350 rebuild questions.

Discussion in 'Small Block Tech' started by Juze86, Nov 9, 2015.

  1. Juze86

    Juze86 Well-Known Member

    Hi! I'm planning to do some rebuild for my 350 engine, and I have some questions. I did leak test and found 7 exhaust valves leaking. I took heads apart and found this:
    https://www.dropbox.com/s/eg1mrler50kwjke/2015-11-07 15.31.43.jpg?dl=0 (sorry tried to upload image here but failed)
    Almost all exhaust seats were widened unevenly. I lapped my valves at last winter and seats looked nice and even then, what possibly caused that?

    How about my main bearings (my first time took any engine apart) how they should look after maybe 5 years?
    https://www.dropbox.com/s/levei75iqb5zk4x/2015-11-07 20.11.41.jpg?dl=0

    My engine should be -76 but somebody has changed older type connecting rods, should I try to find cap screw style rods? Or just polishing and balacing old ones? Or just balancing? Im probably gonna change pistons to get CR to somewhere 9.5-10 (about 8.9 now) for Crower level 3.

    Sorry my english, -Juha
  2. alec296

    alec296 i need another buick

    Measure how far down from deck your current pistons are. I'm finding that the cast 340p hi comp pistons may not have actual compression height they are advertising . just new from others doing a 350. Might want to measure before using. Cheap v8/v6 pistons are at 1.800. The 68-74 stock pistons should be at 1.835(340p also) . valve seats look as they did poor job. .
  3. Mart

    Mart Gold level member

    Doesn't look like you have bronze valve guide liners. Check and see how whopped out your guides are. Exhausts should be about .0016 and intakes about .0012 clearance.
  4. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    That would be in inches not mms.

    Mart you need to watch that and make sure to put the " in the dimensions, the guy is from Europe home of the metric system.(well maybe not the "home" of the metric system but it is what they use there) Those would be some REALLY tight tolerances if that was in mm! :eek2:


    WV-MADMAN Well-Known Member

    :gp: :Smarty:
  6. Mart

    Mart Gold level member

    Yes you read the sizes correctly! I would suspect he would realize we don't do the metric system as much as they do. Sorry for the assumption.:eek: I'm pretty sure he would understand. I dealt with close tolerance machining (in inch) all my life so I guess I didn't give it a second thought. Besides, you wouldn't fit the stem in the hole!
  7. Juze86

    Juze86 Well-Known Member

    Yes I know you speaking in inches :)

    Pistons were 0.100 below deck, 1.805 CH silv-o-lite, I have to measure dish volume to get real CR but it is somewhere 21cc. Combustions chambers are around 46cc (there is some metric for you :))
  8. Juze86

    Juze86 Well-Known Member

  9. alec296

    alec296 i need another buick

    You don't want a piston that far below deck. That's just wasted fuel. It has to burn .100 plus .045 gasket. Lost HP. Silvolite hyperutectic h522 is 1.855 height. Or get autotecs and set your needed specs.
  10. sean Buick 76

    sean Buick 76 Buick Nut

    I am curious to know how the piston sitting in the hole is an issue on an engine with a completely open head chamber design?

    I know of a Buick 350 that ran the flat top V6 pistons sitting way in the hole and ran low 12 or high 11s NA so I am really not sure that there is any problem with having a piston below deck height.
  11. UNDERDOG350

    UNDERDOG350 350 Buick purestock racer

    Get the Auto tec pistons. Money well spent.
  12. Juze86

    Juze86 Well-Known Member

    Yes like Sean said I also think piston is not too far below because no quench areas in head.
    With Autotec 13.5cc dish and 1.850 CH gives me 10.4:1, is it too much for Crower level 3? Also I'm not sure is forged piston best for my use, car is my daily driver at summers, and Im no going to race with it. Forged piston need more piston to cylinder wall clearance right?
  13. 8ad-f85

    8ad-f85 Well-Known Member

    When the piston is running warm, it takes up the clearance anyways.
    The cold clearance is just that, cold. Not running temp.
  14. sean Buick 76

    sean Buick 76 Buick Nut

    I do also think that $600-800 pistons is a little extreme for a daily driver mild 350. The old style forged pistons required the larger clearances and had some issues with pistons rocking in the bore when cold etc however we hear less about that with the new alloys used in the modern pistons like the Autotec.

