So I was getting ready to put my rings on an assemble the engine, when I noticed a small crack in the piston skirt. Pretty sure it happened when my dad pulled them out. I didn't want to chance it so I ordered a new one. The problem is the part # on the old one is now changed. These are lower end speed pro/Federal Mogal. But wanted them all to be the same. The piston that matched says press fit only. How tight are the pins to press? I removed two clips on the old one and pushed the pin out. Are those floating or pressed? Will I have an issue.
Piston Question
- Thread starter 71dart318
- Start date
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BlueDream
Aspiring Mopar enthusiast
If the wrist pins were held in by clips, they're floating. No clips, they're pressed onto the rod. Machine shops can press the wrist pins on for you.
Is there a big weight difference between the new and old pistons? Too much of a difference could cause balance issues.
I don't think so...need to check. They don't list my exact piston number. I have 285NCP and all they list for my 318 is 285AP. Specs looked the same flat top with 4 valve reliefs. I assumed they just updated some numbers. This motor was built by previous owner in 07. It had a bad cam, so I tore it down to refresh. Didn't need to bore so just a deglaze. Will the pin press in or are pins different? Can the rest be floater and one pressed?
You want them all to be the same type. I'd just call that company directly, they may be able to sell you the correct piston yet. Who knows what all differences are with the new vs old. Could be a different weight, different grade of material or even different compression height (remember, we're dealing with decimals here )
nm9stheham
Well-Known Member
The 285 NP/NCP may have a higher compression ratio piston; The Silvolite equivalent has a compression height of 1.741" and the 285AP has a compression height of 1.720".
Put in a pin part way in one type and then the other end of the pin into the other type, so the pin is in both pistons side by side at the same time. Rest the tops of the pistons on a truly flat surface at the same time. See if both piston tops set flat on the surface with absolutely no difference. If one top is not exactly flat on the surface with the other, then you have a different compression height and the compression ratio will be different.
Then weigh both types on a gram scale to see if they weigh the same within a few grams.
If the compression heights and weight are the same, look at the pin bores. I bet the NCP has grooves for the pin locks and the AP does not have those grooves. If that is the only difference, then I would be OK with the AP.
BTW, the 285NP is the same as the 285NCP except the NCP has a skirt coating and the NP does not. So you will be good if you can find an NP.
Have you tried eBay?
Put in a pin part way in one type and then the other end of the pin into the other type, so the pin is in both pistons side by side at the same time. Rest the tops of the pistons on a truly flat surface at the same time. See if both piston tops set flat on the surface with absolutely no difference. If one top is not exactly flat on the surface with the other, then you have a different compression height and the compression ratio will be different.
Then weigh both types on a gram scale to see if they weigh the same within a few grams.
If the compression heights and weight are the same, look at the pin bores. I bet the NCP has grooves for the pin locks and the AP does not have those grooves. If that is the only difference, then I would be OK with the AP.
BTW, the 285NP is the same as the 285NCP except the NCP has a skirt coating and the NP does not. So you will be good if you can find an NP.
Have you tried eBay?
Well here's the problem you'll have.
The rod you have is for a floating pin. What is going to hold the press pin in place? It won't be the rod because that will have a bushing in it. Also you need an oven , a press and, a jig to put pressed pins on properly. You will also need a new rod.
You will be better off having a machine shop put the clips on the piston and make them floaters.
The rod you have is for a floating pin. What is going to hold the press pin in place? It won't be the rod because that will have a bushing in it. Also you need an oven , a press and, a jig to put pressed pins on properly. You will also need a new rod.
You will be better off having a machine shop put the clips on the piston and make them floaters.
So, I checked and both pistons have comp height of 1.720 and pin dia. of .9842. Still waiting for the new one to get here, to see if it has clip groves. I did look on ebay an found exact match but only as a set of 8 and I just needed one. Mine are 285ncp and they say they are both floating or pressed. Well if it doesn't work I guess ill buy a whole set.. But I was trying to avoid that and would of bored the block instead of a deglaze. Going to take more than a few hundred dollar set back to slow down this love/obsession. Thanks Guys
nm9stheham
Well-Known Member
That's the spirit! Did you check the weights being matched? They probably are, but it's easy to be sure.
