Topic: 2.5L Rebuild using a commercial 3.0L stroker?

[CaliGold]

Well RNandKT, the stroke on the 4.0 is 3.413" whereas the stroke of the 4.2 is 3.895".  The 2.5 is at 3.19".  

To offset grind amount for the 2.5 crank to the 4.0 stroke, would be about .110.  I would guess it would bring the motor to about 2.7L.  

With welding and offset grinding costs betwee 400 to 500 dollars, and some reasonable bookkeeping, it's probably not gonna be worth it unless you have a lot of disposable income.   Same with sleeving the cylinders for bigger pistons.  That's probably why there isn't too much information involving these engine mods.  Oh, the voice of reason...

I'm not in the position to pour money into a custom motor with unknowns, so I'll think I will spend the bulk of my expense into a good camshaft from either Hesco or Clifford.  Combined with .030 overbore, milling the head, and a little cleanup of the ports and combustion chambers, and like you mentioned, balancing and knife edging.

I think it could be a nice increase in power and torque while maintaing good reliability.  Even this and all the other parts involved in a rebuild would probably run about $800.  I would be happy with 20 to 25 extra hp.  Hopefully enough to get up those highway grades, and keep up on backroads on my way to the trails.  

Still very interested in hearing about others who've modified the internals of their motors.  I

[RNandKT]

Well I think using the 4.0 con rods and offset grinding as apposed to welding and using the 4.2 is a much better idea. (200 versus 625 for thatmuch of a change). Plus I have done some research and it appears as though 2.7 is the moststroke you can get before some major clearence issues. I am leaning towards this heavily.

I very much apprciate everyones information. If and when I get to my rebuild I will keep everyone posted.

Thanks
Ryan

[YJmechanic]

wow, everybody said I was crazy for spending so much time building my motor but you guys are talking way further out there.  I'm not sure if it is worth all that effort to stroke it but I'd like to see it work.  By the way the 30 bore is probably a bit on the careful side because most four cylinders can actually be bored in the 80-100 range and be ok if run properly.  My machinest told me about himself taking chevette motors and boring them 120 over.  Now mind you that is double what I did but you get the picture.  Four cylinder blocks have thicker cylinder walls that larger motors to start because of the high rpm's.  My motor is 60 over and sporting plenty of other goodies(cam, shaved deck, port and polish head, custom valvetrain, etc..) and I run it to almost 7000 rpms with no problems but I am also the first person I have seen to build this motor that far and we'll se how it does in the long run.  But I outrun 4.0 and I spent under $2000.

[little97tj]

Offset grinding..... You have to start with a rod that has a smaller large end than the rod thats being currently used. Thats how you can offset grind. With that being said... If you can find one with say .100 smaller you can offset grind the crank by an aditional .095. The reason for the .005 less is you have to allow your self enough materal to ensure that all the strokes come out to be the same. Other wise it has to be welded to add enough materal to come up with extra stroke.

I just come across this forum and have spent the last couple days researching more indepth what I can do to my 2.5. Still not much more than I already know...  I've had my TJ for about 6 yrs now and love my 4banger but its time for more power. I will stroke this motor and bore it .060 over and cam it. The rest of the known mods have been done already 4.0 throttle body header stage 3 jet chip bla bla bla.. Its time for all or nothing.

My hat is off to YJMech for going .060 on his motor, I read about how people say you shouldn't do it. If that was the case they wouldn't offer the pistons..... I was a machinist for 15 yrs and 4 of that I had a side job in a engine shop. I have built several motors to say the least and some were strokers and they have ranged from chevy to ford to VW aircooled. Not to toot my own horn I'm just trying to say it can be done. I just need to dive into the books and find the right rod and piston.

I love this forum so far, you guys are great. its nice to have a place were someone isn't saying put a small block in it. I'm with YJMech do what cant be done..... or was suposable cant be done... Lookig forward to haveing some fun here, I'll keep everyone up to date with pics... the only thing that sucks is I travel with work but its looking to be slow for a few months ...hopefully its long enough to get this stroker built lol :-) and have some fun with it.

[Mr_Random]

Offset grinding..... You have to start with a rod that has a smaller large end than the rod thats being currently used. Thats how you can offset grind. With that being said... If you can find one with say .100 smaller you can offset grind the crank by an aditional .095. The reason for the .005 less is you have to allow your self enough materal to ensure that all the strokes come out to be the same. Other wise it has to be welded to add enough materal to come up with extra stroke.

