318 piston questions

-
C

carcrazyguy

Well-Known Member
Joined
Feb 5, 2005
Messages
1,358
Reaction score
16
Location
Amarillo, TX
Okay the machine shop just called. :(

My 318 cylinders range from .012 to .015 wear. So they suggest boring the block .030 over and using 8.5:1 flat top pistons. They have the pistons in stock and the boring and pistons will add $320. So should I use their pistons ($140) or should I find my own pistons?

Another question. If we are using 79 model 318 heads (the machine shop took .010 off those to "square" them up) with 8.5:1 pistons will I still have close to 8.5:1 when it is built?

THANKS IN ADVANCE
 
You really need to know how far down into the bore the pistons will sit, the size of the chambers in the heads and the diameter of the bore of the head gasket and it's compressed thickness. If the "flat top" pistons have valve reliefs machined into them you will need to now the volume of the reliefs.

You can use this calcualtor to figure out what your compression ratio will be. http://www.csgnetwork.com/compcalc.html

If you are trying to build an engine for performance you should be looking to get over 9:1 on the compression.

Also, the advertised compression ratio of pistons almost always winds up lower. The reason is the deck height of factory blocks is almost always higher than the nominal design dimension.
 
I thought it might be helpful to post more of my combo (if you could call it a combo).

The motor is going to be a 2bbl motor (for a while anyway). My original plan called for a 360. But after buying a motor and the block being cracked I decided to just do what my Dad wanted all along. Build a stock spec 318. But now that the machine shop called about the block needing boring I am thinking that I might just use some better pistons and a better cam (with the 2 bbl) for a while. Then next around next summer adding the 4bbl set-up once we get any-all bugs worked out on the car.

So what kind of pistons and cam could I get now (using 2bbl) that would work well with a 4bbl set-up later?
 
I would have your shop order up and use Keith Black hypereutechtic pistons. They are roughly $250/set, but they will give you much better compression. Your factory ones (and the replacements) are not performance parts..They actually yeild around 7.5:1 when installed as the factory did. The newer design raises the top of the piston to very close to the top of the block. Depending on heads and gaskets,. you will end up with a compression around 9:1, maybe a touch more. This will give only benefits even when using a 2bbl. The downside is the cost, and also that the new pistons will need to be balanced. if the budget is firm, then just use his pistons, the price is reasonable, and many shops will not stand by anything if you supply the parts because they did not get them.
 
carcrazyguy said:
{---- :embarrssed: Sorry that is not going to help me. I have no idea on some of those numbers?
Click to expand...

The only two values that you can't get from a catalog or spec sheet is the true size of the chambers and where the top of the piston will sit in relation to the deck.

The chamber volume is easy. Get a 5" square of plexy glass and put a small hole in it off center. Grease it up and stick it over a chamber on your heads. Using a graduated in cc's syringe you can pick up at the local pharmacy measure how many cc's it takes to fill the chamber with water (or your favorite liquid).

The machine shop that has you block should be able to tell you your deck height (if they can't I would get your block to another machine shop fast).

Subtract 1/2 the stroke, the center to center rod length and piston compression height from the deck measurement and you wil have the distance below (or above the deck the pistons will sit). Actually a good machine shop will do these calculations for you.

Plug the numbers into the calculator and you will have your compression ratio.
 
That is what I was reffering to.. I am 350 miles from the motor (and the car). I live in Fort Worth and the car is at my Dads place in Amarillo. I don't know much about the actual numbers but even if I did I would still have 350 miles to drive to look at the motor! :(
 
Ask the machine shop for the brand and part number of the pistons they have and let us know what they are. Have you selected a camshaft yet? If so, what are the specs on it? Do you want to run this engine on 87 octane, or are you okay with running 93 octane all the time? How are you going to use this car? What are your performance goals? Why are you waiting to go to the 4bbl intake? I assume $$ is a big factor for you.
 
I am waiting to find out the part number/brand. My Dad was going to go by the machine shop today and find out.

As far as the cam. Since the motor is going back with a 2bbl we planned to use (?)1976(?) 360 2bbl grind cam. I know that is not much of a cam but I don't want to over cam the motor since my Dad wants the 2bbl. I am restoring the car for him to drive (and I will drive it when I go up to visit them). It will get changed to a 4bbl at some point. The car will never see the track at all (building a 70 Duster later if I decide I want to make a trip to the track). But I still would like to get some decent performance. Maybe just a little more torque. The car will be driven on some 250 mile trip to car cruises and things like that. It will have working A/C and we will be using the A/C :)

Money a factor.. Yeah you could say that!! :) I left my job/career 14 months ago and my wife and I moved to Fort Worth so she could attend medical school. We planned for me to take a year off so I could help with the household duties and try to make it as easy on her as possible for the first year. I will be working again come August!! I am going crazy sitting at home. Not to mention that I have bought 4 cars since I got my last paycheck!! :) Now I need some more money to restore the two that need it!! :)
 
That being the case, I would suggest letting the machine shop use the pistons they have. That is as long as they are not just plain cast aluminum. For that $140 price, they had better be Hypereutectic. If you want higher compression later on, you could always get a set of 302 heads from a mid 80's 318. They have a smaller combustion chamber than your original heads.

What are you planning to use for the exhaust?
 
If money is a big deal, and cruising is all you plan to do, then use the cheapest thing they can get. Dont sweat the details if details are not critical. a bone stock rebuild will suit you fine.
 
I just talked to my Dad and he said the machine shop pistons were Sealed Power. My dad did not ask if they were cast. :(
And I have not found anywhere on the net that I could price Sealed Power 8.5 to 1 pistons for a .030 overbore??? Anyone know where I can find out the price so I can see if I am getting the shaft???
 
Sealed Power, Sterling and Speed Pro are all brand names from Federal Mogual for the same pistons. Hughes Engines web page has info on these pistons.
 
You must log in or register to reply here.
-

Back
Top