You might want to look into finding a set of higher CR ones if you are looking for a whoel set. Like:
1. Sealed Power 526 AP; looks like they are 1.745" compression height and have no valve reliefs. A quick computation shows that raising your SCR from the low 7's to the low 8s.... a VERY worthwhile improvement. They are press fit pins, but I'd put them in the engine in a heartbeat over the lower CR ones that you have. Here are some STD bore ones for sale.
http://www.ebay.com/itm/SEALED-POWE...ash=item2331d72db7:g:uPkAAOxygj5SemBr&vxp=mtr
2. Silvolite HC814P or CP Hypereutetics (check the weight versus stock cat to see if they are close or you will have to rebalance the crank. These will make the same rise in SCR as above.
3. Also here are some old TRW 318 pistons: I don't know the specs, but generally the LxxxxF TRW pistons were forged, not cast as this ads says. But they are .030" over.
http://www.ebay.com/itm/TRW-cast-pi...ash=item23587471c4:g:yMMAAOSw9r1V7iGQ&vxp=mtr
And here a link on some pistons data:
http://www.moparchat.com/forums/showthread.php?t=106712
And a lot more:
http://www.campbellenterprises.com/pistons/318-pistons-mopar-dodge-318-stroker-pistons.php
You might want to look into finding a set of higher CR ones if you are looking for a whoel set. Like:
1. Sealed Power 526 AP; looks like they are 1.745" compression height and have no valve reliefs. A quick computation shows that raising your SCR from the low 7's to the low 8s.... a VERY worthwhile improvement. They are press fit pins, but I'd put them in the engine in a heartbeat over the lower CR ones that you have. Here are some STD bore ones for sale.
http://www.ebay.com/itm/SEALED-POWE...ash=item2331d72db7:g:uPkAAOxygj5SemBr&vxp=mtr
2. Silvolite HC814P or CP Hypereutetics (check the weight versus stock cat to see if they are close or you will have to rebalance the crank. These will make the same rise in SCR as above.
3. Also here are some old TRW 318 pistons: I don't know the specs, but generally the LxxxxF TRW pistons were forged, not cast as this ads says. But they are .030" over.
http://www.ebay.com/itm/TRW-cast-pi...ash=item23587471c4:g:yMMAAOSw9r1V7iGQ&vxp=mtr
And here a link on some pistons data:
http://www.moparchat.com/forums/showthread.php?t=106712
And a lot more:
http://www.campbellenterprises.com/pistons/318-pistons-mopar-dodge-318-stroker-pistons.php
Not sure if I can change the compression height. I'm running 6077 Edelbrock heads, with Felpro 1088 head gaskets. Not sure?? Don't want a piston to meet a valve. They were super specific on head gaskets. I would assume do to how much crush.
nm9stheham
Well-Known Member
That is a good thing to worry about. Buuuut... I guess you mean 60779 heads and a Felpro 1008 gasket? (That gasket has a .039" installed height BTW.) We have those very same heads on our 340 and the pistons are out of the hole about .004", and have valve reliefs. We had GOBS of piston to valve clearance with a moderate cam and 1.6 ratio rockers: around 268 advertised duration and .490/.505 lift.
Your pistons are .100" in the hole and would be .080" down the hole with the taller compression height 526's. So it is only the lack of valve reliefs that you need to be concerned over, AND your particular cam. It is easy to find out: mock up the assembly with the cam and valvetrain and piston/rod for 1 cylinder and turn it over with some clay on the top of the present piston and see how deep an impression it makes relative to the flat top area. Then you can figure out if .020" extra height and no valve relief will be any issue.
