6.21" rod won't work at all with 255 pistons. 1.585" compression height, 6.21" rod, puts the piston 281 thou out ofthe 9.469" block. You'd need a block 281 thou taller to capture that in the block.
I like the custom 6.21 300 rod and forged Racetek or CP piston, but there are other options. It
will work
Some rods, like the stock 1978-1980 D8's a re cast iron, don't use them. Use forged rods. Best option is 4 cylinder Taurus 6" 2.5 HSC rods and a deep dish piston so you can reduce compression. The 9.469" deck gives a 103 thou short. With the HSC rod, the shortfall is gone in one hit. Victor Reinz head gasket or actual deck will propably get the piston clear of the head.
Factory Aussie pistons with 27.9 cc deep dish are avaliable from Silvolite 3332H on page 33 of the catalogue.
6-250 4.1L 3.6807(IN) 93.49(MM) AUSTRALIAN FORD ENGINE Hypereutectic, Dish 2.843” dia. .276” deep 28.7 cc calc, but chamfered, so its 27.9 cc.060” Offset Pin Unleaded Fuel
COMP HT: 1.530
these are available in flat top version and smaller dish # 3328H
2.967” dia. .071” deep
.060” Offset Pin
COMP HT: 1.530 PIN DIA: .912 Offset
Not intended for racing applications
3328H
STD.
.020
.030
.040
.060
PIN DIA: .912 Offset
3332H part number STD. .020 .030 .040 or .060
top ring 2-5/64
Second ring 1-3/16
Assy Interchange no RA4106
can these be used with stock 250 rod and will offset deck to .090 clearance then 3.5cc dish on these ones with 46cc chamber would be 10.26 to 1 CR so what about that option too would that combo work? or still too much deck clearance unless you use the HSC 6 inch rod which i have not been able to locate a source for?
Ak Miller said the 250 piston short fall gave extra room for higher lift cams, but when closed up, you needed a custom Jahns piston. Well, this was Ford Australias cross flow piston, and it will help reduce compression, and clear the lift of a 490 thou cam. And its tough as.
And is US made.
Slot the 51 cc chamber, 27.9 cc #3332H dish, into a 20 thou over 250 with 6" rods and a Chevy LS1 like +16 thou piston stick out, and you still get a fairly good compression ratio.
http://wallaceracing.com/cr_test2.php
(Your displacement is 252.24 CID The Compression Ratio is:
8.72)
Problem solved.