348 / Mondial T engine cylinder leak test

If all cylinders are leaking at 20% or more, perhaps the last valve job was not done properly. If it were my car, I would lap the valves again. That means engine out, disassembled, and redone. Budget or not, that is your call.

 

Viton seals for valve stems.

 

You might not have to resurface the head at all. Measure it.

 

In my experience, this is not a bad number. I have measured many while doing a major service and small percentage leaks are always there. I find it rare for a cold head to seal well. If it improved with a tap, then I would run it. Did you have any running issues when it came in? Don't fix what isn't broken.

Also, cutting the heads on these engines is only needed if the head is warped. I have only had to cut one set and I have been doing this since 1998 and was schooled in automotive machine work with a few years in a race engine shop before I went to a Ferrari dealership.

The one I had to cut was so hot, the springs and buckets had turned purple. The over heat was a previous owner. The actual fault was the engine had softened from heat. The heads were retorqued to the point of squeezing the aluminum into the threads of the studs. (With PO) The main caps actually came loose and the oil pressure went to almost zero. The block was not reused but I did save the heads.

 

I cannot believe some of these answers.
In answer to your original question the very best a valve will ever seal is the day the valve job was done. They never get better.
Cold hot, makes zero difference.........ever.
20% is terrible. Someone did a terrible valve job. Many auto machine shops really struggle with doing the small valves in a 4 valve head. I am redoing the valve job in a QV right now that was done not that many miles ago by someone very well known on F Chat and it looks like whoever did it was blind and drunk.
Don't let anyone tell you its OK. Its not.

 

Yeah 20% is bad on any leakdown tester, some are more sensitive than others, such as the other Snap-on tester mounted in the red box that uses 60psi (pic enclosed). That thing is incredibly sensitive if anyone has that thing......


Anyway, the 4v heads and Nikasil liner engines usually leakdown really really tight don't they? As in, less than 4-5% leak is the norm with them for the few of them I've worked on.

 

I cannot reconcile 225 PSI on one test and 46% leak down on another test.

 

The last sentence of the OP states, “the engine works just fine.” Given that fact, why not use a few cans of Liqui Moly Valve Clean to clean up the intake system. After a few cycles of high speed driving, rerun the leak down test again.

 

Hello

For reference hère are what I had on my Mondial T:


In psi and %:

98/94: 4% | 88/86: 2%
92/85: 8% | 90/89: 1%
93/88: 5.5% | 92/90: 2%
99/93: 6% | 98/96: 2%

Compression test:

12.5(180) | 13.5(185)
12.5(175) | 13(185)
12.5(175) | 13.5(190)
12.5(180) (counter test at the end: 13(185)) | 14(200)

The twentish % leak down test looks weird indeed given the good compression..

 

I only build a motor every few years and a ferrari motor once a decade. I'm also not that into the nuances of building & machining. I send out machining and do my own build. I have caught plenty of errors that have to go back to the machine shop. Every time I build a motor I have to find a new machinist because I have never found one that pay attention to detail and think of my motor as anything more than a "unit" of work.

That said I don't think lapping at 40% leak is a fix. I might use a lapping exercise not even with compound but a sharpie marker just to see what the current seat looked like. My second step would be to put a runout gauge on the valves as I suspect the failure is there. Then it would be off to the machine shop.

Just as an aside leak downs are best done on hot or warm engine. I don't like to get burned and I work slow. My tests are always done cold. I don't know why these tests should be done warm/hot. The machining was all done cold and all the parts are cut cold to match. Measuring later at temp with expansion of parts doesn't that just introduce a new error? I would think all measures should be at room temp for repeatable results. Well no one is electing me president either...

Then there is valve lapping. I don't do this. It is 21st century. The machinist cuts are accurate. I'll spin every valve with sharpie marker and look for the uniform smear. I have never found a need to lap. Any fault would be a trip back to the machine shop not a cover up of an error with lapping compound.