question regarding piston center nibs. L2353 forged Sealed power.

I have a set of +.040 sealed power L2353 pistons that are new but old. They do not have the coated surface on the skirts.

They do however have a center lug/nib sticking up in the middle of the piston. This was used during manufacturing as a center point for the lathe. It does have a little bit of a burr on them but I assumed they would just get a quick smoothing and stay there. Here is a picture off the web that isn't my piston, but is similar. You can see it even looks a little rough.


Today I see a picture of a very slightly used set of the same piston and in his pictures I notice the center nib has been smoothed down flush. I am going to use nickbuickgs's picture. I hope you don't mind Nick but I didn't want to ask this in your for sale thread as I have been told that is not kosher.




So my question is why? I will be running mine with some iron stage 1 heads and a .500 lift cam and I was going to do some sample fitting and if needed, I will notch the pistons for valve clearance on the edges to make sure they don't hit.
Should I also smooth off that center nib?

Doing the work is no problem as I have been a machinist for 30 years and own my own shop, so if Boeing would let me make parts for them, I reckon I can notch a piston.

Thank you for all input.

Jon

 

I've got milled heads and zero decked block with .516/.516 lift with my roller rockers. NOT notched pistons and I am fine. No clearance issues. You should be fine. I think your pistons will lower your compression ratio quite a bit with the depth of the dish. Might want to check that to be sure.

 

I have run the numbers based on zero decking and the cam and heads I have. Came out to 7.8 dynamic compression ratio so a little low but ok. I can raise that a bit by going to a thinner head gasket than the .04 thick one I estimated

Mainly I was just wondering if there is a problem having the nib on there. If it has more edges to it then more likely to be a hot spot and then more likely to ping from Preignition That kind of thing

 

You are correct. Remove the nib to prevent a hot spot. It will hardly increase the dish volume.
You are also correct about checking the valve to edge of dish clearance. You may not have to notch them but it is alway good to check.
It would be better if you did not have to notch those pistons because they are made from a high silicon alloy aluminum and any inside sharp corners creates a place for failure.

Paul

 

I've pushed those pistons hard way into the 11's with notches I've made with an Isky cutter and no problems. You only make a small notch, if necessary, along the side and into the top a little. I usually debur the notch and then run a wire wheel over the area to smooth it out. Friday night I just finished a friends shortblock with them. He's running a TA 413 which probably clear without them but we wanted to make sure things would clear if a cam upgrade happens. Once you get past .500" lift clearance should be measured especially if more than a cleanup has been done on the deck.

I get rid of any sharp spot on the nipple but mostly keep it. In my case I'm usually looking to get the most compression out of them. Early pistons came with them and later ones did not. If you are not looking for maximum compression machine the nib flat. Every CC helps though. You won't be able to use thinner head gaskets because with a 0 deck your piston to head clearance will be about the minimum you can run, .040", the gasket thickness. If you want more than that the heads will have to be milled.

DCR is nice to know for the final setup but knowing what your specs are for calculating static compression ratio is more important to know when asking questions on getting the compression you want. Also, did you actually measure the CC of the piston dish? If you are using the 27CC spec I'd be measuring. My friends total deck CC came in at 26CC with the piston a few thou in the hole and the notches.

When I have a block machined I usually tell them not to deck it until I can take it home and do a test assembly and actually measure the deck at each piston. Usually from side to side there is a noticable difference, maybe .008" or more and even differences from piston to piston. This can make a couple of tenths difference in compression ratio from side to side so it is worth checking especially when zero decking. Lots of people just go in and say take .020" off both side. I know I did up until a few years ago. Of course you can always have them do the measuring for you.

 

Jon

Disregard my reluctance to notch the pistons. Just do as Mike suggested and smooth the cuts.

I just had a minor flash from the past but I'm OK now.
We had bad experiences back in the late 1970s when TRW used a square type groove in the pin bores.
The pistons skirts would break starting at the sharp corner of the groove.
After enough complaints they switched to the round groove in the pin bore that you see now.
Here's 430 pistons with sharp cornered grooves and nothing sticking up in the center of the dish.



Paul

 

Ok let me start another theory. As the compressed gasses ignite don't they want a nice dome to expand in a total circunferance of the piston head to exert the most power downward evenly? I'm just saying. I ground all mine off evenly and ran high 10's with -.020 deck and stock deck, same cut heads and big valve reliefs on 1 engine and a slight cut in the pistons using a cutting tool like a valve in the head with a drill, head attatched, to get like .100 clearance for the 7XR KB cam so that would be the only thing is for the performance of the ignition expansion.

