I started a cylinder head swap today. Taking off my 882s and replacing with a pair of Darts. While doing some clean up, I saw part numbers on the piston tops so I jotted them down to look up later. While I'm at it I tried to measure the piston deck hight.I kept coming up with .045. I kept thinking thats not right, so I looked up the piston number (345np) and discovered those pistons are destroked .020. Now what, my quench is shot. With a shim gasket I'll be at .060 and only 9.5 to 1 compression. The gaskets I already bought were .028 thick but i can,t use them now. Am I screwed or what? What do you guys think?

 

I'm a total noob at this, but can't you have the deck planed?

Would require pulling the engine :-/

 

The engines in the car so thats out of the question.

 

I don't know of anything else. I'm guessing you can't mill the heads enough to get where you need. I'm also currently working on swapping Dart heads (and a new cam) into l-48 and I hope I don't run into something like this...

 

So what exactly is your concern?

Was the car was running okay with the 882 large combustion chamber heads before you removed them?

Are your Dart heads smaller combustion chambers than the 882's?

Did you make a cam change or other mods that require a tight quench?

 

Things Never work out like you want. Roll with the punches.

 

Quench at .060, I've read here that .040 is optimal. And yes the darts are 64 chamber heads. No cam change.

 

If the pistons are good for forced induction put a supercharger on it )

 

GM makes some .015" ones. About as close as you're going to get.

JIM

 

If you put the .015 gaskets on better make sure that deck is clean and totally flat.
Thin gaskets are great to help with compression but leave little room for error on the mounting surface.

 

When you say desdroked Im gonna assume you mean they have .020 less compression heigth.
Way I see it you have 3 chioces, put it together and see how it runs, buy new pistons or have the block decked. Id go with the 1st and if things dont work out the move to the 2nd option.

 

I agree with dugs…..it will probably run fine…just might not get the HP increase you were looking for. Decking the block that much is tricky….the intake might not fit without being machined too.

 

Thanks for the feed back. If the motor was out of the car, I would just change pistons.
I was thinking, easy project, swap heads, I've done it 100 times in my younger years, nothing to it. I didn't figure on destroked pistons. Didn't know they existed. Another lesson learned. I wanted to stay away from a .015 gasket because of the perfect surface requirements but I have no choice now. I'll give it a shot and see what happens. I worry about getting stranded away from home with a blown head gasket.

 

The pistons are not "destroked." They are just a cast stock replacement piston using the factory-spec compression distance.

Using your existing components and without blueprinting the block, rods and crank, you will not get a zero deck height in all eight holes even with new pistons with a .020 higher compression height. I have never seen a factory assembly come in at zero deck and certainly not on all eight. Some by design, some by stacking tolerances but the norm for an assembly line engine is a negative deck height. The cylinders are never exactly perpendicular to the crank centerline, the crank throws are not precisely indexed, the decks are never perfectly square, rod center-to-center distances vary and piston compression distance vary. And then you get to the imprecision in the cylinder heads from chamber to chamber. If you were building an engine where cost was no object, then these are the things you'd attend to.

You are getting wrapped around the axle over what's considered an optimum value. In the real world where cars are driven day in and day out, it's really inconsequential.

 

Hamck, you still haven't answered John's question. What chamber size are the Dart heads that you have? In my opinion, that's way more important than deck height and gasket thickness, because that is the basis for your whole top end build, period. If you have the L48, with the dished pistons, and as an example, you went with 64cc heads, you're only going to see moderate gains over the 76cc heads that are on the L48, 882 heads. If you wanted a really significant gain in compression, keeping the stock dished pistons, you would've chosen the 58cc heads. The aluminum L98 heads are nearly the perfect match for this. 58cc combustion chambers, D port exhaust and are identical to what was on the ZZ4 350 HO motor, that GM sold through their Performance line. That and a good 262 or 268 cam, would've gotten you a very fun combination.

Finally, putting a new top end, on an old bottom end, is only going to delay the eventual event of having to pull the engine anyway. The increased compression will eventually put a strain on the old piston rings and you'll start getting oil blowby into the combustion area. I did just what I described above, to my L48 and got two years of fun out of it, but we found a wiped cam lobe and decided to pull the engine, finding burnt oil deposits in the,combustion chamber and on top of the pistons. And it was still capable of laying 75 feet of rubber on the road! But eventually, the oil consumption and deposits, would've had an effect on the performance. So go ahead and do the top end swap, just be aware, that eventually, you'll be going back. We knew that from the start and had a blast with the car.

 

you may not have the optimum quench you're looking for but a bump in compression over the stock heads will be a plus! The .015 head gasket is risky because you are not working on a freshly machined block but if you really take your time cleaning it and get a true machinist flat edge bar to check the deck for straightness, then you should be ok!
Second option is to deck the heads .020 I personally would go that route just so I could use a better gasket! Don't go much farther on the heads because you will start to run into intake manifold alignment problems.

 

.045" sounds a little big for even a factory engine. alot of guys use the .015" steel shim with the deck height of usually .025" from the factory. I have never even heard of a destroked piston, as you know the crank shaft determins stroke, maybe take a picture of the piston & post it so that we can see it. A quench of .060" is a little big, but if the pistons your running are dished, there's not a lot of piston to affect the quench anyway. You might be able return the gaskets you have and get the .015" shims, but given that the engine is this far apart I'd pull it, chanbe the bearings get it bored and use the new pistons and get the block decked so that you can set the quench to .040". If you do go this route check out the total seal gap less rings.

oops!

 

Just wanted to add, I see you already have the 64cc heads. That will widen your choices, when it comes time to rebuild the bottom end. The 58cc's are usually flat tops, while you get a choice of domed pistons with the 64cc's.

 

Ive never heard of a destroked piston also, but here goes, Piston no. 345np is cast, flat top with 4 valve reliefs, 5 ccs. From top of the piston to center of pin is 1.548. Stock L-82 pistons (Witch is what my engine was origionally) has a distance of 1.567. A difference of .019 thou. 25+19= 44 A local shop put it together for me 5 years ago, may be 4000 miles total. The bottoms good. 882 heads weak. The things you learn with this hobby is amazing.

 

I would like to add, I got a set of 1094s, so I think ill clean the block the best I can, drench em in copper coat, and go from there. I've got AAA