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Topic: Valve Clearance (Read 805 times) previous topic - next topic

Valve Clearance

Engine guys,

I have a set of Windsor JR with 1.940" valves.  They are going on a freshly honed/ringed bottom end with clean stock notch pistons. Shortys, 2.5" exhaust, and GT40 intake, will be mass air.

I was looking at getting a Crane 2030, or 2031 or maybe a TFS1.

While doing my research, I started to wonder if I will need to notch my pistons.  I can't seem to find any data on what size valve/duration will require a cut.  Seems most people end up cutting reliefs with 2.02s and I didn't find anyone who cut reliefs for gt-40s.

I know the only true way is to clay the current notches. 
For reference:
E7 - 1.78
GT40s - 1.84
WJR - 1.94
AFR185 - 2.02

2031 = Advertised Duration 276/282, Lift .513/.529 LSA 112
TFS1 = Advertised Duration 275/279,Lift .499/.510 LSA 112
2030 = Advertised Duration 270/278, Lift .533/.544 LSA 112

Any thoughts?
-dz

Valve Clearance

Reply #1 –
Sorry, checking is the only way to go.  Variables include: Cam timing (when the valve actually open in relation to the piston), combustion chamber size, how far the piston is in the hole and of course valve size.

As an example:  I had a stock 95 Mustang Cobra short block.  The piston was slightly above the deck (.005), my combustion chambers were 54 CC's and valves were 194/160, an E303 cam.  I had to cut the intakes deep (those hyper pistons were HARD) the exhaust was at the limit of acceptablity. 

I've found pistons as low as -.40, a few at -.25, a few at -.005 and the one +.005.  Where that piston sits will greatly influence compression!

An aside on the cam specs you have listed for the 2031 cam.  That cam is designed for 1.7 ratio rockers.  The lift number advertised will be less with a 1.6 rocker.

A custom cam can be designed to eliminate any piston to valve interference.  You need to provide the type of head, combustion chamber size, valve size and how far the piston sits in the hole.  The cam designer will make sure the valves open at the right time.

Valve Clearance

Reply #2 –
Quote from: 86T-bird;268652
. 

I've found pistons as low as -.40, a few at -.25, a few at -.005 and the one +.005.  Where that piston sits will greatly influence compression!

.


all that on the same engine?  some were -.4 and others at -.005?

did you have the block line bored?
or
the rods broached?

Im not sure how this wide of a variance could happen on one short block set.  I can see this happening if the rods were broached but not if the block were line bored just a .0005 off or less.  Your pistons would never go up and down correctly if the block line bore was off.  If the rods are punched again and they are off, thats where your coming up with pots having different installed heights.

Valve Clearance

Reply #3 –
In the old days(pre '80s), wasn't uncommon to have .015-.020 deck/piston variance in production engines... Was the reason the Hi Po guys always blueprinted their engines, the factory parts  left a lot to be desired... Pistons were all between .001 & .003 out of the hole on my '86 HO short block, figured that was good enough...

Valve Clearance

Reply #4 –
"all on the same engine?"

God no!

The -.40 was a rebuilt 289 with Silovlite cast pistons.  The others were various engines I've owned and rebuilt over time.  After discovering how far in the hole a piston could be, my machine shop costs on rebuilds increased due to squaring and decking of the blocks. 

Of course those extra steps are not be needed for the average garage ring and bearing replacement. 

Knowing where the piston sits in relation to the deck helps determine the real static compression ratio and explains why someone has what appears to be an identical combination but one has P/V clearance issues.

Just gets back to: you have to check.

Valve Clearance

Reply #5 –
Well, I will plan on doing a clay check.  I look up how to do that tonight.

Whats the best way to measure TDC to deck height?  Flat edge and feeler gauge?  I have a dial indicator and magnetic mount that I use for setting up gears if that will help.

-dz
 

Valve Clearance

Reply #6 –
The dial indicator is most accurate.  First you it will tell you when you are at the true top of the throw, then you can measure from the deck.

Hope you have good luck with the P/V clearance.