Hemi piston pop-up

[HankN]

In a 426 street Hemi for stock the pop-up on piston from deck is 0.536” and gasket is told to be 0.018” and dome cc should be max 88.3 cc.
As I got it this motors it is not allowed to adjust the pop-up if a thicker head gasket, as in a wedge.
I has use steel heads and use the MP steel gaskets before.
Reason I ask is I like to use aluminium heads now on this combo ( who is practically dead horse with steel heads in A/SA ) and use Cometic’s MLS gaskets.
This are sold at 0.027, so from 0.018” diff is 0.009”.
Is there news on this today, if so I let pop-up be another 0.009” when I deck my new MP block.
Keep posted.

I waited 5 years on this block now. Mopar has not sold them in years and to get here was not a easy one.
But at #297 LBS it’s heavy but with aluminium heads it will be ok in AA/SA and not that front heavy.
I had the topic’s up last summer on oil pan who I see many use 8Q but after calling NHRA they told me oil pans who is numbered is legal, so not the 8Q.
On use big bore cam tunnel I got to know it’ been legal for years in babbit bearings, but as I has contacted Pat C at NHRA he told me for several years it’s not Ok in stock eliminator.
If one read the rule book it’s not permitted.
So how is it ?
Has it ever been checked at a NHRA race ?

[ss3011]

I thought in Stock they only measure the dome height , not the dome CC . That's how I was planning on building my engine . Was also planning on using the oem shim gasket .

[HankN]

Again, as I got it before it was not legal in a AB engine as the Hemi to do this.
I can’t understand why.
Keep posted.

About dome cc.
If you read the technical specifications its clear say dome cc ( max ) is 88.3 cc, and can’t be more if one like play it legal anyway.
But who’s going to check that in the barn, and this is coming from manufacture with credit to has a NHRA numbered piston.
So to get legal pistons done by manufactory whit correct data and numbered correct, it can’t be more cc on dome anyway.

I has never understand why the street Hemi never could had the std 425 rated HP as the 427 Chevy has it at the std HP.
That combo are equal to the Hemi in many ways but now way lighter, more comp, more cam, lighter rotating etc.
I know there has been racers using the street Hemi destroying this combo ( and HP on Monday but never been in tech after ) and it been over 450 and now 449 HP.
-I has never heard when it was rated std HP.

[Tom Broome]

I'm struggling to follow your post, so I'll attempt to explain how I would approach your situation. I apologize if this gets a little long winded.


I would not even think about decking the block before I qualified a few other dimensions first. Let me acknowledge that I'm accustomed to the Super Stock process where piston machining is allowable. So I'll alter my thoughts to keep the pistons unmodified.



Before I surfaced the block, I would have the cylinder heads completely finished and know the actual piston to head clearance. On a Hemi, piston to head clearance is equivalent to deck clearance on a wedge engine,so it becomes the critical dimension. If you don't have enough piston to head clearance you will have to run the thicker gasket regardless of dome height. I would anticipate that different "Stock" piston manufacturers have slightly different dome radius profiles. Set your engine up with the crank, rods, pistons, and (preferably used) head gasket you plan to run. Install the cylinder heads, screw a dial indicator fixture into the spark plug hole(s), rotate each cylinder to TDC. Zero the dial indicator, remove the rod cap, and bump the piston(s) against the head. Your dial indicator displays the clearance between the piston and cylinder head. Do that seven more times and determine how much (if any) you need to deck your block. This needs to be done after the heads are surfaced to your specific chamber volume, if you chose to surface the heads it will decrease this clearance dimension.



Then check piston to valve clearance. I know you can't do that because your block has cam bearing issues. Has NHRA checked cam core diameter before? I don't know. If you get caught, are you willing to accept the consequences? I'm certainly not going to encourage you to stray too far into a gray(ish) area. Are you planning to race the car in the States?


As I said earlier, I wouldn't deck the block until you have everything to pre-assemble the short block with heads. It seems you need to reconcile your camshaft situation first, then proceed with your build. Like you said, these parts are difficult to acquire and you need to achieve a successful outcome the first time.

[HankN]

Tom,

This car had a Hemi in it on early 70’s in California and I raced it with a Hemi since 2009.
It was former raced with a 440+6 for many years in Div 6, but I it converted the car into a Hemi combo, and a engine I had for my former car - and had Don Little get me the combo
The car was at PMR to get new Lambs, new cage and headers done etc 2008.
I has went 9.84 at 434 HP in A/SA with steel heads before in Sweden, and it has not seen any super weather or best lambda.
I has not raced since season 2015 and waited on see what rules went and get the new block and roller rockers update.
As I sold the my old MP block I got into a issue some years now, but I had no hurry back racing on the other side.

I has good rods from Crowers so I can go rather tight in piston to head clearance.
Deck is now at OEM specs and I will use my old combo in crank, rods cam, headers etc.
But it will be a new solid steel cam since I like remove the Schubecks.
I will also use Total Seal 0.7 mm rings, coat the skirts and the legal 7Q oil pan ( I fixed ) and Manton pushrods.

Over to thread.
I will use a legal piston so dome is what it came from CP.
Reason I ask as I said now it will be aluminium heads and I like to use the Cometics.
A guy from Texas totally killed the steel head combo 2 years ago now.

The Cometics is thicker than the MP steel gaskets that are in the specs 0.018".
If the rule say I can’t has the piston pop-up more than 0.536” above deck I will lose compression by use another thicker gaskets.

But if allowed to has a thicker gasket, I can deck block more.

-But is it legal, that's the question.

[nolongerracing]

Ok, This is what he is asking.... Can I adjust the piston out of bore (above deck) to compensate for a thicker head gasket.
Yes you can.
Your running clearance as per NHRA is .518. That is .536 above block - .018 gasket. 518 is the number you must meet.
If you use a .027 gasket, your piston may be above deck by .545.

Piston CC's do need to be correct. There are some that are not and are still being used. Will they ever check? Most likely not!!!

[HankN]

Yes but this did cost me a block/pistons before.
As many years before I did use the Cometic's but one guy here in Sweden with connection to Gregg L said this was not Ok for a AB motor, so that did cost me a bunch ro fix it.
As I got, that was the rule before.
I know this is all Ok for wedge, but I wounder is this is still the case for a Hemi.
-If so, it's very odd.

What you are saying on the deck, piston/gasket's make all sense to me.
That's the question.

[Race Clean]

As I understand the rule as it once where "if it sticks out of the hole you can't compensate for the gasket" as you can if it's down in the hole.
I could be wrong, it may have changed, or they dont care.

[HankN]

Jepp, so who knows, do anyone cares ?
I like to read the book, if not why in ’stock’ eliminator class.
This is why I put helmet off, as I could not get the sence out if this and I been in it since mid 80’s.
Maybe one can do as the fat cam tunnels..

[Tom Broome]

Originally Posted by Race Clean

As I understand the rule as it once where "if it sticks out of the hole you can't compensate for the gasket" as you can if it's down in the hole.
I could be wrong, it may have changed, or they dont care.

I agree on all points.

I haven't had to worry about dome height for quite a while. But as I recall it was "above deck"......nothing said about gaskets.
Apparently no one that currently races one of these combinations wants to give any "tips".