    I am of the belief that with the stock heads the best bet is to minimize the decking of the block, and minimize the milling of the heads, and get the compression ratio you want with the pistons to match the cam. The GM 3.0 liter flat top piston would give you about 9.8:1 static compression with no block or head milling so you could fine tune from there by milling the heads or block from there.


    Gary can help you pick the ideal compression for the crower level 3 cam, but my guess is 10.2:1 static is about right.

    If the heads or block are milled excessively it makes for issues of the intake fitment, and then you end up with intakes and heads that are "married" together as the machine work needed makes interchanging parts later a hassle.

    An often overlooked issue with the 350 is that there is a roughly 1/8" ring around the outside of the head that overhangs over the cylinder walls creating a hot spot. Always put the head on the block after it is bore and before you put the pistons in the engine and look to see this overhang. Then mark it off and remove this ridge.
  15. pmuller9

    pmuller9 Well-Known Member

    The head needs to be repaired first before a piston is chosen so you know what the final combustion chamber volume is.
    If the valve job was good before and the valve stem clearance is ok then I would say you are looking at valve recession.
    If this is the case then exhaust valve seats will need to be installed.

    What was the valve spring pressure with the valve closed?

    I would not go more than 9.5:1 on the static compression ratio. It will give you a 7.5:1 dynamic ratio

    The normal chamber volume is around 58cc. Was the head surfaces machined?

  16. alec296

    alec296 i need another buick

    What's the highest octane fuel there? Open chamber or not the smaller you make the total combined area of combustion the faster it pushes on piston . physics. Your octane should be higher there and no ethanol.
  17. sean Buick 76

    sean Buick 76 Buick Nut

    Paul is right check the volume of the heads after they are rebuilt and then pick your piston from there.

    So if an engine has an open head chamber and there is no quench area does it matter if there is more volume in the head or piston dish for a given compression ratio? Obviously higher compression ratio makes more power all things being the same but like people are pointing out the octane of fuel will come into play. I do agree a piston with a dish might be an advantage over a flat top all things being equal but it is tough to say.
  18. pmuller9

    pmuller9 Well-Known Member

    There is always squish velocity as the piston approaches the head, it is a matter of how much and when.

    Lets look at the new 350 head with a closed chamber design and a piston that has a round dish leaving a 1/2 squish ring along the outside of the piston top.
    With the piston at zero deck and a .040 head gasket or squish distance, 9:1 compression ratio, when the engine is at 4000 rpm

    The Max squish velocity is 25 m/sec at 8 degrees BTDC and around 15 m/sec at 20 degrees BTDC.

    Now switch to the open chamber 350 head angle milled so there is around .140" squish distance including the head gasket.

    The Max squish velocity is 8 m/sec at 13 degrees BTDC and around 6 m/sec at 20 degrees BTDC.

    If I drop the piston in the hole .100" with the open chamber head, max is 3 m/sec at 15 BTDC

    I would say that it is still worth having the piston at zero deck with the open chamber head.

    Last edited: Nov 11, 2015
  19. Juze86

    Juze86 Well-Known Member

    Machine shop took .040 from heads last winter, and there was already taken some. Old very experiencent shop owner told me about guides they are fine, but anyway I will get new guides in this winter.

    We have 2 types fuel here 95 and 98 octane, but Im not sure are ratings same than there. 95 has 10% ethanol and 98 has 5%, I always use 98.
  20. UNDERDOG350

    UNDERDOG350 350 Buick purestock racer

    Paul you described exactly what I was thinking (without the math). That there is still some effect of squish even if open chamber. Sort of like scavenging that a tuned header provides or the ram effect of intake port velocity.

    Sean, I don't know why you keep pushing those cheap cast pistons. If you're building a performance engine, use performance parts. Raising the compression allows use of larger camshaft which in turn raises the RPM range. If you do use those cheap (probably Chinese ) things keep your RPM under 5500 or they will fail.

    My new Autotec forged run at .003" clearance.

    Your valve seats have recessed possibly due to weak/incorrect valve springs. Time for hardened inserts or better yet new heads. If those heads were cut so much to have a 46cc chamber I guessing you almost have a closed chamber now. Can you post a photo showing the entire chamber?

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