The cam is what you have to look out for. BTW, I re-ran the SCR numbers with your parts (assuming 63 cc's on the head chamber), and got 7.51 with the 285 pistons and 8.55 with the 526 pistons. That 1+ point in SCR will be worth a good low RPM torque boost in street use.
What cam are you running? And what kind of running will you be doing? Cruise? Pure drag race??
And who is 'they'? The machine shop?
Your pistons are .100" in the hole and would be .080" down the hole with the taller compression height 526's. So it is only the lack of valve reliefs that you need to be concerned over, AND your particular cam. It is easy to find out: mock up the assembly with the cam and valvetrain and piston/rod for 1 cylinder and turn it over with some clay on the top of the present piston and see how deep an impression it makes relative to the flat top area. Then you can figure out if .020" extra height and no valve relief will be any issue.
The cam is what you have to look out for. BTW, I re-ran the SCR numbers with your parts (assuming 63 cc's on the head chamber), and got 7.51 with the 285 pistons and 8.55 with the 526 pistons. That 1+ point in SCR will be worth a good low RPM torque boost in street use.
What cam are you running? And what kind of running will you be doing? Cruise? Pure drag race??
And who is 'they'? The machine shop?
pishta
I know I'm right....
get yourself a $8 HF digital scale that goes to 1000g. Weigh the old and new with pin. you can take out weight from the pin if needed, just grind a little out of the ends. Many pistons are both floater and pressed, just having a clip boss machined in them for floaters. The pin is held to the rod via an interference fit, the piston floats on either pin. Its the rod that is different. You could get away with 1 pressed and the rest floating but they will have to weigh the same (piston pack, total rod, then little end and big end)
They as in edelbrock. And running their 7177 cam. Your right 1008 gaskets. If this single piston don't work, then I will have the shop bore it out to .060 over and buy a hole set. Would love to get more torque. My plans are driving and weekend drag. Also stomping some punks on the street. One other thing if I go with these 285ncp would I be able to run a small shot of NOS. Or will they grenade.
nm9stheham
Well-Known Member
O yeah darn.....sorry, I forgot. As OldManMopar said, without the grooves inside the pin holes in the piston, then you could not use the rods made for floating pins unless you had the pin holes in the 526 pistons grooved for the locks at a machine shop... I have never priced out that work. Otherwise, you would have to change out rods. My son might have a set of the lighter rods for press fit.
One more thing to be aware of: Again assuming the heads are the Edelbrock 60779's: (Is your head PN in post #9 incomplete?) The heads have valves that are .24" larger in diameter than the original 318 valves. I suspect that the valve reliefs in the 285 pistons are too small to clear the larger Edelbrock valves. So you really NEED to partially assemble the cam and chain, time the cam, install one head, one set of valves and lifters, with light springs installed, along with the crank, bearings and one piston and rod set, and check the valve clearance.
JIMHO, and not to irritate you, but your low compression ratio and that moderately large cam are going to make the engine lacking in torque down at the lower RPM's. The engine is likely going to need to be spun pretty high in RPM.
I can't say on the NOS.....
One more thing to be aware of: Again assuming the heads are the Edelbrock 60779's: (Is your head PN in post #9 incomplete?) The heads have valves that are .24" larger in diameter than the original 318 valves. I suspect that the valve reliefs in the 285 pistons are too small to clear the larger Edelbrock valves. So you really NEED to partially assemble the cam and chain, time the cam, install one head, one set of valves and lifters, with light springs installed, along with the crank, bearings and one piston and rod set, and check the valve clearance.
JIMHO, and not to irritate you, but your low compression ratio and that moderately large cam are going to make the engine lacking in torque down at the lower RPM's. The engine is likely going to need to be spun pretty high in RPM.
I can't say on the NOS.....
The heads are 60779's. I would assume they were bought as complete heads.(springs , valves, ) I'm with you.. I want more low end torque. I only have the springs on the head no light ones. any other piston options close in price.
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