Mary'sMyBaby

I'm rebuilding my lg4 with a larger cam, matching valve springs, new valve seals and ported 416 heads. Just curious if there is a set of pistons that is relatively cheap, that will boost my compression a little. I have found some but they run $4-500. I know the 305 isn't a great performance motor but I'm doing this to see what I can get out if it. Also any other suggestions on parts to throw on for some power would be appreciated. Thanks

big hammer

here's some .030 over keith black hyper's, flat tops. $240

http://www.summitracing.com/parts/ue...rview/make/gmc

they don't give an actual CC, but using a -8 CC for the 4 valve reliefs, a .020 deck height and 58cc chamber, that will put you around a 9.6:1 static compression ratio which is about nice. unless someone can correct me on the actual CC's of the piston. but -8 should be ballpark.

Mary'sMyBaby

That's the thing though, I've seen some of those on sites, but my bock is standard bore. Is there any that would fit a standard bore?

big hammer

you would have to get your block bored out. which is a good thing to do on a 305, it helps unshroud the valves.

big hammer

here you go. stock bore flat tops. $200

http://www.summitracing.com/dom/part...rview/make/gmc

Mary'sMyBaby

Okay, thank you. Will either of those up my compression from its current ~9.5:1?

big hammer

if you're already 9.5:1 then I would leave it there.

Mary'sMyBaby

Okay. Thanks for the insight. Much appreciated

big hammer

you're welcome. as you approach 10:1 your engine will be much more fuel sensitive and tune sensitive. especially with iron heads. you HAVE to run 91 or better.

blueprint your engine to ensure you are 9.5:1. check your block deck height, make sure you use the proper head gaskets, etc. block deck heights.

for example, if you use the stock pistons, (12cc?) 58cc heads, 4" bore headgaskets, and end up having a .025" deck height, you could end up at under 9:1 compression. all these factors come into play.

Mary'sMyBaby

Okay thanks, I'll check up on all those. What I've previously found says that an 86 LG4 should be at 9.5:1 but like you said, it could be less

LB9GTA

iTrader: (1)

Just remember that if you get new pistons it will cost you about $15/piston to press off the old piston and press on a new one.
Then you'll need a new set of rings.
Are you putting on new main and rod bearings too?

Mary'sMyBaby

Well I figured if I do this pistons i will have to replace those, but if I don't, then I'll probably just leave them. I'm most likely going to stick with the top end build for now, due to cost reasons, and I will do the bottom end later.

drdave88

iTrader: (3)

It would be wise to have the cylinders measured before you just go buy pistons and slap them in. You may have out of round cylinders and need the block bored.

ASE doc

What I have found is that new pistons makes a big improvement in piston/bore clearance all around and helps new rings work much better. The aluminum piston, especially at the skirts wears faster than the cylinder wall does. You need to be sure to cross hatch, de glaze the cylinders when you replace rings. This includes pistons and rings. The new rings will never work properly if the cylinder walls are glazed over from many miles of wear. To do this you need a ball hone, especially if your cylinders are worn and you have the typical ring ridge at the top. This ridge will prevent the cheapy 3 stone cylinder hones from making full contact at the upper portion of the cylinder, where you really need to remove glazing. Ball hones are expensive. You might see if a local parts store will loan or rent you one. A ridge reamer is also available for removing the ridge from the top of the cylinders but I broke mine years ago and have done quite a few in car overhauls without it. Never had an issue. The only way to properly build a motor is to send it to the machine shop. If I find severe ridges, the block is coming out and going in for machine work.

rusty vango

I would zero deck the block and use flat top pistons. when I rebuilt my 305 I blue printed the block. and I ended up with a good, strong, long lasting engine. this area is where attention to the details, wins the day.

big hammer

this.

ASE doc

Couldn't agree more.

LB9GTA

iTrader: (1)

What's it cost to zero deck a block?
Do you need to get shorter push rods once it's decked?

big hammer

could get the crank offset ground instead....

rusty vango

not generally. it really does not cost more to take the block to the factory blue prints that it was designed with. than it does to over bore one. and the benefits are well worth it IMHO. generally speaking you are making the cylinder decks a true 90 degrees from the crank center line. along with making the rods all the same length. as well as making the cranks throws true, which most stock cranks I find not to be. the end result is you get a smoother running, longer lasting, more efficient engine for your money. a win , win !!

ASE doc

I think it's important to note here, both quench and final compression ratio are largely affected by deck height. Decking the block gives you the opportunity to set quench. The ideal quench is .040", but anything less than .050" is good. .040" quench will produce better detonation resistance when you start pushing compression upward. Quench will be the total of deck height, and head gasket compressed thickness.

Once you determine deck height, you can factor that in to determine compression ratio. For compression ratio, you need to know deck height, head gasket thickness, total dome volume of the pistons(negative or positive), and the chamber size of your heads. A good quick way to determine final compression ratio is to use a calculator. Summit Racing, for one, has an easy to use CR calculator on their website. You can access it by selecting Engines and Components and then pretty much any piston, crankshaft, or connecting rod. CR calculator will be at the bottom of the part information page.