Newb, I need some strange help

[Mr_Random]

Okay, first of all I understand this site is devoted to the "six cylinder" jeep stroker, but I have some questions on it's little brother... the AMC 2.5L... oh and be prepared for a long post!

First off, I have an 86 XJ 2.5 that I have been planning *forever* to upgrade to MPI (and yes I believe I can do it, it has been done before), unfortunately the engine crapped out about the same time I finished gathering parts. I have since acquired a replacement engine with 200K on it, the owner had just barely bought the vehicle and was only using the axles (for the 4.10 gearing), he said it ran great (despite some jurry rigged wiring).

To the point: I got the engine on the stand and started stripping (it, not me!), and quickly noticed it had recently been rebuilt and has larger pistons installed. Now, not only does this excite me (because I don't have to spend as much cash), but it also intrigues me; what more could they have done to it?

Right now the ONLY thing I know for sure is that it has .030 over pistons (as labeled directly in the dish). The dish itself is pretty deep, larger than stock, but I have no clue (or guess) on size. For now I will assume this is the only difference from stock.

Okay, first question: how can I determine whether the block has been shaved? Trouble is, among other things, I don't know the stock gasket thickness OR piston-to-block depth (can't remember name right now), so I can't calculate quench. I know a depth gauge will work, but I have no baseline (stock) measurement to compare it to...

Also, wont the oversided/dished pistons bring down the compression ratio? Stock is 9.2:1, which seems pretty high for being able to run 87 octane (85 in high alt). Could this mean the stock head gasket on the 2.5 is thinner than the 4.0 (.051), in order to have a better quench? I know for sure the XJ 2.5 has a .1:1 difference compared to the YJ 2.5 (9.1:1), so either the head is shaved (a possibility), or the gasket is smaller (more likely)

The Crank: does anyone know (or care) if there is an aftermarket "stroker" crank available, other than one-off pieces? IF I'm INCREDIBLY lucky, the PO/previous rebuilder installed one... but if not, I'd like to instead. My original budget for the rebuild is nearly five times what I need now that I know everything is VERY new (within 25K for sure, if not less). I would like to invest the money saved into performance, so a larger crank would make me happy... anyone have advice here?

CAMS! I want the Hesco RVOB4 cam, but hope the PO has already installed something aftermarket. I have heard great things about this cam, and know that aftermarket 2.5L cams are much more reliable than 6 cylinder ones (something about rotational mass and amount of torque put on the length of the 6 cylinder cam makes aftermarket ones unhappy, I believe). Anyone know of viable alternatives?

Lastly, am I just crazy? I don't think I am, but am I truly? I could make a 3.0L stroker from this with only 600 dollars! I don't think an engine swap, or new vehicle would be nearly as rewarding, or as cheap!

Thanks for bearing with me... Oh, and Muad'Dib, I love Dune... I'm rereading it RIGHT now, I read it when I was young and didn't quite understand it... now I can actually *read* it, it's quite enlightening!

Thanks again!

-Random

[Mgardiner1]

I don't want to rain on your parade, or sound like a downer.....

Wouldn't a stock 4.0 put out more power then a worked 2.5? I know you wouldn't have NEARLY the bragging rights...

Also, don't assume that the engine is loaded with performance items because it has .030 pistons. Chances are, it may have had a really bad failure, or been rebuilt more then once. Only way to be certian what "other" parts are in there will be either from the previous owner, or casting numbers off of the parts.

[Mr_Random]

yes... but refer to the second to last paragraph; the cost of a swap or buying a new jeep (which I actually wouldn't mind) is much more than the upgraded engine.

I stated that the only thing I'll assume (because it's true) is that the pistons are larger, anything else will have to be figured out with more digging. I highly doubt that anything else is changed, that's why I'm researching and setting up a budget... This was a dream a couple years ago, but it's looking more possible now, so I'm excited (but a little apprehensive).

Hum... well, even if It doesn't go anywhere, it'll be a learning experience!

[John]

Okay, first question: how can I determine whether the block has been shaved? Trouble is, among other things, I don't know the stock gasket thickness OR piston-to-block depth (can't remember name right now), so I can't calculate quench. I know a depth gauge will work, but I have no baseline (stock) measurement to compare it to...

