NHRA Stock Replacement Piston Question

[Bernhard]

Hi.
When a piston is submitted to NHRA for review is it an exact copy of the original?
specifically ring land design?
Thanks
Bernhard

[SSDiv6]

Originally Posted by Bernhard

Hi.
When a piston is submitted to NHRA for review is it an exact copy of the original?
specifically ring land design?
Thanks
Bernhard

Although some non-compliant pistons are out there, in a nutshell, Yes, they should mirror the OEM piston according to the rule book.

Compression distance must be identical to OEM.
Ring land location must be in the same location and distance.
Ring groove width must be OEM.
Dome size, (height/volume must be OEM)
Valve reliefs must be the same size and depth.

[oldskool]

Here's a Pontiac 350 factory piston that was used to get a CP piston approved.

http://forums.maxperformanceinc.com/...8&postcount=77

As you can see, they're not EXACT duplicates. Main thing is that they are forged, & they are NHRA accepted & legal, for some Pontiac 350 engines.

[SSDiv6]

Originally Posted by oldskool

Here's a Pontiac 350 factory piston that was used to get a CP piston approved.

http://forums.maxperformanceinc.com/...8&postcount=77

As you can see, they're not EXACT duplicates. Main thing is that they are forged, & they are NHRA accepted & legal, for some Pontiac 350 engines.

Correct, they do not need to be exact duplicates, however, they must maintain the specifications I mentioned in my previous post.

It appears that some engine builders may have some close friends in piston companies because I have taken apart engines that although they have the approved part number stamped,they do not meet the approved spec sheet.

I have found the following discrepancies in some of the Stock Eliminator engines I have torn apart:

- Ring grooves that are thinner than spec.
- Shorter distance from top of piston to top ring land that shown in spec.
- Deeper valve reliefs than spec.
- Smaller domes than spec.
- Smaller dish volumes than spec.

During a teardown, does NHRA assume the piston was built and approved in that configuration or the engine owner blames the piston company?

I assume since there are less intrusive teardowns and less piston removals, that many are taking the risk by doing so.

[Bernhard]

Thanks for the replies.
Very informative!

[jmantle]

[QUOTE=SSDiv6;619698]Although some non-compliant pistons are out there, in a nutshell, Yes, they should mirror the OEM piston according to the rule book.

Compression distance must be identical to OEM.

This is one I've always questioned, approved piston section lists "minimum compression distance" which in my case was considerably less than a stock piston.

Jim Mantle V/SA 6632

[Jackie McCracken]

Originally Posted by SSDiv6

Correct, they do not need to be exact duplicates, however, they must maintain the specifications I mentioned in my previous post.

It appears that some engine builders may have some close friends in piston companies because I have taken apart engines that although they have the approved part number stamped,they do not meet the approved spec sheet.

I have found the following discrepancies in some of the Stock Eliminator engines I have torn apart:

- Ring grooves that are thinner than spec.
- Shorter distance from top of piston to top ring land that shown in spec.
- Deeper valve reliefs than spec.
- Smaller domes than spec.
- Smaller dish volumes than spec.

During a teardown, does NHRA assume the piston was built and approved in that configuration or the engine owner blames the piston company?

I assume since there are less intrusive teardowns and less piston removals, that many are taking the risk by doing so.

To quote Wesley Roberson, "it is the racers responsibility"... in NHRA D4 if you are torn down you will be held accountable no matter where it came from or how long you've had it if it does not match the spec's on the blueprint sheets for your combo and this applies to anything on the car/truck. there is some tolerance but only if it is not to your advantage. for example if you are supposed to have a 11cc dome and it measures less you will be given a pass unless it is way off however if it checks over by POINT ANYTHING you will be DQ'd along with other actions by Glendora depending on the nature of the infraction.. for combustion chamber volume you can be over but never under because it is an advantage for you. something that the old guys know but most newbies don't, if a valve contacts a piston and leaves a mark on it, you'll get tossed for it if torn down.



Jackie

[dwydendorf]

NHRA has a way to check how far down the top ring land is without removing the piston. They have a small piece of steel almost thin like sheet metal that they put between the piston and cylinder wall. They know the size of the piece so they simply measure how far it sticks up over the top of the piston and subtract that from the known size of the metal piece and you know how far down the ring land is. Travis Miller showed it to us a few years ago.