Aluminum (big block) clearances vs Cast iron

69427

69427

The Artist formerly known as Turbo84
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Clinging to my guns and religion in KCMO.
Depending on who I talk to, I get all sorts of comments about how the clearances need to be or will be different with an aluminum block. Can anybody point in a direction so I can figure out what is fact and what is BS?

thanks,
Mike
 
Maybe CNCBlocks can chime in. I would assume you'd need a little more clearance on the mains.
 
Yes, I goofed, the main bore will expand more in an alu block than an iron one. Expansion is 7 ppm per deg. F for cast iron and 12 for aluminum. So, if you start out with the same clearance you'll end up with more due to thermal expansion on an alu block. Then there's the added result of main web distortion which I'm sure is more significant in an aluminum block.
 
69427 said:
Depending on who I talk to, I get all sorts of comments about how the clearances need to be or will be different with an aluminum block. Can anybody point in a direction so I can figure out what is fact and what is BS?

thanks,
Mike
Click to expand...

are you going to pin the mains? The Rodeck aluminum 350+ i have has the mains and cam bearings pinned.
 
Twin_Turbo said:
Yes, I goofed, the main bore will expand more in an alu block than an iron one. Expansion is 7 ppm per deg. F for cast iron and 12 for aluminum. So, if you start out with the same clearance you'll end up with more due to thermal expansion on an alu block. Then there's the added result of main web distortion which I'm sure is more significant in an aluminum block.
Click to expand...

OK, I gotta ask what I think is a good question.....I figgered all aluminum blocks has steel sleeves in them so installed frozen and the block hot, therefore the piston is riding on the same iron/steel? that it would more normally BE....

so why extra clearances for anything???

I maybe understand a main bearing for the top 1/2 of the bearing.....assuming a steel cap....

so???

Last all aluminum block I heard of without steel liners was that disaster from the Vega yet....

even the Buick/Olds 215 Land Rover V8 had steel liners....

:stirpot:
 
It's not just about thermal expansion, there's the main web distortion also. I'm pretty sure the mains on turtles rodeck block are pinned to keep them in place as they will have less "bite" in the honed pattern of the alu main webs of his block due to expansion and distortion.

As for all alu blocks, there are lots more, for instance the porsche and bimmer alusil and nikasil blocks (and not only porsche, japs have used those techniques too)
nikasil is a german invention (mahle)
it's a nickel, cadmium (kadmium in German) and Silicon carbide coating on the cylinder bores. It's an oil retaining hard film and it's very wear resistant. Obviosuly when scratched or needing an overbore it's a problem. The benefits however are that you can have very tight tolerances, it has better conductivity than a cast iron linered block.
The 911 mainly benefited from the nikasil's thermal conductivity benefits because it was an air cooled engine. Not sure if the water coolers they now use are still nikasil blocks.


The alusil is a cheaper option, it's a hypereutectic alu/silicon alloy and when etched leaves a hard porous (oil retaining) silicon surface layer with excellent wear characteristics.
There are many successful high end engines that have used this, in fact I'm pretty sure the Cayenne still uses an all aluminum alusil V8 block
 
Twin_Turbo said:
It's not just about thermal expansion, there's the main web distortion also. I'm pretty sure the mains on turtles rodeck block are pinned to keep them in place as they will have less "bite" in the honed pattern of the alu main webs of his block due to expansion and distortion.

As for all alu blocks, there are lots more, for instance the porsche and bimmer alusil and nikasil blocks (and not only porsche, japs have used those techniques too)
nikasil is a german invention (mahle)
it's a nickel, cadmium (kadmium in German) and Silicon carbide coating on the cylinder bores. It's an oil retaining hard film and it's very wear resistant. Obviosuly when scratched or needing an overbore it's a problem. The benefits however are that you can have very tight tolerances, it has better conductivity than a cast iron linered block.
The 911 mainly benefited from the nikasil's thermal conductivity benefits because it was an air cooled engine. Not sure if the water coolers they now use are still nikasil blocks.


The alusil is a cheaper option, it's a hypereutectic alu/silicon alloy and when etched leaves a hard porous (oil retaining) silicon surface layer with excellent wear characteristics.
There are many successful high end engines that have used this, in fact I'm pretty sure the Cayenne still uses an all aluminum alusil V8 block
Click to expand...

BUT don't they all have that same problem the Vega did?? on trip up the temp scale, and it's toast??? :hissyfit:
 
Twin_Turbo said:
It's not just about thermal expansion, there's the main web distortion also.
Click to expand...

The LS engines must have some tricks to minimize this to achieve bearing longevity. Probably the reason for 6 bolt mains and the deep skirts and reinforcement on the lower end.

The GEN 1 small block in aluminum may just not be an animal that will be practical and live a long time in street applications.

I think i'm down to just saving this Rodeck block for a possible build for my jet boat and just going with another GEN 1 iron block crate engine when it's time.
 
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Yes, indeed 6 bolt mains and extended block skirts like you said.

1282340546.jpg
 
I read someplace the Katech guys build the C-5R engines with real tight clearances on the LS race engine. So much tighter they built in a requirement to preheat oil and coolant prior to start.

