318 piston height/deck clearence 1971 Deputy Challenger

[Nick McGee]

Hi guys,
I'm new here, located in Melbourne Australia and building a '71 Challenger from the ground up...

It was an original 318 car (a Deputy Challenger which turns out to be kind of rare, who knows).

Anyway the motor has been rebuilt (never run) but with low comp pistons that I calculate at about 7.9 to 1.

There is about 3mm measured from the deck that the pistons are sitting down the barrels.

It's be bored to .030" over so my question is: I want to run higher comp pistons that have a longer length from the pin to the flat top.

If there are any engine/machine guys reading this will there be any way I can order .030" pistons (such as what Keith Black offers) but not have to rebore the engine entire deal again, hone only perhaps?

Also, the pistons in it (that sit way low down) have the 4 valve type cutaways... What other piston options will I have that will get them higher without having to shave a stack off the deck – 3mm is a long way down obviously. It's going to be a daily driver so I'm looking for pistons that are not super expensive...

Thanks in advance for any thoughts,

Nick McGee.

 

[challenger6pak]

If the motor has never been run, you can just switch pistons without any other machine work needing to be done. There are several companies that can supply the pistons you need. I believe summit racing has a good overseas shipping policy. www.summitracing.com You could start there and see what they have for pistons. What else are you doing with the motor; cam; 4bbl; etc?

 

[Adam]

Easy to do like 6pak says, but before you order new pistons measure the bore diameter and see what the new pistons require for cylinder wall clearance. Forged pistons require more clearance than hyper-eutectic. Your engine was likely built with cast pistons, so do your research.

 

[Nick McGee]

Thanks guys, if I'm going from the factory style pistons to a higher comp set (or KB hypereutectic) will I have to change rods? That is to say will the wrist pins differ in style: press fit I imagine for Keith Blacks?

Thanks again, Nick.

 

[Cranky]

Right, not all pistons are manufactured to be exactly the same size however, they usually don't vary much. If you get a set that's a bit over what you have, you can hone out the difference. You main problem might be the balance. Since pistons with a higher compression height usually weigh more, you will have to rebalance the assembly.

 

[aussiemark]

I wouldn't waist money on the 318 perhaps mill the heads for a bit more comp and look for a 340 or 360.

 

[Nick McGee]

aussiemark,
Think I'll stay with it brother, it has hi-flow Edelbrock alloy heads, an air gap manifold, 700 Edelbrock carb etc etc... They rate these combinations at 400HP. It'll blow the doors of a stock 360.

Not sure if I can mill the Edelbrock heads? And a zero deck height with combust better as I understand it?

 

[aussiemark]

Hi Nick all the 318's I've worked on had floating piston pins so a piston swap is something you can do at home usually they try and keep piston weights close to a spec but maybe weigh the new ones and compare them to the old. I like the bigger bore blocks because on these motors the valve follows the cylinder wall as it opens and this blocks some flow that's why Cleveland and big block Chev valves are on angles the further the valve opens the further it is away from the cylinder wall, the 340's are great because they have a 4.040"bore std compared to the 4.000"bore of the other big bore small blocks including the 360. With your 318 there wouldn't be much clearance between the inlet valve and the cylinder wall especially with 2.02 valves trying to find a good 340 or 360 may be worth the hassle for this reason.

 

[Aussie Challenger]

Nick the KB's do have a higher piston head but you may still have to machine the block or heads, I would do the block myself.
Either way when heads or blocks are machined you need to check the inlet manifold alignment or there will be a miss match and leaks.
The KB's have the option of using the fully floating gudgeon pin but you will need to check the weights of the pistons and a re-balance may be needed.
If your block has been bored already then buying new pistons with the same OS will work however you will need to check final fit and a light hone may be needed or they may even be too loose.
Now something you may or may not have thought about and that is the valve to piston clearance if you are using a high lift cam, don't go too wild with the 318 as they become a "pig" to drive on the street.

 

[Nick McGee]

Hi Guys,
The cam I've installed from Summit is the following, do you think it will cause an issues with valves hitting the KB pistons if i install them? (I think the KBs are 1.810 high from pin center to flat top).

Summit Cam: Duration 276/288, Lift .441/.441, Chrysler, Small Block #: SUM-K6901

ALSO - I've pulled one of the existing piston and rods out: there are no cir clips on the wrist pins - obviously a press fit as the piston and pin (gudgeon) are sliding together as one? Will the KBs require reaming and clips for their pins?

Best, Nick.

 

[Aussie Challenger]

The KB's should have come with circlips and they don't need machining as if you have the bushed rods then the pin will float, if press fit then you don't need the circlips.
I would check the piston valve clearance with that amount of lift, it is not too much but better to be safe than sorry, that valve timing if it's the advertised duration should be OK on the street in a 318.
Warning ** Make sure you use cam lube ** and break-in the cam properly or you will wipe a lobe very quickly, sometimes after the initial start up.

 

[aussiemark]

Hi Nick usually a press fit piston pin is retained by the small end not the piston, to fit them you heat the small end of the rod with a blow torch then quickly slide the piston pin through the piston and rod then when it is cool it can't be removed without damaging the piston. I don't see how a different piston can change this the clearance between the piston pin and small end bush will be the same and a rod with a bushed small end is not designed to be used in a press fit application.

 

[krazykuda]

Hi Nick usually a press fit piston pin is retained by the small end not the piston, to fit them you heat the small end of the rod with a blow torch then quickly slide the piston pin through the piston and rod then when it is cool it can't be removed without damaging the piston. I don't see how a different piston can change this the clearance between the piston pin and small end bush will be the same and a rod with a bushed small end is not designed to be used in a press fit application.

The press fit pistons are held by an interference fit between the connecting rod and wrist pin.

You have to match the pistons with the rod type that you are going to use, press fit or floating.


I do not recommend using a torch to heat up the connecting rods to install the pistons. It is too hard to control the heat, and if you heat them up too much (over 512° F) they will turn blue and the microstructure will be weaker.... Have them done professionally unless you got better equipment to prevent overheating the rod...

 

[krazykuda]

Here's a link to a thread on how to check your true compression of your engine/parts....

http://www.forabodiesonly.com/mopar/showthread.php?t=278215

 

[moper]

The press fit pistons are held by an interference fit between the connecting rod and wrist pin.

You have to match the pistons with the rod type that you are going to use, press fit or floating.
I do not recommend using a torch to heat up the connecting rods to install the pistons. It is too hard to control the heat, and if you heat them up too much (over 512° F) they will turn blue and the microstructure will be weaker.... Have them done professionally unless you got better equipment to prevent overheating the rod...

x2.
Given the pressed pins I'd keep that you have for pistons unless you have $$ in the budget for machining. I'm not sure how common they are over there, but I seem to recall 318s being fairly easy to find and cheap. It might be wiser to find another one and rebuild the lower end the way you want. At least then you can sell the lower performance one after, but drive it in the meantime.

 

[aussiemark]

Get a 340 or 360 and put all your good bits on it and be done with it. Got a 318 in my shed only because it is the original block to a car I have otherwise I would send it to a scrap yard. 340's are cool because they've got the biggest bore got 2 of them and a 440 wouldn't waist my time with small bore big blocks either unless they are original to the car.