Dismiss Notice

Welcome To CK5!

Registering is free and easy! Hope to see you on the forums soon.

Score a FREE t-shirt and membership sticker when you sign up for a Premium Membership and choose the recurring plan.

Replacing 305 with 350 questions

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by zach78k5, Jun 10, 2003.

  1. zach78k5

    zach78k5 1/2 ton status

    Joined:
    May 14, 2000
    Posts:
    406
    Likes Received:
    0
    I was planning on replacing a 305 from and 86 with a 350. I have a 78 350 in the garage. Will that work or will i need a 350 from a newer year. What else will have to be replaced. I assume that the alternator, steering and water pumps should be fine. The headers too i think. will I need a new carb? anything else. any input would be very helpful. will i need a bigger radiator? thanks for the help.
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    26,976
    Likes Received:
    189
    Location:
    Roy WA
    If you've got the one up and running, you should be fine swapping everything over.

    Radiators are typically a sticky point, but it's cheaper to stand by with a garden hose than to just replace it without knowing if it works for your 350 or not.

    Carb, headers, etc., just swap them over. Some carbs are going to be picky, but from what I've seen, a Q-jet (what are you running?) will "mask" a lot of problems relating to engine size differences, pretty well. Again, it's one of those things that won't hurt to try first.
     
  3. TxK5Blazer

    TxK5Blazer 1/2 ton status

    Joined:
    Aug 13, 2000
    Posts:
    593
    Likes Received:
    0
    Location:
    Floresville, Tx
    I swapped a 95 350 into my 82 in place of a blown up 305 everything swaped over except the intake manifold I had to buy a new one with that new bolt pattern if you have any questions just let me know I have a good mechanic that can answer most questions that you have if I can't answer them good luck with the swap.....Jason /forums/images/graemlins/usaflag.gif /forums/images/graemlins/k5.gif /forums/images/graemlins/usaflag.gif /forums/images/graemlins/pimp1.gif
     
  4. txbluethunder

    txbluethunder 1/2 ton status

    Joined:
    Jan 29, 2002
    Posts:
    835
    Likes Received:
    0
    Location:
    Austin, TX
    everything will swap over fine. just realize that your dipstick tube will be on the opposite side and you need to use the flexplate that came on the 350 and not the one on the 305. the 305 had a one-piece rear seal and a different flexplate because of that. otherwise it's a pretty easy swap.
     
  5. bigblock44k5

    bigblock44k5 1/2 ton status

    Joined:
    Jun 8, 2003
    Posts:
    396
    Likes Received:
    0
    Location:
    Hurlburt Field, FL
    everything already stated was great advice, especially about the flexplate. 305 uses a much smaller flexplate, 153 tooth, however the starter will work with both engines. the only thing you might have to work around is the mounting brackets for the accesories. even though chevy used pretty much the same brackets and hold patterns on the block and cylinder heads, sometimes the holes are not drilled. no need to worry though, you can punch and drill them out CAREFULLY. Only go as deep as you need to for fear that you might hit a water jacket. hopefully they are all there for you. radiator will probably be fine, as long as the 350 is mostly stock. might be a good idea to upgrade to a good fan if you dont have one now. i would go with an 8" harmonic damper, all you 305 pulleys will work fine. your intake maifold will also work, the only reason 82k5blazer had probs is because in 87 chevy changed the center 4 bolts on intake maifolds to a more up and down pattern, which you could just have redrilled anyway. since you have an 86 you wont have to worry about this anyway. any more questions feel free to ask.
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    26,976
    Likes Received:
    189
    Location:
    Roy WA
    [ QUOTE ]
    305 uses a much smaller flexplate, 153 tooth, however the starter will work with both engines.

    [/ QUOTE ]

    I thought the difference between the 153 and 168 tooth setups were the starter mouting position? The 168 being "out" farther to accomodate the larger ring gear diameter. I know there are two starter patterns, and two bellhousings for manual trannies (153 vs 168) but I'm not sure on the automatic side of things.
     
  7. bigblock44k5

    bigblock44k5 1/2 ton status

    Joined:
    Jun 8, 2003
    Posts:
    396
    Likes Received:
    0
    Location:
    Hurlburt Field, FL
    The starter is out farther for the larger flywheel but it is still the same mounting pattern. 168 tooth flywheels/flexplates have 2 different style starters. One is an offset bolt pattern.
     
