zero decking the block

[Dark_5.0]

In my quest to get my little naturally aspirated 302 in the 11's I am getting my block reringed and honed with new bearings and was wondering if zero decking the block to gain more compression was a good idea.

[Gearhead999]

Depends on what your deck measurement is now. Also head configuration. Valve to piston clearance.

How about a thinner head gasket??

If it is the typical deck that is a couple of thousands down in the block, I don't think you'll gain a whole heck of a lot. I could be wrong.

[Dark_5.0]

Yeah, your probably right.

[Jeff Chambers]

Zero-decking is an excellent way to increase the compression. You get the full effect for every 0.001" taken off the block compared to the head since a Windsor head is only a semi-open combustion chamber. I bring every block I've got to zero. Besides, it really evens out the cylinders. Its not uncommon to see 0.007-0.008" variation from corner to corner of a stock block.

[HOTRODJIM1]

Zero decking is a good idea for raising compression and cyclinder ballance, don't forget to test fit your heads and intake before final assemblly. I had a block that was .022 from front to back. It took .025 to clean it up. They had to shave a little off the pistons to get the zero. I have been running that same block for 8 years. It is in my car now as a .030 over 351 and makes 405 to the wheels on motor. Good luck with your build!!!

[tmoss]

My factory deck ranged from zero to negative (piston out) .009". Don't forget that piston pin placement and rod centers affect the deck (even crank machining) - it's why racers select fit pistons and rods in the assembly stage

[andy669]

For maximum compression its better to run the pistons .005~.007 in the hole and select a small bore head gasket thats .030 thick. Or with an iron head you can have the pistons .020 in the hole and use a .015 steel shim gasket. Zero decking with a .037 gasket can give up a little because the bore size of the gasket is always bigger than the piston. But to do this accurately you need to have the block deck trued. This brings the deck perfectly parrallel with the mains, and makes sure both decks are set at 45 degrees. I have seen them be out as much as .020 from front to back and top to bottom.

Andy

[~The Jester~]

This is really interesting, two of the guys opinions I respect the most on this board (Jeff & Andy) have different ideas on this.

I ALWAYS take the "twist" out of my blocks, and since it's at the machine shop on the right machine, I've always taken them to zero deck, talking hot rods here, not daily driver engines I've built.

Andy brings up a good point though, if you read that closely and invision what he's saying there, it makes sense. Funny what comes out of the woodwoork when you're talking to guys that are in a heads up class, have rules to follow, and are trying to squeeze every ounce of power out of an engine!

Great thread going on here!

[Jeff Chambers]

Andy may correct me here, but some of his experience is based on the NMRA requirement that they maintain a minimum quench of 0.038". Over in FFW, we have no such requirement so we do everything we can to get the compression ratio as high as we can. My TW heads have been milled nearly .200" to 38cc chambers, with oversized valves to the point that the intake valve is actually proud (hangs below the head surface). I use a Yates-style gasket and relieve the block a little locally to clear the valve, a 0.035-0.040 head gasket and zero-deck the block. At 9000 rpm, the rod stretch is actually enough to bring the piston 0.003" or more out of the block for even greater dynamic compression.

Heads up racing baby! If you want the trick stuff, that's where you'll find it.

[Dark_5.0]

Originally Posted by Jeff Chambers

Andy may correct me here, but some of his experience is based on the NMRA requirement that they maintain a minimum quench of 0.038". Over in FFW, we have no such requirement so we do everything we can to get the compression ratio as high as we can. My TW heads have been milled nearly .200" to 38cc chambers, with oversized valves to the point that the intake valve is actually proud (hangs below the head surface). I use a Yates-style gasket and relieve the block a little locally to clear the valve, a 0.035-0.040 head gasket and zero-deck the block. At 9000 rpm, the rod stretch is actually enough to bring the piston 0.003" or more out of the block for even greater dynamic compression.

