4.2 liter main caps on a 4.0 liter motor

[strokin96xj]

ok so i have a slight problem i think... I am just beginning on my motor build 4.0 liter block and 4.2 liter 12 counterweight crank. when i was putting my main bearings in they would fit in the main caps just fine but when i went to install them on the block side the tangs on bearings didn't line up with the notch in the journal on the block. so i went to the parts store and they ran the part number on the bearings and i have 4.2 liter bearings .030 undersize. which means i have 4.2 liter main caps and a 4.0 liter block. So my question is if i buy 4.0 liter main bearings .030 undersize to use on the block side and just keep the 4.2 bearings on the cap side will this work?? need to know so she doesn't grenade on me when i fire her up for the first time. sorry about the length and any help would be much appriciated thanks.

[dwg86]

You need to buy mains for the 4.0 block. If your crank is .030 under, buy .030 bearings for the year block you are using. DO NOT mix the 4.2 and 4.0 bearings.
I'm assuming the bearings you are trying to use came with the 4.2 crank. They will not work in a 4.0 block.

[Cheromaniac]

You need to buy mains for the 4.0 block. If your crank is .030 under, buy .030 bearings for the year block you are using. DO NOT mix the 4.2 and 4.0 bearings.
I'm assuming the bearings you are trying to use came with the 4.2 crank. They will not work in a 4.0 block.

Correct. If you have a '91+ 4.0 HO block, you need to use 4.0 main bearings that have the locating tang in the center. The 4.2 bearings with the off-center locating tang will only work in 4.2 blocks and in '87-'90 Renix 4.0 blocks.

[amcinstaller]

which main caps are you trying to use? someone here has mentioned using 4.2 main caps on a 4.0 block, due to the added strength, but im not sure if its just a straight line bore and go for it or if you need to use anything special.

[SIXPAK]

I was going to try this route when I replaced the block in the race car. I was unable to fing a set that would be close on the bore from side to side. If I was going to try it with what I had I would have had to shave the one side then weld up and regrind the other to align the bore. Then after that was done (and bolt holes checked for bolt clearance) I would have to shave the mating side of the caps and then have the block align bored.

[strokin96xj]

ok thanks for the answers i'll go buy some 4.0 main caps. the only reason i was asking if i could use both 4.2 and 4.0 bearings is because my 4.2 main caps bolt up to the 4.0 liter block with no problem and there is no like edge where the two meet. it is almost perfectly smooth. is that normal or has my block been line honed to make the two work? motor was givin to me with machine work already done just needed to put it together.

[IH 392]

The main caps match a block, they DO NOT! interchange between blocks!, even IF they feel close they ARE NOT!
Use the caps that were on the block you intend to use, if you don't the crank will bind in the bearings and even if it does turn you will run into issues!

[strokin96xj]

alright sweet thanks for clearin that up for me

[lafrad]

If you get the 4.2 caps properly line honed at a quality machine shop you should not run into any issues.

[strokin96xj]

ok well i took some pictures of the main caps and the block and one of the 4.2 liter caps installed with no bearings. does it look like its been line honed? i don't know and i can't get a hold of the guy who gave this headache to me. and if it looks ok can i go ahead and get 4.0 bearings for the block side and 4.2 bearings for the caps? just make sure there the same size? Or should i just go the safe way and pull the 4.0 out of my jeep and build the stroker with it so i have the correct main caps? thanks for the help.

jeep motor 2.png

[Bodo]

only way to know is to stick a dial bore gauge in there and see if it's the right size and if it's round and straight.

[IH 392]

If a block has been line honed the finish of the main bores will have a cross hatch finish just like a cylinder wall.
If there is ANY! dought if the caps match the block have it checked with a dial bore gauge!

[amcinstaller]

yea, if theres doubt, dont use them. too expensive to chance i think. maybe can you get a picture of the other side of those main caps? then maybe someone can tell you if in fact they are 4.2 main caps. i have a set of main caps from a 4.2 and considered the use of them in my engine, but i think thatll be another idea that ill scrap