Cast vs Forged Pistons for Moderate Street Build

Dan1950

Dan1950

1974 MK II Roadster
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I had a very reputable Norton engine builder suggest using cast pistons for my street build. He suggested head skimming to raise compression ratio.

His thoughts were that cast pistons would be less "heat sensitive" than forged.
 
It's the alloy that determines heat expansion. Forged pistons come in 2618 alloy (higher heat expansion rate) or 4032 alloy (lower heat expansion rate). The 2618 alloy such as J.E. uses is the strongest, has the highest melting point. You can get less heat sensitive 4032 alloy in forged (Wiseco) or in billet (Gibtec). Those brands are US made.

The low heat expansion cast pistons are commonly JCC and are made in Tiawan. But the JCC circlips can be junk, come loose and ruin the bore. If you go JCC then you should get different circlips. You should also consider higher quality rings such as Hastings. The JCC pistons use thick heavy rings that tend to pound out and loosen up the ring lands. When that happens the motor starts smoking.

There are previous discussions in this forum about problems with the JCC circlips, loosening ring lands and ring substitutes. If you research those threads you can find recommended circlip and ring replacements to improve reliability.
 
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jseng1 said:
It's the alloy that determines heat expansion. Forged pistons come in 2618 alloy (higher heat expansion rate) or 4032 alloy (lower heat expansion rate). The 2618 alloy such as J.E. uses is the strongest, has the highest melting point. You can get less heat sensitive 4032 alloy in forged (Wiseco) or in billet (Gibtec). Those brands are US made.

The low heat expansion cast pistons are commonly JCC and are made in Tiawan. But the JCC circlips can be junk, come loose and ruin the bore. If you go JCC then you should get different circlips. You should also consider higher quality rings such as Hastings. The JCC pistons use thick heavy rings that tend to pound out and loosen up the ring lands. When that happens the motor starts smoking.

There are previous discussions in this forum about problems with the JCC circlips, loosening ring lands and ring substitutes. If you research those threads you can find recommended circlip and ring replacements to improve reliability.
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As I understand it, the graphite skirt coating on the JE pistons takes up some of the cylinder clearance until it "burnished".

Does all of this coating wear away or does some remain?

If some remains, would this contribute to longer service life by reducing piston slap.
 
Here's a photo of one of Yves 920cc pistons with the graphite coating. He checked them at low milage. When Bore Tech was still in business they recommened this coating and told me it doubled the life of the piston.

Cast vs Forged Pistons for Moderate Street Build
 
Dan1950 said:
The cam grind I am going to use needs at least 9.5:1.
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Rubbish ! Race cams work well in motors which have comp. ratios as low a 7 to 1. There are four factors which are in balance - comp. ratio, ignition advance, jetting and fuel octane ratio. Within reaon, the ignition advance and comp. ratio can be almost anywhere and the jetting can be changed to suit. The combination of the four factors needs to be optimised, and you will end up at the same place. CHanging the comp. ratio does not do much.
If I had the choice, I would always use forged pistons in preference to cast. In a forged piston, the grain structure is usually better and you are much less likely to get acseize or a piston collapse - and they are usually lighter. Piston weight has a substantial effect on performance. If the pistons are lighter, the motor usually spins up faster.
 
acotrel said:
Rubbish ! Race cams work well in motors which have comp. ratios as low a 7 to 1. There are four factors which are in balance - comp. ratio, ignition advance, jetting and fuel octane ratio. Within reaon, the ignition advance and comp. ratio can be almost anywhere and the jetting can be changed to suit. The combination of the four factors needs to be optimised, and you will end up at the same place. CHanging the comp. ratio does not do much.
If I had the choice, I would always use forged pistons in preference to cast. In a forged piston, the grain structure is usually better and you are much less likely to get acseize or a piston collapse - and they are usually lighter. Piston weight has a substantial effect on performance. If the pistons are lighter, the motor usually spins up faster.
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I've been hot rodding for for over 50 years and I call BS on this post.

Cam grinds and static CR work hand in hand, one complements the other.

Decreasing the volume of the combustion chamber at TDC also promotes evacuation of the exhaust gases and better induction on the downstroke.

I have seen a 13% increase in HP by increasing static CR from 9.6:1 to 12.7:1 with not other changes whatsoever.
 
Not wishing to create additional stress to this thread. Let's look at common mechanical reasoning. Any cam with high lift and overlap is going to reduce compression at low RPM's To over come this and recover some lower end you could raise compression with pistons. You can overcome low compression by purchasing a "compression building " cam shaft, instead of changing pistons. Sifton used to market these for Harley's! Might be BS for Norton's but I know what works with Harleys. Big block Mopar,13:1 makes them go! (OOPs maybe should wait for Access Harley forum) LOL
 
Plus higher compression exhaust sounds much better, especially on a V8. ;)

Is that motor apart yet? Better get that head off and in the que if you want it back in spring of 2023. Just saying

Forum police are very sensitive. Jerry will let you know when you step over the line. It's a fairly high bar to clear.
 
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mean gene said:
Not wishing to create additional stress to this thread. Let's look at common mechanical reasoning. Any cam with high lift and overlap is going to reduce compression at low RPM's To over come this and recover some lower end you could raise compression with pistons. You can overcome low compression by purchasing a "compression building " cam shaft, instead of changing pistons. Sifton used to market these for Harley's! Might be BS for Norton's but I know what works with Harleys. Big block Mopar,13:1 makes them go! (OOPs maybe should wait for Access Harley forum) LOL
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At the other end of the spectrum are cams meant to retrofit high compression engines from the '60s to allow them to run on unleaded pump gas by lowering cylinder pressure at lower RPMs.
 
Dan1950 said:
I had a very reputable Norton engine builder suggest using cast pistons for my street build. He suggested head skimming to raise compression ratio.

His thoughts were that cast pistons would be less "heat sensitive" than forged.
Click to expand...
I wouldn't skim the head if you can avoid it. Irreversible.

If you want cast pistons, cant you get 10:1 with valve pockets?
 
gortnipper said:
I wouldn't skim the head if you can avoid it. Irreversible.

If you want cast pistons, cant you get 10:1 with valve pockets?
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I think I'm going to stick with the JE forged pistons with graphite coated skirts.
 
Dan1950 said:
I've been hot rodding for for over 50 years and I call BS on this post.

Cam grinds and static CR work hand in hand, one complements the other.

Decreasing the volume of the combustion chamber at TDC also promotes evacuation of the exhaust gases and better induction on the downstroke.

I have seen a 13% increase in HP by increasing static CR from 9.6:1 to 12.7:1 with not other changes whatsoever.
Click to expand...
You tell 'em Dan!
 
Eliminate the base gasket and machine down a copper head gasket (what I did), or go with JS's head gasket rings, this brought me to 10:1 static with the web 312a advanced 5 degrees
 
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