 

Ron, if by "dome" you mean the crown of the piston, then yes, the flatter the better. The use of the word "dome" when your talking about pistons conjures up wild images of massive lumps of material on the crown (top) of SBC/BBC/Hemi pistons in my head.

With the later BBB and SBB, we are blessed with small combustion chambers, so a flat top piston is typically all that is needed in a racing application, and then CC size of the heads is manipulated to achieve desired compression.

In street applications, a conical dish is an excellent choice for flame travel propagation.

AS far as the production nubs on the pistons in question, I believe the current generation of those parts has it already removed.

Taking it off was more of a "Standard Operating Procedure" to remove that casting nub. It was just "how it was done".

Concrete reasons to remove it?... Well, let's just say I have never taken a failed motor apart, with those pistons, and saw the nub, and went "AH HA... that's why it failed "

:grin:

I have taken some off, and left some on.. it's really no big deal.

Typically, street cams need those pistons notched just on the outer ring of the crown. Rarely will they need to be so deep that the valve relief does more than just touch the recessed portion of the crown.







JW

 

You're right Jim. I should have said "Dish" You got me! :beers2: That always made me think when I use flat tops I would loose some of the benifit of the Dish but I guess more compression cures that. :TU:

 

Couple followups. What is the minimum piston to head clearance? I guess if you say I can't run a .027 gasket but can run a .040, it must be in that area.

No, I did not actually measure the piston dish CC. I was going by their spec. The way you phrase that it seems you are stating you measure the volume in the hole. I assumed you measure the dish volume of just the piston. Can you clarify?

 

Generally about the minimum you can run is about .040". So if you have a 0 deck with a .040" gasket you are at the minimum. I'm sure with everything just right you can get away with a few thou less. On a street motor with carbon buildup things could get closer than a race engine that only runs a few minutes.

What I'm pointing out is that with a piston that I've added some small valve notches to and with a so called 27 CC dish and the pistons sitting below deck a few thousandths I still only got a measurement of 26 CC. If the piston truely had a 27 CC dish my reading would have had to be greater than 27 CC. Basically I'm saying actually measure the dish and then make your calculations. You can always add in the deck height when you actually measure the height or even better measure the actual deck volume with the piston at TDC. I'd measure the piston dish CC first to confirm its actual CC to confirm or deny what is written and then do your preliminary calculations. Go from there.

 

Keep in mind the piston is nearly 2 inches down in the bore at max lift. So its not max lift, but cam duration that causes interference issues.

 

Good point. I generalized too much. My thinking is that once you get around .500" lift you are starting to talk about cam with decent duration although Jon never did mention what cam he plans on using.

 

the cam I am using has 280/290 duration and .488/.496 lift on a 108 intake center line. It is focused on some low end power since I have the 2.78 rear and stock unported stage 1 heads.

 

From those specs I wouldn't worry about valve notches. What are the .050" specs? 214/228? I'm thinking you don't really need to get up 10:1 either with a 0 deck. 9.5 should be fine if the above numbers are close. It will have plenty of torque anyway if I'm in the ballpark. With only 2.78 gears out back you don't want to push the limits of cylinder pressure with high DCR's.

 

I figure your .050" specs to be 224/230

With a 280 advertised duration, ICL @ 108 and 10:1 compression the DCR will be at 7.8

What Brand cam is it?

Paul

 

Mark Remmel provided it for me, it is an Erson cam and lifters. At .050 it is 224/229 and 110 lobe separation.

Well it sounds like a good polish / rounding of that center nib then to eliminate edges and hot spots will do everything I need then.

My only hot-rod experience goes back to a sbc 350 in my old 4 speed 69 Camaro in the 80's and I really don't know what the cam/engine spec were from that.

I'm hoping for somewhere in the 400+ hp and 475 lb/ft torque when all is said and done. My neighbor's 70 GS 455 convertible used the comp cams 260 and his head mechanic recommended I go slightly larger with the comp 268 cam but after a bunch of talk last year, I decided to bump up larger than that to this custom grind cam to work with the rest of my setup.

I'm getting so antsy for this to be an engine and a car again.

 

Bringing up the piston nibs again, after cleaning the tops of the pistons that were in the 10 second race car they are discolored where the plug was firing and behind the nib they cleaned up better like it was blocking the firing of the air/fuel not spreading it across the whole top of the piston. So if I grind them off do you guys think that there will be enough weight loss to cause problems with the balance being off? I will show pics tonight.

 

Here's a pic of the tops of the pistons.

 

If that cam has alot of overlap, check the clearance. More overlap/fast ramps makes for interference issues. Nibs should not be an issue, but you HAVE to mock it up to be sure! I would leave the nibs if possible, just smooth out the sharp edges.