help me ID this block

VTStang66 · Aug 3, 2007

It's a mexican block 302. I found this thread about decoding numbers, but according to that, the ID number is supposed to start with a letter. Mine starts with a 7.

75ZY-6015-AA above the starter

5-J5 on the other side

and "HECHO IN MEXICO" on the top

Do the mexican blocks have a different coding system than the domestic blocks? What can I tell from these codes?

fasttback · Aug 3, 2007

The "7" does seem odd. Both Mexican blocks I have had started with C8 or D1. That block does have the "Hecho in Mexico" in the valley, as did both of mine. The big ticket item would be the main caps, pop off the oil pan and take a pic of caps.

D.Hearne · Aug 3, 2007

Other than it's clearly a 75 Mex block, not much.

VTStang66 · Aug 8, 2007

What is the deck height for a 75 mexican 302? I just talked to speed pro (pistons) and they said that the pistons that I have in my block (L2484 0.030) won't give me 10:1 compression no matter what my combustion chamber volume is because the deck height is too high. He said that the compression figure in jeg's (9.09 w/ 63cc head) is for a newer, lower deck block.

To me that sounds like these pistons shouldn't get all the way to the top of the cylinder. But I measured them at TDC and it's about .035" away, which seems pretty close. If the piston were designed for a lower deck block, wouldn't it be sticking out of the top of this one? They're flat top pistons. He said the valve reliefs are 2.7cc, which seems too small too. Here's a pic.

SoCalCruising · Aug 8, 2007

.035" is a lot. What you shoot for is .040-.045" from the piston to the deck of the head WITH the head gasket in place. If you are using a typical .040" (compressed thickness) head gasket, then that's .075" you have, and that's a LOT. This lowers compression and leads to detonation sooner than a motor with .040-.045" piston-to-head clearance.

If you want to raise compression properly, you should have the block decked to give you +/- .005" piston-to-deck clearance. Then you'll be right around .045" from the piston to the head after you torque the heads down.

To see the effect of this, go to a compression calculator and enter in the .075" number where asked and see what the compression is. Then keep everything else constant and just change the clearance to .045". It's a little over half a compression point. Or, you will be down to about 8.8:1 with .075" clearance and nearly 9.5:1 when at .045". THEN, you can have the heads shaved to 55-56cc to get to 10:1. With your present clearance, too much would need to come off the heads.

Got it?

D.Hearne · Aug 8, 2007

You've also got to remember this in choosing 302 pistons, not all 289/302/5.0 flat tops are the same. The compression height varies from 1.585 to 1.610, that's a .025 difference. Deck the block .020 and the heads another .010 and you'll move em closer to where you want. Or go more, you can remove about .040 total before having intake fitment problems, and then it's usually just a matter of elongating the intake bolt holes to compensate for the head bolt holes moving toward each other.

SoCalCruising · Aug 9, 2007

Yeah, good point about intake fitment. I've been fighting that with the three intakes I've used and my deck is just 8.195". Re c-h, the same is true for 331 pistons. They can be had with 1.165-1.175" c-h, depending on supplier.

VTStang66 · Aug 9, 2007

Well according to the guy I talked to yesterday from speed-pro, these have the 1.605" compression height, so that's good. How do I measure deck height? Thanks for all the help.

It's crazy that all this time I thought I was running around 9.5:1 compression and it was closer to 8. I was having detonation problems with less than premium fuel and it turns out it was caused by the distance from the piston to head? How about that.

D.Hearne · Aug 9, 2007

If those pistons in the Mex block have a 1.605 pin height, then your comp ratio is at least 9 to 1. But the final ratio depends on all the other factors. Deck clearance (measure how far below the deck the piston top is at TDC, you can use a straight edge and feeler gauges for this)For each .010, you'll have about 1.5 to 2 ccs volume to add. A .040 thick gasket yeilds a 9.5 cc volume, figure a bit less for thje smaller bore of the cylinder. Add the gasket volume, chamber volume and then another 5 ccs for the piston top volume. With 1.605's you can easily get to a zero deck or even as far as having the piston poke up out of the hole, depending on how tight you want to stretch things. I'd guess with 54 cc heads,on that short block you have pictured, you'll have a 10.5 to 1 ratio easily. To get it down to 8 to 1, you'd have to have the 70 cc D8 heads on it.

SoCalCruising · Aug 9, 2007

D.Hearne: he already measured it to be about .035". Seems to be a lot for that piston's C-H.

VTStang66: I don't know how to easily measure deck height. I took mine to a machine shop, after measuring piston-to-deck height, and told them where I wanted it.

D.Hearne · Aug 9, 2007

If he's got a -.035 deck clearance, something's not right. Add all together and you come up with a total of 8.200. Nominal deck for a 302 is 8.206. For him to have a deck clearance of .035, you'd have a block with an 8.235 deck height. Of course, being as it's a Mex casting, that could be right, I've never looked at one personally, but have read that the quality was sloppy on these. His pics don't look like it's got a piston .035 down the hole, looks more like .0035, same as the slugs on my 331.

htwheelz67 · Aug 9, 2007

I think that is a mexican block out of a mexico car..........those are not normal casting #'s

VTStang66 · Aug 10, 2007

D.Hearne said:
His pics don't look like it's got a piston .035 down the hole, looks more like .0035, same as the slugs on my 331.

Nah, 35 thousandths on the dial caliper. The speed pro guy said that with a 75 302 block (I didn't tell him it was mexican), I have the taller deck height, and my pistons should be about .034" from the top, which is in line with what I measured. Maybe even closer. I should have asked him what that height is, but I didn't.

D.Hearne · Aug 10, 2007

The only book I can lay hands on at the moment lists it as 8.229. Take it apart and mill the decks. It might need that anyway to get em square to the bores.

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