305 to 350 swap info.

[crazymudder]

Hey guys I'm new here.  Awesome forum.  I'm going to swap a 305 for a 350 on my 86 gmc c15 2wd.  The 305 runs reliably but is about shot.  The rear main leaks but even if fixed will blow out again because of high oil pressure.  It also blows oil out of the valve cover breather.  I think its the valve train mainly but also many other contributing factors.  And my question was what all can I reuse when I drop in the 350?  I know I can reuse many of the externals (alternator,compressor, carb, etc.) but what about distributor?  I know the timing is different on a 350 so I'm assuming I'll have to get one for a 350. Any help would be appreciated.  I'm going to wait until summer to do the swap, but want to start getting some information on it.
Thanks

[VileZambonie]

Welcome

Your rear main seal is not affected by oil pressure, it's a non positive seal. If it keeps leaking you have a crankshaft imperfection or excessive crankcase pressure. Is the PCV and breather clear?

Your distributor and accessories will swap over to the 350 without a problem.

[crazymudder]

Yes the pcv valve is clear and with good suction.  The breather is also clear.  Did a compression check and got anywhere from 50 psi. to 185 psi.  The piston rings are good and it doesn't burn a drop of oil.  I think it is the cam lobes are rounded off.  It has a rough idle and lack of power.  With 110,000 miles on it I would rather save up for a new motor than replace the cam.  I have no idea what has been done to it in the past but know the previous owner has messed with it.  My feeling is it could be more than the cam.  It kind of bothers me that the motor is like this when the truck is so immaculate.  Not a scratch or dent on it.  Has had a bedliner since it was brand new.  Has had seat cover since it was brand new.  Thank you for your help! Any further input would be great!

[Lt.Del]

I think your starter will need to be replaced as well.  The flex plate too i believe.  

The starter for a 350 should have off-set (staggered) bolt pattern, intead of paralell with the block end.  

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

[DixieOffroad87]

i will try to remember as best i can i did the exact same swap 3 years ago in my 86 k10. all of the accessories will swap over. get a new fly wheel and all. you can use the one off the 305 but your better off to replace it with a new one for a 350. and as for the starter i recommend a starter off a new 5.3liter motor they have alot more torque than the old starters, weigh 1/4 of the weight, and bolt right up. any other questions just ask.

[VileZambonie]

The only difference in the flexplates is if the engine is internally or externally balanced. You'll know when you are looking at the back of the crankshaft. If it's tound and have a one piece rear main seal it's externally balanced. If it has a counterweight on the back and uses the two piece rear main seal its internally balanced.

Your compression test results will tell you what your problem is. Your results are way too far apart. Do your compression test as follows:

Warm the engine up
Remove all spark plugs
Disable the coil
Install a battery charger to maintain consistent cranking speeds.
Install your compression gauge and crank the engine 4 compression strokes on the cylinder you are testing.
Make sure you have the throttle wide open and the chokeis open.
Record your results for each cylinder.
If there is a difference of more than 10% between cylinders, perform a wet compression test.
Shoot a squirt of oil in the spark plug hole and repeat the test. Record your results.

The engine I'm putting in that 86 k10 is a 350 taking the place of a 305. I did have to get a new flexplate because the 305 was externally balanced.

[crazymudder]

Thanks for the info guys.  Also, I did the compression check EXACTLY how you described.  When I got the results I did a wet comp. test and got the same results.  I'm losing compression some how in the valvetrain.  As I said the motor is about shot.  Here are the psi. readings:
cyl. 1  185
cyl. 2  180
cyl. 3  180
cyl. 4  170
cyl. 5  105
cyl. 6  50
cyl. 7  100
cyl. 8  130
                   And thats not a typo.  I did this a couple times and again got the same results.  

[Leadfoot]

by doing the wet test and it not improving the results,then yes,the problem is most likely in the valvetrain..if it was rings it should have showed a significant improvement when the oil was added..

[VileZambonie]

Turn the Carnkshaft so the timing mark line up on zero. this will put you on TDC #1 and TDC#6. Make sure #6 is on TDC compression, without rotating the crank put some regulated air in the cylinder and see where it's going.....if you hear it leaking into the intake= bad intake valve, in the exhaust = bad exhaust valve, into the cooling system= possible head gasket problem, no leaks= broken rocker arm stud, bent pushrod, wiped cam lobe.

[crazymudder]

Thanks for the info.  I might do that just so I know whats wrong, but like I said I'll still probably do a swap.  Also, anyone used an ATK motor?

[Lt.Del]

reconditioned cylinder heads will do wonders

SgtDel
aka "Andy"

www.delbridge.net

1979 Big 10 383 stroker
1991 Blu 'Burb 3/4 ton 4x4

[vince311]

Hey guys I'm new here.  Awesome forum.  I'm going to swap a 305 for a 350 on my 86 gmc c15 2wd.  The 305 runs reliably but is about shot.  The rear main leaks but even if fixed will blow out again because of high oil pressure.  It also blows oil out of the valve cover breather.  I think its the valve train mainly but also many other contributing factors.  And my question was what all can I reuse when I drop in the 350?  I know I can reuse many of the externals (alternator,compressor, carb, etc.) but what about distributor?  I know the timing is different on a 350 so I'm assuming I'll have to get one for a 350. Any help would be appreciated.  I'm going to wait until summer to do the swap, but want to start getting some information on it.
Thanks <p></p><i></i>

I know this is an old post....but had a follow up question. I've got an '86 Silverado as well, with a tired 305. I've called around, searched the internet for a GM Crate replacement 350. Seems that they offer everything BUT 1986. Anyone have a source for a 86 350 crate engine? thanks!

[Captkaos]

PN 12499529 is the 290hp 350.  You can put any motor in there you want to...

[TexasRed]

The exhaust manifolds should be different too. Your spark plugs should be below the manifolds on the 305. What kind of budget are you on and how nice is the truck because you could just fix the 305 and keep it original?

[1976Scottsdale]

If all else fails, pull valve covers and look at valve stem heights.  Seen a lot of sbc heads that pulled the valves into the seats and ran as you describe but still did not miss, just blew a lot and acted tired.