  8. bigblock44k5

    bigblock44k5 1/2 ton status

    Joined:
    Jun 8, 2003
    Posts:
    396
    Likes Received:
    0
    Location:
    Hurlburt Field, FL
    The starter is out farther for the larger flywheel but it is still the same mounting pattern. 168 tooth flywheels/flexplates have 2 different style starters. One is an offset bolt pattern. thanks for making me thing dyeager,even though its too early for that /forums/images/graemlins/thumb.gif
     
  9. sled_dog

    sled_dog 1 ton status

    Joined:
    Sep 3, 2002
    Posts:
    16,870
    Likes Received:
    0
    Location:
    Austin, TX
    I did a similar swap and I am pretty sure a lot of the info you guys are giving him is incorrect. First off 1 piece rear main seals didn't come till 87 so that has nothing to do here cause his is an 86. As for the flywheel I replaced my 84 305 with a 78 350 and used the 305 flywheel with absolutely no issue. I have over 4000 miles on the motor without any issues except some crappy carbs. The 305 carb will work fine, I recommend rebuilding it when you do the swap though. I didn't rebuild mine and my carb turned out to be a source of power loss. Replaced it with a 600 Holley that needs a rebuild and it kind of sucks cause it really needs to be rebuilt but it produces much more power on the butt-crack dyno. I am using my 305 radiator with a factory 78 7 blade fan and it cools great. In mild weather(say 60s) it stays at 190 on the dot, in weather like we have now(80s) it sits at around 205+ but never gets over 210 under driving conditions. No matter how long I hold it out. Giving it gas cools it down thanks to that fan. Only problem I could kind of notice is it goes to like 230 when I shut it off in this weather, I should let it idle a couple minutes but 230s not bad. If you are running manifolds that can be an issue! I forgot to mention that if you have manifolds the newer 86 ones won't work. The location of the oil dip stick on the older motor is oppossite of your factory one and will take some creative use of older manifolds if you want to retain your Y Pipe. If you have headers ignore this.
     
  10. txbluethunder

    txbluethunder 1/2 ton status

    Joined:
    Jan 29, 2002
    Posts:
    835
    Likes Received:
    0
    Location:
    Austin, TX
    every 1986 K5 w/ 305 V8 I've ever seen had a one piece rear main seal. don't believe me...i'll go take a picture of my old 305. /forums/images/graemlins/thumb.gif
     
  11. wrathORC

    wrathORC 1/2 ton status

    Joined:
    Apr 27, 2002
    Posts:
    908
    Likes Received:
    0
    Location:
    Hancock, MI
    Use the 350's harmonic balancer but everything else on the front of the motor should come from the 305. If you're keeping your EGR, AIR, and ESC you'll need to keep the 305's water neck. It has controls on it for the EGR, AIR, and torque converter lockup (it prevents the torque converter from locking up if the motor is cold). You can keep your ESC distributor and carburetor but keep in mind the carburetor is designed for a 305 with small main jets. I recommend using the carburetor off the 350. The ESC distributor is a good idea to keep but it's curved for a 305. If you keep it you must keep the knock sensor on the passenger side of the block.

    I recommend getting rid of the exhaust manifolds but they'll work. Since you say you have headers this is a nonissue.

    You will need to keep the 350's flexplate on the 350. The 305 one has a different pattern because of the different rear main seal. There is a 99% chance the rear main seal is a one piece unit. I have heard some two piece ones made it all the way into the 87 model year but I doubt it.

    The radiator will probably be fine. I ran my two-core with my 350 out of a 77 3/4 ton and it kept it at 210 with the air conditioning on.

    Your dipstick tube will be on the driver's side. I suggest getting a new one that you can bend to where you want it. Mine was a PITA to get to.

    I think that's about it other than sensors. PS: you'll probably have problems with the water temperature sensor for the gauge. It's on the driver's side of the engine on the cylinder head. I can't remember but I'm pretty sure the fitting is a different size on the 350 and your 305. I think one has a 3/4" NPT and the other is 1/2" NPT but don't quote me on it. With any luck the 78 will have a gauge type sender and not a idiot-light type sender so you won't have to worry about it.
     
  12. txbluethunder

    txbluethunder 1/2 ton status

    Joined:
    Jan 29, 2002
    Posts:
    835
    Likes Received:
    0
    Location:
    Austin, TX
    good call about the temp sensor unit. use the one that came with the truck...it's calibrated differently than what came with the 70's engines and will read incorrectly when attached to your gauge. go to napa and get an adapter to make it fit. for some reason i believe its 5/8" fem to 3/4" male.
     

Share This Page