I just come across this forum and have spent the last couple days researching more indepth what I can do to my 2.5. Still not much more than I already know...  I've had my TJ for about 6 yrs now and love my 4banger but its time for more power. I will stroke this motor and bore it .060 over and cam it. The rest of the known mods have been done already 4.0 throttle body header stage 3 jet chip bla bla bla.. Its time for all or nothing.

My hat is off to YJMech for going .060 on his motor, I read about how people say you shouldn't do it. If that was the case they wouldn't offer the pistons..... I was a machinist for 15 yrs and 4 of that I had a side job in a engine shop. I have built several motors to say the least and some were strokers and they have ranged from chevy to ford to VW aircooled. Not to toot my own horn I'm just trying to say it can be done. I just need to dive into the books and find the right rod and piston.

I love this forum so far, you guys are great. its nice to have a place were someone isn't saying put a small block in it. I'm with YJMech do what cant be done..... or was suposable cant be done... Lookig forward to haveing some fun here, I'll keep everyone up to date with pics... the only thing that sucks is I travel with work but its looking to be slow for a few months ...hopefully its long enough to get this stroker built lol :-) and have some fun with it.

Wow dead thread, although I've read it once before... some cool stuff here!

Please explain more on offset grinding, I'm intrigued! Got a second engine, too!

My current engine is .030 over, most of the power difference between it and the last is just the fact that it's fresh... i wanted a performance cam and springs, but I needed it done fast and I was short on money... next one will be a more serious build.

[little97tj]

Yea I noticed after I posted the answer the thread was old, I seen you was asking alot of questions when you did go threw yours. Nothing wrong with that. I guess I'll start a new thread on this subject. I've spent the past few days reading pretty much everything to offer on rebuilds in this forum just thought I would throw in my 2 cents.

Well basically to off set grind you have to have a rod journal thats smaller on the rod your going to use in the build. By doing this it allows for moveing the stroke. Draw a circle then draw another smaller one inside and put the end of the smaller one on the edge of the bigger one. This will give you a better idea of what I am trying to get across.

Also with useing a smaller rod journal you have less friction than the larger stock journal, on these motors it isn't a issue your not spinning it to 9k like a race motor, we are pretty much wanting to make our max power around 4 to 4500. I'm running around 3200 on the highway, and thats not very often. I mostly use mine jeep offroad and very seldom see's anything over 2500 to 3000. We mostly woods ride and hill climb bla bla bla. I just want to be able to put it in 5th and not down shift the thing when I do decide to take it for a cruise.

I picked up another motor to go threw so the jeep dont have any down time while I'm doing this project, Ended up finding a motor, trans and tcase for 300 only had 80k on it but was out of a 1990, mine is a 97. I dont think there will be many issues witht the year differance but we wil find out.

[Mr_Random]

Yea I noticed after I posted the answer the thread was old, I seen you was asking alot of questions when you did go threw yours. Nothing wrong with that. I guess I'll start a new thread on this subject. I've spent the past few days reading pretty much everything to offer on rebuilds in this forum just thought I would throw in my 2 cents.

Well basically to off set grind you have to have a rod journal thats smaller on the rod your going to use in the build. By doing this it allows for moveing the stroke. Draw a circle then draw another smaller one inside and put the end of the smaller one on the edge of the bigger one. This will give you a better idea of what I am trying to get across.

Also with useing a smaller rod journal you have less friction than the larger stock journal, on these motors it isn't a issue your not spinning it to 9k like a race motor, we are pretty much wanting to make our max power around 4 to 4500. I'm running around 3200 on the highway, and thats not very often. I mostly use mine jeep offroad and very seldom see's anything over 2500 to 3000. We mostly woods ride and hill climb bla bla bla. I just want to be able to put it in 5th and not down shift the thing when I do decide to take it for a cruise.

I picked up another motor to go threw so the jeep dont have any down time while I'm doing this project, Ended up finding a motor, trans and tcase for 300 only had 80k on it but was out of a 1990, mine is a 97. I dont think there will be many issues witht the year differance but we wil find out.

Different cam at least, the 90 might have a higher compression ratio... Let me try to remember how it goes...

TBI 86-90 2.5 has 9.2:1 compression ratio, YJ OBDI is 9.1:1, XJ is 9.2:1, OBDII XJ and TJ 2.5's are 9.1:1. I'll check out my FSM soon to confirm...

Start an engine build thread of your own and I'll chime in on external differences you'll have to deal with.