The aluminum BB 69427 has is a re-do of the old ZL-1 block. What were the clearance specs for that one? They did do some improvement(s), and I'll wager the alloy is better than they used in 68-69, but I suspect they didn't do a whole lot else relating to durability.
FWIW, I've always heard they could make more HP with iron parts because of the flex of the aluminum or something like that. And the early ZL engines were very prone to valve train failures, again that was 40 years ago too. Lots of things have gotten better since. Some even got worse!
 
TimAT said:
I read someplace the Katech guys build the C-5R engines with real tight clearances on the LS race engine. So much tighter they built in a requirement to preheat oil and coolant prior to start.

The aluminum BB 69427 has is a re-do of the old ZL-1 block. What were the clearance specs for that one? They did do some improvement(s), and I'll wager the alloy is better than they used in 68-69, but I suspect they didn't do a whole lot else relating to durability.
FWIW, I've always heard they could make more HP with iron parts because of the flex of the aluminum or something like that. And the early ZL engines were very prone to valve train failures, again that was 40 years ago too. Lots of things have gotten better since. Some even got worse!
Click to expand...

I used to hear (read) that on occasion, also. Usually in the monthly hot rod type magazines, although they rarely, if ever, gave an actual number or percentage. Given that the block and heads of an aluminum BB would reduce the engine weight by about 235 pounds (160# off the block, 70 or 80# off the heads), this is about 7% of the vehicle weight (235/3400). I can't believe the engine hp difference would equal or exceed 7% (35hp on a 500hp engine). And regardless, even if the vehicle acceleration was the same with both engines, the better braking and cornering with 7% less weight should yield better lap times. And maybe better gas mileage! :rolleyes:
 
The '80s era Chevy Powerbook contained a chapter on big block Chevys and included tolerances for both iron and aluminum engines. They knew what they were doing.
 
pcf_mark said:
The '80s era Chevy Powerbook contained a chapter on big block Chevys and included tolerances for both iron and aluminum engines. They knew what they were doing.
Click to expand...

Thanks. I'll have to look through a few boxes to see if I still have any of the old Powerbooks. I threw a lot of stuff away a couple years ago when I moved south, and other stuff is just lost in the pile of moving boxes.

Thanks again,
Mike
 
Hen's teeth

Finally managed to procure some pistons for this build. A 427 is such a "redheaded stepchild" these days that getting pistons is a PITA. I couldn't find anybody who had off the shelf standard bore pistons in stock with the C.R. I was looking for (in the 10 to 10.5 range). Finally had to break down and call WiseCo for a special build. Assuming I didn't screw up my C.R. calculations, I should have a nominal 10.25 compression ratio.

IM001229.jpg

Just have a couple additional parts to order (cam bearings and rings), and I can drop it off at the machine shop for final honing, balancing, and getting the pins pressed into the rods/pistons.
 
I love Wiseco - nice quality and very light. Price is ok too considering it is a custom piston. When I used them last I think each piston was 200 grams lighter than the TRW forged I took out!
 
Ring compressor

I apparently lost my old ratcheting/adjustable ring compressor during the last move, and been looking at the one size (4.250), non-adjustable type that Summit sells. It looks like a tapered cylindrical affair. Any opinions (good or bad) on these versus the adjustable type? I'm looking at ease of use and time savings, and not necessarily the flexibility of using it on future engines.

thanks,
Mike.
 
I have one for 4.280 bore and will NEVER, EVER use anything else. It as $30 or so 10 years ago but you can drop a piston in one handed and never catch a ring or lose the orientation of the rings. After a few rings pop out of the ratcheting compressor and you curse yourself hoarse you will want one! Summit has them for $26 for std SBC.
 
Picked up the block from the machine shop this afternoon (sleeves bored and honed). Had a learning experience a couple days ago when the machine shop owner called me and said the cam bearings I dropped off wouldn't fit this block. After a bit of explanation I found out that iron blocks have a couple different ODs on the cam bearings, where this block is bored straight through (all ODs the same). So, I had to buy a couple spare bearings to get the job completed. These blocks are machined by an outside shop, whereas the iron blocks are done in-house by GM (Tonawanda, I assume).
The rods and pistons weren't done when I stopped in, though. Found out one of the (definitely not cheap) Wiseco pistons had a wrong size pin bore in it, and the pin (or any other pin I tried) wouldn't fit. Fortunately it looks like it can be honed a touch to get the pin to fit.
Also, the small end of the Scat rods are a touch too small to have the usual press (interference) fit. Even after the first couple rods were heated up for pin installation, they still wouldn't slide together as they're supposed to. The pins measured to spec, but the rod bores are smaller than normal. The machinist is going to do some honing on the rods, and then I'll call Scat next week and chew some ass.
The present engine I have in the car went together so damn smoothly years ago (Chevy crank and rods, and TRW pistons) and I originally wanted to go this same route, but it's just getting difficult to find standard-bore, 427 stuff anymore. I kicked around a while back just boring this block .030 over and transplanting the guts out of my engine into this block, but later on I decided to just buy new stuff again, and recoup my costs by selling the iron shortblock when I get this engine put together. Given how much extra cost I've run into so far, I don't think I'm going to break even on the shortblock internals.
 
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