720 question

[filthy forever]

hey everyone first off im new to the forum (been lurking for a while B) ) and I picked up my dream truck a while ago, a 1980 720 with an L20 for 500 bux :thumbup:  clean title, straight frame, barely any rust, and running but poorly. The motor has good compression on cylinders 1,2, and 3 but very low compression on cylinder 4 (bad rings and a hole in the piston). I have been doing the math on how much it will be the rebuild the motor (bore out the cylinders, new pistons, rings, bearings, deck block and head, gaskets etc) and unfortunately I cant afford that right now :sneaky: . Now for the main question (and sorry if it sounds dumb) but I came across a very nice L18 (with good compression) for 400 bux and I was wondering if that would bolt up to my transmission, and if I can use the manifolds off the L20? any help would be appreciated.

 

im trying to figure out how to load pictures on here but when I do ill post some of the truck

 

have a good day guys

[datsunaholic]

The transmission will bolt up, but be aware that the majority of L18s are import engines and use a different (smaller size) clutch.  This will cause issues unless that L18 came with the throwout bearing collar, OR it happens to be a (fairly rare now) '74 620 truck L18 which would have the correct flywheel for the larger truck clutch (which means you'd use your existing throwout collar).

 

Manifolds are a small issue- the '80 L20B has a smog head and hence smog manifolds.  L18s did not come with that, and it is a very bad idea to use the small, roundport exhaust manifold on the square-port head that all L18s came with as it will likely leak as well as flow poorly due to the partially covered ports.  The other way (using square manifolds on the smog head) works, but that's not what you have. 

 

The other issue is that the L18 is physically shorter in height, which in my case caused the tailpipe to hang low enough up front to hit every damn speed bump until it was fairly flat.

[Z-train]

A L-18 is less than the best choice for moving a 720.

[datsunaholic]

A L-18 is less than the best choice for moving a 720.

 

True, but if ya don't have anything else, it's better than a tow truck to the wrecking yard.  I had an L18 in my '76 King Cab and it was one of the best engines I've ever had.  The '80 isn't THAT much heavier unless it's a 4X4.  That said, by the time you find all the requisite parts to make an L18 of unknown lineage work in a 720, you may have spent enough to have rebuilt the L20B.

[Z-train]

Or find a KA in a wrecked D-21.

[datsunaholic]

That's a lot more work, though.  With a NAPS-Z 720, it's a little easier but you're still fabricating motor mounts, adapting the wiring and the fuel tank for EFI. etc etc.  With an '80 720 it's the same amount of work as fitting a KA to a 620 as you have to change the transmission as well.  If he can't afford to rebuild the L20B and can't rebuild it himself (which can be done well under $1000 for a basic rebuild that doesn't require extensive machining), I can't see that happening. 

[Dirttrack510]

L20b engines are fairly common, why not just find another good one and slap it in there, everything will just bolt up and i'm sure someone on ratsun has one they'd sell for $400 or less....

Post an ad in our classified section ;)

[filthy forever]

thank you for your advice guys I really appreciate it. It looks like I just have to suck it up and put her on ice for a little while longer until I can find an L20 or rebuild mine. I take auto classes at a community college (and so does my brother) so we have enough knowledge to rebuild the motor, but as you guys stated if I could find an L20 on here for sale that would be awesome. hope to have her rolling sometime soon.

 

have a great day