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Cyl. Wall Skid Marks

 
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journeyman
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Joined: 12 Jul 2007
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Location: Moscow
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PostPosted: Mon May 12, 2014 5:06 pm    Post subject: Cyl. Wall Skid Marks Reply with quote

My friend, KV is trying to register on this board with no luck, so he asked me to help him out.

He has 500 miles on a 454 that is now apart again.
It was clattering something terrible when cold and then a lot less when warm. I figured it was piston slap, but not entirely sure.

Here is a pic:
[img]https://drive.google.com/file/d/0B8kKPJNjpNkYY3FQSnk1SWZOTHc/edit?pli=1[/img]

What could these marks mean?

The block was bored .040 over and apparently he ordered +.040 pistons (KB hypuretic).

How much clearance is normal between piston OD and Cyl ID?
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journeyman
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PostPosted: Mon May 12, 2014 5:07 pm    Post subject: Reply with quote

Trying to upload a photo so that it actually shows up!

https://drive.google.com/file/d/0B8kKPJNjpNkYY3FQSnk1SWZOTHc/edit?pli=1
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af2
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Location: grassvalley, ca
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1933 Willys Coupe

PostPosted: Mon May 12, 2014 9:26 pm    Post subject: Reply with quote

Looks like no champfer on rings. Not enough to make the noise though.

Look at the fuel pump and rod......
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journeyman
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PostPosted: Tue May 13, 2014 7:50 am    Post subject: Reply with quote

Curious on this that it makes the clanking noise especially when it is cold.

Seems to me that maybe the pistons are "growing" closer to the correct size and thereby slapping less. Does that even make sense?

Definitely want to sort this out before reassembly! Nothing worse than putting it together without knowing what happened.
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Big Dave
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PostPosted: Tue May 13, 2014 11:35 am    Post subject: Reply with quote

You are correct in your assumption that pistons grow when heated. This is why the Hyper piston is so popular. They offer the strength of a forged piston without the problem that of forged pistons growing more than a cast piston.

In a 454 piston manufactures recommend 0.0030 to 0.0035" piston to cylinder wall clearance to account for piston growth. Block manufactures specify 0.004" to 0.005" piston to cylinder wall clearance. The looser the clearance the more noise and harder time sealing the cylinders, as well as the sooner you will be rebuilding the motor due to excessive wear.

Big Dave
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af2
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1933 Willys Coupe

PostPosted: Tue May 13, 2014 11:59 am    Post subject: Reply with quote

af2 wrote:


Look at the fuel pump and rod......
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af2
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1933 Willys Coupe

PostPosted: Tue May 13, 2014 6:46 pm    Post subject: Reply with quote

Big Dave wrote:
You are correct in your assumption that pistons grow when heated. This is why the Hyper piston is so popular. They offer the strength of a forged piston without the problem that of forged pistons growing more than a cast piston.

Big Dave


Dave....... really?????? I have 4 pistons that will argue that statement....
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Big Dave
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PostPosted: Thu May 15, 2014 12:12 pm    Post subject: Reply with quote

af2 wrote:
Big Dave wrote:
You are correct in your assumption that pistons grow when heated. This is why the Hyper piston is so popular. They offer the strength of a forged piston without the problem that of forged pistons growing more than a cast piston.

Big Dave


Dave....... really?????? I have 4 pistons that will argue that statement....


All pistons can withstand the pressure of combustion without breaking, even a cast piston will run in a stock Chevy for 150,000 miles without failing under normal driving conditions.

When a piston breaks (either physically breaking apart or blowing a hole through the top) something out of the ordinary has happened to cause the failure. This is generally a result of improper fuel air mixture or from detonation (which has host of causes). It is what happens when a piston breaks that determines your choice of piston material.

Cast pistons fall apart when they break leaving a forged steel connecting rod loose in the cylinder attached to a rotating crank. The destruction caused is generally total breaking the block and possibly the heads and many parts attached to them. Cast pistons are used by the factory because they are cheap, that is the only good thing you can say about them.

Hypereutectic pistons are also cast pistons but because they alloy of the aluminum used has a higher silicon content than a standard cast piston they are stronger. Able to withstand more deflection than a brittle cast piston without breaking. There strength approaches that of a forged piston because the forged piston started life as a cast hypereutectic piston before being hot forged and CNC machined. However the hypereutectic piston is still a cast piston so when it eventually fails it also breaks apart. (it doesn't shatter into small pieces the way a cast piston does, but the chunks left can still seriously score or break the block).

Like I said a forged piston used the same alloy of aluminum to build the piston. The hot forging process compresses the grains of the metal and removes stress points that could lead to failure under stress. When a forged piston fails it generally stays mostly intact but could hole or loose the ring lands rendering it useless and in need of replacing.

The other common piston uses a different alloy that is much stronger but subject to scuffing for use with extreme combustion chamber pressures and temperatures. These pistons are designed to work with a blower or nitrous in a racing situation. They have a shorter life span and will quickly wear away loosing their seal over time, so they are not normally used in endurance racing. These are also a forged piston so they fail the same way as other forged pistons but because of their extreme use application the failure is generally total.

Big Dave
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