[little97tj]

Fly wheel is a issue do to the crank sensor. Cr is going to have to be ajusted due to the .060 over, that added volume will raise it alone, the added stroke will raise it again. Deck height will need to be adjusted for proper quench, but wont know untill I CC the head to find that out. Pistons might have to be cut a little to put it were i want it. Cam sounds a little big but you have to remember the bigger the displacment the more it tones down the cam. Its not going to be easy........

[Mr_Random]

Fly wheel is a issue do to the crank sensor. Cr is going to have to be ajusted due to the .060 over, that added volume will raise it alone, the added stroke will raise it again. Deck height will need to be adjusted for proper quench, but wont know untill I CC the head to find that out. Pistons might have to be cut a little to put it were i want it. Cam sounds a little big but you have to remember the bigger the displacment the more it tones down the cam. Its not going to be easy........

Reuse the 97 flywheel, it'll bolt right up no problem. I'm fairly certain there is more than one size head gasket, and there is a WIDE selection of pistons and rods thanks to the 4.0/4.2 interchangeability.

Wont offset grinding to the 4.0 stroke and using 4.0 rods get you the same CR as the 4.0? I'm pretty sure the head volume and head gasket are the same between the two. The CR will actually go down to 8.7:1 if they are, but the quench height will go up, so you'll want to deck the block and raise the CR higher than 9.2:1, so you can run dished pistons to increase volume and bring the CR back to stock... correct me if I'm wrong, but I believe that is the correct train of thought?

[little97tj]

yea your on the right path, provided all jeep parts were used. It could end up being a nissan rod or anything at this point. This is what is the hold up is. I'm basically looking at every rod on the market to see whats the easiest and what will provide the most stroke. A short rod puts more side load on the piston, thats causes friction, premature wear, over heating issue's. Plus it limits the rpm range to 5k. A longer rod could take away from bottem end TQ, but hold up very well at higher rpms. Some were there is a happy medium, its just a matter of finding the needle in the hay stack.

I'm pretty sure the head will work with the TJ injection, I have one on the way, another intake that is picked it up for 20 bucks. I also figured the flywheel and bell houseing can be used from mine unless I can find a deal on one. These are minor issues, but there is easy fixes and work arounds. The pullies and externals will all be swithed over, I'm really just interested in useing the block and head basically and what ever internals can be rebuilt or reused.

[Bikerjr1]

This thread came at a good time.  Thursday evening I was talking with a long time mech that works at the Dodge/Jeep dealership, he also builds hi-per 4x4s, about more power/displacement for the 2.5L.  he said that it was possible to use the crank and rods and flat top pistons out of the 2.8L Mercruiser engine.  I did a search and this engine is a diesel.  I've just got back into town and haven't asked him if that's the right one.  If it is I can see a second engine being built.  I have a friend at a marine shop that could get me an old engine.  Hopefully I'll be able to check for sure this week.  I'm like ya'll I'd like to keep the I4 just build it up.
JR

[Mr_Random]

This thread came at a good time.  Thursday evening I was talking with a long time mech that works at the Dodge/Jeep dealership, he also builds hi-per 4x4s, about more power/displacement for the 2.5L.  he said that it was possible to use the crank and rods and flat top pistons out of the 2.8L Mercruiser engine.  I did a search and this engine is a diesel.  I've just got back into town and haven't asked him if that's the right one.  If it is I can see a second engine being built.  I have a friend at a marine shop that could get me an old engine.  Hopefully I'll be able to check for sure this week.  I'm like ya'll I'd like to keep the I4 just build it up.
JR

If this is true, all the offset grinding talk isn't really necessary (depending on how much extra stroke the 2.8 crank provides). I'm VERY intrigued.

I'm running a later model intake with later model pullies and accessories, there were only minor problems with valve cover mounts and such, but I resolved them... make sure to swap your later-model engine mounts, or you wont be able to mount the voltage regulator on the alternator. I'm sure there are other minor challenges that you'll need to figure out, good luck!

Wow, just read that the mercruiser stroke is 3.94! I'm fairly certain that with .030 that would make a 3.1L engine... I'll do some real calculations, that was just off the top of my head...

[little97tj]

3.149 @ .060 bore   motor would be 3.94 x 3.94 thats a square motor my friends, when your talking max TQ out of  a package thats how its done.

I'm still on my first cup of coffe, need to look intot to this 2.8 a little more, but this is VERY interesting to say the least. I've been looking at more involved options.

[little97tj]

192.149 cubes 

[little97tj]

Ok this is the storey its the 2.8l merc made from 1987 to 1989, its a gas motor and based off the gm iron duke... thats all pretty promising. I cant find the cranks and rod details on line though. So it looks like a trip to the machine shop in the morning to go threw there books.