Look on the top of the block and the bottom of the head. We stamp the amount removed. .005 .010 etc. should be present near the edge of the castings if material has been removed.

Also, wont the oversided/dished pistons bring down the compression ratio? Stock is 9.2:1, which seems pretty high for being able to run 87 octane (85 in high alt). Could this mean the stock head gasket on the 2.5 is thinner than the 4.0 (.051), in order to have a better quench? I know for sure the XJ 2.5 has a .1:1 difference compared to the YJ 2.5 (9.1:1), so either the head is shaved (a possibility), or the gasket is smaller (more likely)

CC the piston dish and measure the the distance below deck at tdc and we can help with the CR estimates. measure the gasket height when you tear down.

The Crank: does anyone know (or care) if there is an aftermarket "stroker" crank available, other than one-off pieces? IF I'm INCREDIBLY lucky, the PO/previous rebuilder installed one... but if not, I'd like to instead. My original budget for the rebuild is nearly five times what I need now that I know everything is VERY new (within 25K for sure, if not less). I would like to invest the money saved into performance, so a larger crank would make me happy... anyone have advice here?

CAMS! I want the Hesco RVOB4 cam, but hope the PO has already installed something aftermarket. I have heard great things about this cam, and know that aftermarket 2.5L cams are much more reliable than 6 cylinder ones (something about rotational mass and amount of torque put on the length of the 6 cylinder cam makes aftermarket ones unhappy, I believe). Anyone know of viable alternatives?


That would be a good possible choice for a cam.

I read about someone building a NA 2.5 and going bigger, I will see if I can find it. IT wa on another forum.

John

[Mr_Random]

Look on the top of the block and the bottom of the head. We stamp the amount removed. .005 .010 etc. should be present near the edge of the castings if material has been removed.
CC the piston dish and measure the the distance below deck at tdc and we can help with the CR estimates. measure the gasket height when you tear down.

That would be a good possible choice for a cam.
I read about someone building a NA 2.5 and going bigger, I will see if I can find it. IT wa on another forum.

John

The gasket went to pieces when we pulled the head, so no dice on measuring it. Would the stamp be on the perimeter of the engine? Or on the inside? Once I clean up everything I'll take a look for that stamp.
I'm pretty set on the cam choice...

Yeah, I can think of one other person who has done similar work, yjmechanic from 4bangerjp.com, but I don't know if he actually did any crank work... There is someone else who actually stroked his 2.5, with .060 over and a new cam, he got it to 3.2L I believe, but no one has heard of it for a couple years. He was on the same website, but I don't remember the name.

Well we know it has been done, now to figure out how... I'll research some on a crank...

Edit: found a little bit, but it seems iffy (from the 4banger site):

Building on the earlier stroker thread on this board;what about the following combinations.

block bored .060 over
crank journals offset ground to 1.97" dia and hardened--but not a straight off one side offset ground--an offset ground with a stroke of 3.55"
2.4l dodge rods 5.945" length
custom pistons with .8271" pin (or stock piston w/bushing?)

Resulting displacement would be ~172 CI
I'm not sure of the feasibility of grinding the 2.5L crank down that far or of the width of the 2.4l rods.


Another combo I was kicking around is as follows:
block bored .060 over
crank journal straight offset ground to 2.000" (3.606" stroke)
(6" sbc rods and 2.5L piston with .083 milled off piston dome)
or
(5.9" sbc rods and the block .015" decked)

This combination would also net about 175 CI but would of course require aftermarket rods. However I think a slightly longer and otherwise stock sbc rod should have a .9281 wrist pin 1" pin end width and .940" big end width;I believe these are the same as AMC 150 parts.

Any ideas?

This might be beating a dead horse but there has to be a cost effective way to make this engine bigger.

I also remembered that the last person I talked about, who also bored .060 over, said he went through a company and basically told them he needed a 258 crank with the middle two pistons removed. I believe he got an 8 counterweight out of it... but I'll continue researching...

[John]

"The gasket went to pieces when we pulled the head, so no dice on measuring it. Would the stamp be on the perimeter of the engine? Or on the inside? Once I clean up everything I'll take a look for that stamp.
... "

Normally on the deck surfaces, that way if you remove more, you erase the old numbers, add the value to the newly removed amount and restamp.
John

[Mr_Random]

Hah, sounds right smart... well thanks I'll check when I clean it!