deck

[brokenujoint]

ok my machinist called me and said that my shortblock was ready for pickup. we started talking and he said that he didnt like how my pistons when at all the way up on there stroke didnt sit flush with the deck... they were under . didnt say how far.. is it correct that i want there to be zero deck clearance as in the piston and the top of the block to be perfectly even? zero deck height, then the gasket height will be the given deck height to allow for quench and cr measuring right?

not a race engine, but do i still wan zero deck height? 707 rods, 677 pistons, stock 96 cherokee block....

[Bodo]

depends on what compression you want and what octane fuel you want to run.

[SilverXJ]

Find out how far below deck the pistons really are then use the Compression calc here to figure out that the compression is then go from there. If you didn't have your pistons dished you may be pushing the pump gas ping level. Ideally you would want to get your quench as close to .040" as possible, which cam be done by 0 decking and using the .044" gasket.

[brokenujoint]

i want as close to 87 as possible. pistons arent dished. will be polishing the bowls of my head though.

[Flash]

i want as close to 87 as possible. pistons arent dished. will be polishing the bowls of my head though.

Were did you find flat top piston? i thing it vary important to have a dish piston, and then, be able to have a much better piston to deck or Quench height.

[SilverXJ]

I think he means that he didn't have them dished himself.

[brokenujoint]

they are sealed power 677p +0.020 #STL-677P20 pistons.. no machine work done to them. mated to amc 258 rods. 707 casting .. pn 3180444. bolted to a 258 12 weight crank. block is a 53020569 cherokee/tj 1996-98 block that was resurfaced but not decked.

should these pistons sit below deck height or be flush or above or what?

[RAPTORFAN85]

they are sealed power 677p +0.020 #STL-677P20 pistons.. no machine work done to them. mated to amc 258 rods. 707 casting .. pn 3180444. bolted to a 258 12 weight crank. block is a 53020569 cherokee/tj 1996-98 block that was resurfaced but not decked.

should these pistons sit below deck height or be flush or above or what?

The pistons can sit below or at zero deck. The optimal is to have zero deck but by doing so you are going to raise the compression. The pistons that you have are dished to 14cc and with zero deck you are going to be around 8.5 DCR with the stock cam. Thats going to be pushing it for pump gas. Your only options if the block has already been decked are to dish the pistons or add some cc's to the head to get your compression back down into the pump gas range, around 8.0 DCR. That's going to be around 6cc's if my math is correct.

It's not a big problem with your build if the pistons don't sit at zero deck, the stock pistons are in the hole around .020 if I remember correctly so it's not a big deal as long as they aren't like .040 in the hole. Then you would end up with a really sloppy quench.

[brokenujoint]

so if the pistons do sit at around .020 if i polish the combustion chamber down on the head, that should give me sufficient quench and deck height? btw i have a victor reinz head gasket.

[omegatron]

I'm a noob so I could be completely wrong....but, I thought that the beneficial effects of quench decrease significantly @ .060 and above. It seems to me that the people on this sight that have their quench dialed in around .040, even with higher compression, don't really experience pinging. I know there is a lot of other factors, but it appears that people shooting for a dcr of 8-8.5 benefit from a quench as close to .040 as possible.

Where a few have a dcr of 8.5ish, quench above .080 and ping....

[SilverXJ]

so if the pistons do sit at around .020 if i polish the combustion chamber down on the head, that should give me sufficient quench and deck height? btw i have a victor reinz head gasket.

With the clearance at .020" and if you have the thinner VR gasket (.044") your quench will be .064"

[brokenujoint]

just got back from the machine shop. deck height is at .021

[RAPTORFAN85]

just got back from the machine shop. deck height is at .021

.065 is a pretty decent quench. I would polish the chamber to help with preignition and to add a few more cc's. Right now you would be around 8.25 DCR witch may be OK for mid grade or premium gas.