaks.

What about the shorter path from oil supply to bearing edge? Should allow more oil flow but probably not much.