Heads up racing baby! If you want the trick stuff, that's where you'll find it.

Nice post Jeff.

Your set-up is very hardcore.

"Heads up racing baby! If you want the trick stuff, that's where you'll find it."

I love this quote.

Ok with a zero decked block with .566 lift 224 duration Exhaust and .566 lift 218 duration Intake @.050 cam, Stock pistons and 2.02 1.60 valves on my Canfields.

The machine should be able to open up my valve reliefs enough for everything to work, right?

[andy669]

Kinda yeah, kinda no, Jeff. I never really understood why they put that rule in there. Btw, its gone from this years rules. Even at .035 quench everything I have built still has a combustion chamber mark on the piston. I dont feel comfortable running them any tighter.

All I was saying was, since you have to have a minimum quench distance, for max compression its better to have that distance be of as small a bore as possible.

Andy

[tmoss]

I thought you both have been very clear about what you were saying..........

[~The Jester~]

Originally Posted by tmoss

I thought you both have been very clear about what you were saying..........

Me too!

Jeff's second post actually clarified a few things on how those Street Warrior's run like they do!

Then again, Andy has a valid point as well. Just for the record, and those of us that DON'T know, how does NMRA measure quench distance?


I DID NOT intended to put Jeff and Andy "against" each other, but I gahrun-dam-tee yah, I'm gonna sit back and learn what I can! Osmosis at it's best right here guys! Pay attention!

[andy669]

Originally Posted by ~The Jester~

Just for the record, and those of us that DON'T know, how does NMRA measure quench distance?

When it was in the rulebook they measured it the same way you or I would: Mic the headgasket and use a bridge to find the relationship between the top of the piston and the deck surface.

Andy

[Rev]

On my engine with forged TRW flat top pistons, I think the pistons (cold engine) are in a slightly different position each time I check with the bridge and with the dial iondicator. I get a .000 to .005 " down in the hole after I crank it a few turns. That's for the same #1 piston at TDC on the balancer. Also get some slight variation depending on what part of the piston top I measure from. I don't think that variation actually makes any performance difference, but it it is difficult for me to say exactly what the deck height measurement (how far down in the hole) my pistons actually are at TDC.

Rev

[Jeff Chambers]

This is because there is actually a dwell at TDC. You need to check for TDC at some distance before and after TDC....say .050 before and .050 after (coming back at it). Then split the difference and you'll really know where TDC is. Otherwise, you can any where from 3 to 8 degrees of dwell at TDC depending on tolerances, clearances, etc.

[tmoss]

I check for TDC on the short block with a dial indicator right on the flat of the piston. Note when the dial stops rising and where it starts falling and TDC is center of those positions. I have a large degree wheel set up too. I also find TDC starting from both sides of the curve. When I have the dial on true TDC, I index the timing marker.

[Gearhead999]

Have you pushed the connecting rod up to zero out the bearing oil clearance?

[~The Jester~]

Since I'm back down to a bare block, I'm LOVIN this thread.

HARDCORE gear heads talking about things that I never would have even thought of 5 years ago!!!! Way kewl!

Since I'm already at "0.000" deck, I really have no choice on this motor, but the next one..... Hmmmmm!?!?!?!???

BOTH sides have made a substantial arguments in their favor, and I respect both sides of the equation here. This is literally a tough call.

Like I said before, pay attention guys, you're gonna learn yourself something on this thread!

[Jeff Chambers]

No argument here. There's quite a few different theories on setting up a motor, but if you really want to see the tricks pay attention to heads up racing. Not intending to offend anyone, but bracket racers don't normally go to the extents that a heads up racer will.

Just dropped my pistons off at the engine shop today. Beautiful JE customs with all the tricks, titanium casidium coated wrist pins and Total Seal diamond finish ring set. Hell, the ring set alone cost more than most people spend on a full set of pistons. Just had the block punched for roller cam bearing.....sweeeeeeeeeet.