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531 sadness :(

ShadowofBob

ShadowofBob

Revived...
Joined
Nov 30, 2004
Location
Palo Alto, CA
531crack.jpg

So I decided to take my head off after having a consistant idle problem and low vacuum. What really made me do was after I found that my cam was shimmed improperly(.008 feeler wouldn't fit between the shim and lobe) was when it failed a leakdown test horribly on 1-3 and still didn't do to well on cyl 4.

Well upon taking the head off I found that^ on cyl 1-3.:-( I already had cracks repaired on this head when I got it too. Had cracks between intake valve seat and spark plug hole on cyl 2 and 3.

Head had been ported and had 38mm stainless exhaust valves.
Guess this head was trash from the start.:grrr:
 
Those cracks are no big deal. Definitely not going to cause leakdown issues. In fact, there's so much meat there and no water or oil for quite a ways down, it's not going to harm you at all. I've got a head with cracks in all four cylinders that'll be going on my car, and it pressure tested just fine at the machine shop before any work was done.

Your leakdown issue lies elsewhere.
 
Yeah man stick it back on and pretend you never saw it. Wouldn't the problem lie beneath the head if it's failing a leakdown? Like rings?
 
cracked head

I had a crack between all the seats and on two cylinders from the seat to the spark plug hole on my scp 405. My machine shop told me they could fix it but it would be a lot of money and effort that could be expended elsewhere on the head. So, I gave them my 531 and stashed the 405 head for later.

I think Eric hit the nail on the head. You should have adjusted your valves and then checked again. Because I had a head with valve clearances that were too small like around .008" on every valve and that caused bad leak down and low vacuum when idling. A valve lash adjustment cured it and it's been fine ever since. This was on my 82 turbo when I installed the ported bcp 405 on it. I just swapped over all the buckets, shims and cam from the old head. Since the new head had a valve job with new valves installed that made the clearances all smaller than before.
 
EricF said:
If the .008 shim wouldn't fit under the lobe, I wonder if the valve was just not fully closing.
Click to expand...

I took the leakdown test with the camshaft out of the head.

What's weird is that the 3 cylinders with the cracks did the worst in the leakdown test. And cyl 4, which I had found no cracks in yet, did the best(although still not what it should be, 40psi held when charged with 50psi) out of them all.

I know cracks like that can be fixed. Cyl 2 and 3 had cracks like that between the intake valve seat and spark plug hole when I got the head. I had it ground out and welded.

Leakdown results when charged with 50 PSI:
Cyl 1: 18 PSI
Cyl 2: 27 PSI
Cyl 3: 26 PSI
Cyl 4: 39 PSI

Did compression test with my old 530 before I changed it and got 155 across the board.
 
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leak down and compression yeild completely different numbers that shouldn't be compaired that way. did you do a compression test on the 531 before ripping it off.

drop off the head at a machine shop and have them pressure test it. you might be right you might be wrong but hell if its o.k. slap thatpuppy back on the car.
 
The tight clearances might have just ruined the valve sealing surface too. Like a little buildup, or deposits on them. Maybe they just need a little lapping. I know of a couple places that can pressure test it for you while you wait if you just show up with the head. They didn't even charge me for a quick check.
 
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