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Old 08-29-2020, 09:56 AM   #1
dave5287
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engine swap

i have a 1971 c20 with a vary tired 350 i want to swap it with a 305 from a 1986 monty ss is ths possible
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Old 08-29-2020, 10:15 AM   #2
jayoldschool
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Re: engine swap

Possible, but a 35 year old 305 won't really be much of an upgrade. I'd either refresh the 350 according to budget, or drop in a fresh crate engine.
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Old 08-29-2020, 11:04 AM   #3
kwmech
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Re: engine swap

If that is all you've got at the moment, then go for it. Everything will interchange as long as the Monte is car'b and not TBI
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Old 08-29-2020, 11:38 AM   #4
68Gold/white
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Re: engine swap

I agree with Jay about the 305, but the swap I think is pretty much do-able. Being a 71 the heads should have holes in the ends for accessories to be bolted to, just like the 305. So the PS pump, alternator, AC stuff should all bolt up (if you decide to re-use them. Of course the short water pump has to stay with all the other 71 stuff in the swap.

If you decide to keep the accessories on the 305 for the swap, you will have to have custom power steering hoses made, the ends are different, AND the 305 has the long water pump, that could cause issues with pump to radiator clearance...

I believe 86 was the first year for the one piece rear main seal. You should be able to retain the 305 fly wheel (flex plate). I assume both are automatics?...

THose later 305 heads had two holes drilled forward of the front exhaust port on the drivers side, and ovbiously behind the rear exhaust port on the pass side. I don't believe those will be a problem..

Also, back then the center 2 intakes bolts were changed up some. The angle that they go into the head was changed. On the earlier heads ALL the bolts go into the head at the same angle. On the later heads, the center 2 bolts on each side are vertical with the ground, meaning, if you installed studs in the center holes, and removed all the other intake bolts, the intake could be removed by pulling straight up!!!

SOme things to think of.........
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Old 08-29-2020, 01:31 PM   #5
HO455
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Re: engine swap

When I bought my Burban the PO said the 305 in it was out of an 85 El Camino. When I took it out everything that was on it bolted directly to my new 74 350. It is a 67 that had power steering added but I fairly sure it did not come from the El Camino as I didn't encounter any metric fasteners or fittings. I replaced the 305 as it had lost a main bearing.
The truck wasn't a powerhouse with the 305 but it wasn't the slowest rig I've ever owned. Definitely faster than your average 60's VW.
One thing I noticed after the swap is with the 305 and 3.07 gears I got 12 mpg around town and with the fresh 350 and 3.73 gears I now get 12 mpg around town. Who knew?
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Old 08-29-2020, 01:52 PM   #6
68Gold/white
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Re: engine swap

None of the bolt holes on the 305 will be metric. I know that engine fasteners (that attach TO the engine ONLY), will NOT be metric. I know this was till 95, have not worked on enough Vortec's to tell you about them. Of course the LS's are ALL metirc, but were not going there, thankfully.

The 305 balancer can be left of the 305, along with it's timing tab...crank pulleys will bolt to 305 balancer, no problem...
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Old 08-29-2020, 03:25 PM   #7
gmc684x4
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Re: engine swap

Balancers are interchangeable unless your old engine is a 400

Acc bolt holes will be the same
No acc bolt holes in heads predate 71 truck

305 should be tbi intake bolt pattern
If swapping to make it carbrated look for tbi carb intake

Pre 86 engines are 2 peice rear main seal
Post 86 engines are 1 peice rear main seal

Flywheel can be re used if auto with correct starter
Manual trucks use 168 tooth flywheels and there starter bolts to the bell housing

Yes vortec heads acc bolt holes are still standard thread bolt heads

You will need to change temperature sending unit out as the one from the 86 will not read right in your truck

Depending on what position you might not be able to use long water pump setup with the engine in 2 wheel drive stock position
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Old 08-29-2020, 03:37 PM   #8
68Gold/white
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Re: engine swap

All RWD Monte's were carbed, till end of production of RWD Monte's. 71 Quadrajet could be bolted up to 86 intake. Only unknown would be how to engineer a choke using that intake.

Distributor from 71 would bolt in. 86 distributor can be converted to the first gen "4" prong module. It takes some work, Distributor HAS to come completely apart. Replace 86 magnetic pickup with early version, early module bolts right in, harness to coil/cap has to be replaced with early version. Doing all this leaves a notch in the "wall" of the distributor (where the cap seals), that's no problem, JB WELD, or something similar...
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Old 09-16-2020, 11:55 AM   #9
dave5287
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Smile Re: engine swap

thanks to all almost ready to start 305 swap thanks for all imput another 1971 saved from scrap yard
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Old 09-16-2020, 01:24 PM   #10
gmc684x4
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Re: engine swap

we need some Pictures
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Old 09-16-2020, 08:14 PM   #11
hugger6933
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Re: engine swap

That isn't a "normal" 305. That is a L69 305 and they were the strongest[oxymoron] 305 with a 4 barrel carb I had one I bought new I loved that car hard loaded silver with light grey int and even T-tops. It had a bit more HP than the other 305's, It had more cam more compression yada yada bla bla bla.
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Old 09-17-2020, 09:50 AM   #12
j_cst_10
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Re: engine swap

My 85 monte carlo bone stock with a shift kit and no air cleaner ran 15.3@89mph in the quarter with 100K+ miles.

Still a low hp 305 but would do fine in a c10 I would think
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