Pistons....Interchangeable?

[TylerCharger69]

I'm assembling this 440 HP most of you know I've acquired.  I want to get the power of a 69 440 as this block is stamped 1974.  I know the biggest differences are in the pistons.   These are flat tops, however, will the 69 style pistons interchange onto these same connecting rods??  And if so.....what are the differences between the two?  Wrist pin placement?  Taller piston??.....Ace

[firefighter3931]

The SpeedPro #2355 forged pistons are identical to the factory 6-pack slugs and are a pressed pin design. They work with a stock rod and are very close in weight to the factory cast pistons....that makes the balance close to stock. 

The major difference is the compression height....the distance from the pin to the top of the piston is taller with the replacement (2355) 6-pack slugs. ON a factory spec block (10.725 deckheight) the 2355's will sit ~ .020 below deck. More than likely the pistons in the later block you have are .150 in the hole which kills static compression.


Ron

[TylerCharger69]

Okay....so....those Speed pro pieces you mentioned will press right on then.....right?   And that will bring the engine to 69 specs??

[firefighter3931]

Okay....so....those Speed pro pieces you mentioned will press right on then.....right?   And that will bring the engine to 69 specs??

Yep, they are a press fit piston, just like the stockers. The compression will be the same as as a 69-70 HP engine.

The best way to determine "actual" static compression is to calculate using the piston to deckheight + combustion chamber volume + head gasket thickness. The factory ratings were often lower than advertised due to production tolerances....which were sloppy at best.

That being said, the 2355's will put you into the 9.3-9.5 range with an 88cc open chamber head....much better than the 7.8-8.2 that the later engines came with.


Ron

[TylerCharger69]

okay....ill order up a batch!!   Where did you get yours?

[firefighter3931]

okay....ill order up a batch!!   Where did you get yours?

Mine came in the engine when i purchased the car....the engine was freshened with rings & bearings with a rehone of the cylinders then reassembled. Several vendors carry the speedpro line (Mancini/Summit etc...)

I would disassemble the block and have the cylinders measured for taper before ordering. You might need an overbore so inspection of the block is priority #1 before ordering parts, inmo. 


Ron

[TylerCharger69]

Actually when I received this engine, I tore it down and realized that it had a fresh build already meaning, it was bored .040 over already and there was still cross-hatch pattern extremely clear still in the cylinder walls, so  I  just gave it a quick hone to clean it up.  The bearings had virtually no wear on them.   In a nutshell, I probably could have just dropped the motor in, but I wanted to be sure since the exterior of the engine was butt-ugly and dirty, not to mention it was a police car engine and those are driven pretty hard.  But, I'll just get a set of .040 over pistons with the rings and I should be good to go.....what rings to you recommend?  Cast, Chrome.  Moly???   I appreciate the help by the way....Ace

[Chatt69chgr]

Would the balance be the same since he said he has a 74 block and that would have a cast crank instead of the forged crank that would have come in a 69.  Obviously, this assumes he is using the cast crank that was in the 74 block originally.  Or is the difference in the cast crank and a forged one accounted for in the cast crank balancer (and possibly weights on the torque converter).

[firefighter3931]

I like chrome moly....they last long and seat up quickly. 

You will still want to measure the bores. A forged piston expands with heat and it's very important to get the clearances correct. You will want .005 piston to wall clearance to allow for thermal expansion. Cast pistons are run "tighter" than forged so if the current slugs are .040 over you will be able to hone out the additional clearance. 

It's very important to get this right. You can abuse those pistons forever and they'll keep going....basicly indestructible, like Superman 

I'm going to recommend that you install new rod bolts (ARP) unless it's allready been done. 

This shortblock, properly assembled will easily support 500hp and provide years of service if properly maintained. 


Ron


Ps. You're welcome Ace 

[firefighter3931]

Would the balance be the same since he said he has a 74 block and that would have a cast crank instead of the forged crank that would have come in a 69.  Obviously, this assumes he is using the cast crank that was in the 74 block originally.  Or is the difference in the cast crank and a forged one accounted for in the cast crank balancer (and possibly weights on the torque converter).

Technically, the cast crank won't balance with the stock components internally unless you add some mallory which is cost prohibative. Much cheaper to buy a forged crank if you're looking to go that route. The external balance damper & flexplate + weighted converter take care of the balancing. 

The weight difference between a factory cast piston and the aftermarket SpeedPro 2355 is negligible. On a budget street build that won't see 6k rpm it's not worth worrying about, inmo. The Speedpro slugs are an excellent option because of their static weight....basicly a drop in and go deal. 


Ron

[TylerCharger69]

Would the balance be the same since he said he has a 74 block and that would have a cast crank instead of the forged crank that would have come in a 69.  Obviously, this assumes he is using the cast crank that was in the 74 block originally.  Or is the difference in the cast crank and a forged one accounted for in the cast crank balancer (and possibly weights on the torque converter).

Thanks for asking that question because that was my next one....since i needed to hone "cleanup" the bores anyway...I'll probably be right where I need to be with the forged hardware....very good question for everybody to consider in the future!!!  Thanks!!!....Ace

[TylerCharger69]

firefighter.....it says in the Summit online catalog that these pistons are dish top???   The ones i have from the engine originally are flat tops!!!!   Any thoughts?

[John_Kunkel]

I like chrome moly....they last long and seat up quickly. 

Pet peeve alert!!!

There's no chrome in moly faced rings.

[firefighter3931]

I like chrome moly....they last long and seat up quickly. 

Pet peeve alert!!!

There's no chrome in moly faced rings.

Correction....plasma moly. Sorry JK 


Ron

[firefighter3931]

firefighter.....it says in the Summit online catalog that these pistons are dish top???   The ones i have from the engine originally are flat tops!!!!   Any thoughts?

The 2355's are a flat top with valve reliefs. The critical dimension is the compression height....compare yours to the 2355 6-pack replacements and you will notice the difference. 

A flat top that sits .150 below deck is still a low compression flattop. 


Ron

[TylerCharger69]

So...valve reliefs....meaning  the "eyebrow" indentions?....Sorry for the questions....but I want to make sure I get the right stuff for maximum......happinessw....lol....Ace

Yeah....the "Chrome moly" response kinda threw me off too....I knew there was a chrome faced, and a moly faced and when you combined the two....i was like.....WHAAAA?    lol

[Nacho-RT74]

valve reliefs

[Nacho-RT74]

another kind...





all three on flat tops

[TylerCharger69]

does someone have a photo of the speed pro 2355s???   the photo i saw was a dish top and not a flat top

[LeadfootBob]

Say... I've got a lead on a set of TRW 2295's (with the heavy pins), how do they weight match compared to cast slugs? Still in the original box, each piston wrapped in newspaper from 1973. Would be AWESOME to use in my '78 shortblock, even if they are about as high tech as an ox cart 

[John_Kunkel]

The 2295 with the heavier pin is still lighter (1067 grams) than the 2355 or 2266 with the lighter pin (1083 grams).

[firefighter3931]

does someone have a photo of the speed pro 2355s???   the photo i saw was a dish top and not a flat top

This is the 2355 with valve reliefs....it's a flat top 



Ron

[firefighter3931]

For reference ; this is what a dish (Inverted dome) piston looks like. These are the slugs in my 572ci pump gas motor.


Ron

[TylerCharger69]

okay....that's what i want right there!!!