Starting my build

[dwg86]

Don't the OE pistons have the tallest pin height of all the 4.0 pistons?[/quote]

I believe they do. Something like 1.603 compression distance?

[SilverXJ]

I believe they do. Something like 1.603 compression distance?

According to my 2000 FSM that is correct.

[John]

Don't the OE pistons have the tallest pin height of all the 4.0 pistons?

I believe they do. Something like 1.603 compression distance?[/quote]
4.2 and early 4.0 have compression height of 1.651 - 1.655"
the later 4.0's are 1.599 - 1.603"
This is the 89 FSM and the 99 FSM as source.
John

[oletshot]

Deck height for the newer block is 9.450-9.456, so the average height would be 9.453. The piston, rod and crank assembly adds up to 9.426 using the stock piston, which comes out to .027" deck clearance. Now the renix block has an average height of 9.432, which would come out to .006" deck clearance.
My deck height numbers come from the FAQ section on blocks in this forum. My piston assembly dimension is 1/2 stroke plus rod length plus piston compression height or:
1.9475 + 5.875 + 1.603 = 9.4255
It sounds like you may have an older block than you think. Check your casting #'s to the FAQ section on blocks.

[oletshot]

4.2 and early 4.0 have compression height of 1.651 - 1.655"
the later 4.0's are 1.599 - 1.603"
This is the 89 FSM and the 99 FSM as source.
John

I can't see how the early pistons had a compression height of 1.653", the early block had a shorter deck height. Either I'm posting while drinking (which I am) or that combo would be coming out the top of the bore. 1.7065" + 6.123" + 1.653" = 9.4825", that's about .050" bigger than the 9.432" deck height of the older blocks and .030" bigger than the late blocks.

One nice thing is, if I'm missing something here I can always blame it on the beer!

[Rob92XJ]

OK a stock replacement piston should in theory have the same pin height as a OEM piston or am I wrong.

[gradon]

If you look in the available piston thread/sticky in the FAQ subforum, you'll see quite a few different compression heights.

[Rob92XJ]

The block is out of a 96 XJ the original owner never had an engine replacement(they only had it serviced at the dealership I work at). The block is stamped NVH and has a main girdle. When I get it back from the machine shop I'll take pics of it I should have before I dropped everything off.

[oletshot]

From my '90 FSM, 4.0L piston has a compression height of 1.599-1.603", the same as the late piston. The 1.651-1.655" did match the 4.2L piston.


So that leads me to my next problem. To figure out theoretical deck clearance I'm taking 1/2stroke+rod length+piston compression height and subtracting that from the deck height. So that gives me this:

3.413/2 + 6.125 + 1.601 or 1.7065 + 6.125 + 1.601 = 9.4325"
The '90 FSM says deck height is 9.429-9.435. 9.432 would be the middle of the range. Is it me or is that zero deck clearance??????
The FSM states clearance is .0215, anyone see where I have gone wrong?

[Rob92XJ]

Like I said when I get everything back from the machine shop I'll bring my camera to work and post some pics of it.

[John]

Oletshot I just checked 3rd edition of Mopar performance book and also find the 1.651- 1.655" inch listed for the 89 motor there, so it looks like that was a 1990 change.
John

[rradford9]

Have you double checked the measurements with anything else other than the tool you linked to? Are you sure the tool is calibrated correctly? My engine builder used the same thing to measure my deck clearance after installing the pistons. He measured .013 and according to my calculations, similar to what oletshot is posting, it should have been around .0385. Well, I left that day thinking I'd f'd up somewhere and I'd have to send my pistons back out to be dished more to compensate for the increased CR. I called him back and had him re-check with another tool. Turned out the original tool was out of calibration (maybe due to the cold weather????) but after checking with a dial gauge and using a feeler gauge under a straight edge, we determined that the actual deck height was .040. I know its kinda a slim chance, but I thought I'd throw it out there...

[Rob92XJ]

OK here is a little background on me. I have been a service tech for Chrysler/Jeep/Dodge dealers for the past 16 years, I have also been a service advisor and asst manager but would rather fix vehicles more then customers. I used to work at a machine shop after school then after work to pay for my drag racing habit. I have built many strokers before mainly 347 small block Fords some Chevrolets too. My tools I buy are the highest quality because if I am measuring something I want it right the first time, I do check my tools calibration not many people send there torque wrenches(all of mine are Snap Ons) in every 10000 cycles like most good companies recommend.
So I know what I measured is 100% accurate. If I still lived in NY I would be doing the machine work myself, but since I don't really know many shops where I live now I need to rely on someone else.

[rradford9]

When's your stuff coming back from the machine shop? I'm curious to see how this works out. You may have found the golden combo! Have you used the compression calculator to check the SCR/DCR?

[Rob92XJ]

When I used the calculator I was going to be using a different block and I had originally planned on using the 4.0L rods(had them polished and balanced) but I got the 96 engine for free swapped a 98 ZJ engine into my friend Davids 96 XJ(his like mine was bought and sold through the same dealership during it's life) but my build has kind of been hurried but not really hurried(since I have my Ram) because my XJ's engine decided to finally blow, not bad considering it has 210000 miles on it and has been hydro locked 3 times the last time it had a light knock to it but ran good and the past 3-4 months has been developing a misfire that has now turned into a cloud of smoke.
As far as when my stuff is coming back I'm hoping it will be ready when I get back from Florida, since all next week I'm going to be in Orlando for training on the Durango/ Aspen hybrids. I have a list of things I need to have done on my XJ before April 1 since I didn't wheel all that much last year.