Decking a block.

[100% Chevy]

I have a 350 engine that I assume has been decked(no numbers).
What is the usual amount shaved?
Reason I ask brings up another question-If the block is decked,do I need a thicker intake to head gasket?Or,just add more sealer?
I'm trying to determine why my vacuum is so low-15"hg at idle,and rough idle.
Thanks,Mike.

 

[grumpyvette]

decking a block is done to square up the blocks head mount surface with its crank center line, the machine shop will normally remove the MINIMUM it can to get the deck square and .010 is a common deck cut depth, the SBC has a normal deck height of about 9.024 and in theory a deck height of 9.0"-9.024 on the block should be between those extremes ,most shops will do a line hone on the main crank journals to get those absolutely strait before measuring for a deck mill operation to be very sure of measurements, but remember many re-builder pistons are designed to fit the 9.00" milled deck block height, you can obviously still use a block that has a slightly shorter deck height with thicker head gaskets, which are commonly available in .015-.060 in steps like .015, .018, .021, .030,.034,.040. .060 theres several other thicknesses and custom made up to .120 in some brands of solid copper head gaskets
HERES A QUOTE FROM A WELL KNOWN ENGINE SHOP
Four blocks recently that I have records in front of me for.
I kept them because they were all pretty bad & a good example of the reasons to square things up
Two were 400 small blocks & two were BB's
The nominal deck height for a SB is 9.025 & for a BB 9.800 for the standard deck
The two 400's
Worst one was 9.011 on one corner, 9.031 on the opposite end of the other side.
Sweeping this block from intake to exhaust was over .010 from a 45° using the cam as a datum.
The other was better, 9.019-9.027
One 454 was 9.797 to 9.815
The other was 9.786 to 9.801
This is why I don't build performance engines without decking the block square from the mains to whatever piston height I require.
Usually zero to .005 down.

The only exception would be a numbers matching block for a restoration but I have a program I wrote to leave the numbers now so I usually do these too.
This is also why almost all common combinations have a c/h of pistons that come out less then the nominal deck of the block.
It is a hell of a lot easier to adjust the block than to adjust the pistons,,"

It might be a surprise to many guys but a $10, METAL CARPENTERS SQUARE can be used , to verify a block thats is significantly warped on the deck surface

have suspect parts mag checked

obviously the only way to know exactly what your dealing with is careful measurement or allowing the local machine shop to do it for you.

related info
viewtopic.php?f=50&t=2988&p=7845&hilit=angle+mill#p7845

viewtopic.php?f=51&t=2692&p=8201&hilit=edge+block+strait#p8201

http://www.circletrack.com/techarticles ... index.html

 

[100% Chevy]

Thanks Grumpy.
I measured the bore across the top of the deck and came up with 4.060.
So,it's been bored too.
The pistons are like the stock pistons but without the valve reliefs.Look like 305 pistons.It had 305 heads on it when I bought the car.So,I replaced the heads with 350,and also put an iron intake on it.The lifters look new(compared to the new lifters I have),so I'm guessing the cam is good too.Wasn't planning on a rebuild.
If the block has been decked,then it may help explain my vacuum loss.
I've tried 3 carbs,all of which run fine on my truck,but the car still has low vacuum and runs like crap at idle and not that good under load.
I'm going to put it back together with new gaskets and see what it does.
Thanks again,Mike.

 

[grumpyvette]

decking the block will have almost zero effect on the vacuum the engine draws at idle,
it is more than likely that the cams been installed without degreeing it in correctly,
and its indexed a few degrees out from its ideal location,
or the ignition advance curves wrong , thats almost certainly the most likely source of your problem, (the cams indexed wrong)
or the valves are badly adjusted, YEAH I KNOW your 100% sure they are properly adjusted, well I've rarely found that to be true on most cars so, just be sure, check it carefully at idle,
and dropping back to basics and doing a leak down test and verifying TDC and timing tabs sure won,t hurt either
if you check those three issues your likely to locate the problem
where are you located??

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[100% Chevy]

Only thing I'm 100% sure of is I'm losing my patience!
I'm in south Mississippi near the beach.
Reason I was asking about decking is if it was shaved too much,then I thought I may have too thin intake to head gaskets allowing it to lose vacuum.The intake squeezed out almost all the sealer at the front and back of the intake.And was leaking oil at the front.
Cars been running bad since I bought it 5 years ago!
I'm going to make a piston stop and check the balancer.
I may just change out the cam since I don't know what it is.(I can see my face in the lifters!)
I put a new timing set on it last year,along with the heads and yes, I set the valves.No noise then 1/4 turn more.
Have never dialed in a cam!
And,I have a new cam set.It's a Crane Cams #C-274.I don't know what it is and it don't come up on their website.
Any idea?
Thanks for your knowledge and help.Please bare with me,I can be slow!
This is my favorite car too!
Mike.

 

[grumpyvette]

having a vacuum leak will cause some of the issues your describing,you may need thicker intake gaskets,to get the intake to seal correctly if the heads have been milled, but if it was my car Id do some testing and find the source of the problem, and I would think that a new cam and lifters and timing set would be an option here.
as always a logical step by-step test & verify approach will help.

post your cars info, weight, trans, rear gear,tire diam. compression,ignition advance curve and every thing else you know about the engine and drive train and I,ll try to select a reasonably close match in a cam for you.

 

[100% Chevy]

The car is a 1965 Caprice.(4 door hardtop)
Weight?Guess around 3500 lbs.
350 engine,no numbers,but I'm guessing late 70's-early 80's.
350 trans,3:36 rear,P-225/14" tires.
Compression before I tore it down-
1-125
3-125
5-125
7-125
2-125
4-121
6-125
8-130
I did the test on a hot engine,throttle tied open,all plugs out,no ignition power,and recorded highest reading on my tester.Comp ratio around 8:0 or better?
Gm,HEI distributor with full 12 volts,no resistor wire!
I don't have enough vacuum for the power brakes to work right.Also have p.s.,a/c,and factory level-air suspension(requires vacuum too).
I'm not looking for higher performance-Just a good dependable drive train for daily driveing.
I need to cure this vacuum/idle problem though because right now it will load up and foul the plugs.
It could be the intake was leaking on the inside and a new gasket will fix it.
Since I don't have any way to tell what the present cam is,I was thinking I'd replace it with stock?
Thanks,Mike.

 

[grumpyvette]

http://www.cranecams.com/product/cart.p ... il&p=23790

Ive used this cam in several combos similar to yours with good results, and yes the cam REQUIRES new lifters
and a NEW CLOYES TIMING SET, and yes Id carefully inspect the old intake gasket for indications of a lifter gallery leak and replace them with thicker gaskets if thats the case, normal intake gaskets are about .060 thick, they are made .120 thick

http://www.summitracing.com/parts/CRN-114132/

http://www.summitracing.com/parts/CLO-9-1100/