Head questions.

lyeinyoureye · Post by **lyeinyoureye** » Sat Feb 09, 2008 8:42 pm

Besides a pressure test, is there any other way to check if there are cracks in the waterjacket? Also, if the head was cut down too much for a rebuild and the CR increased proportionally, could this lead to overheating?

Quantum-man · Post by **Quantum-man** » Sat Feb 09, 2008 9:55 pm

I suggest you read the zillion threads explaining this very point... Skimming head does not increase compression ratio significantly due to the chamber being buried inside the head.
Wots the point in retaining all these threads if noone reads them?

lyeinyoureye · Post by **lyeinyoureye** » Sat Feb 09, 2008 11:21 pm

Easy killer, a decent comment goes a lot farther than that there line. I suppose if we all knew how to search we'd all have answers to all our questions and there would be no need for this forum. That being said, if the head was milled w/o anything done to the valves, that could influence the CR, correct? And... Anything I can inspect for via sight/smell/touch/taste in order to see if the waterjacket was cracked around the combustion chambers?

Quantum-man · Post by **Quantum-man** » Sun Feb 10, 2008 5:01 am

Sorry mate you just happened to be the innocent [almost] victim of some 'Euro venom' At least you get Camp Granada points for knowing where you live unlike a lot of people using the forum

Checking for real cracks? If head was off I'd get some putty build a birds nest round the crack (awkward ) and drop kerosine onto the crack and check with finger for seepage.

Protruding valve heads would add 'something' to the compression ratio but also increase the chances of a drastic reduction in pressure by twacking the piston if timeing not accurate

I think we all need a little practice with the search function. Its trying to get specific words when posters call same item by different names

and reducing the billion hits

libbybapa · Post by **libbybapa** » Sun Feb 10, 2008 9:28 am

You can add "and" between each search word or check the circle "Search for all terms" to limit the results you get back. Just because many don't use the search and many don't read past threads, doesn't mean that none do. I have often linked to threads or copied posts that I myself have written. Having the archive handy can save a lot of redundant typing even if no one else searches.

Andrew

woodrat · Post by **woodrat** » Sun Feb 10, 2008 12:17 pm

My understanding of internal combustion engines is that yes, if you decrease the combustion chamber size, then yes, the compression ratio goes up.

As was pointed out here, though, more major worries in a diesel are what happens when the valve opens and touches a piston because you've lowered the head too much. That's what the different thickness gaskets are for. The Bentley manual describes how to measure your block and piston setup to get the right head gasket. It also states that you should not mill the head any more than .004".

I would just guess that you would have valve clearance problems long before your compression ratio got high enough to be the cause of overheating.

I've never understood why forum dwellers get so grumpy if you ask a question that may have been answered 14 months earlier. New questions and questioners are what keep forums fresh and interesting, right? Different people will respond each time and different details will emerge, all to the good.

Quantum-man · Post by **Quantum-man** » Sun Feb 10, 2008 2:59 pm

The reason why I don't think the compression ratio would go up by much is because of the flatness of the head and near enough all the combustion chamber is buried below the head surface and skimming pretty well only reduces length of the entrance to the 'ants nest' which has only 1/2 a cm area ... 'ball park'

I think we should all promise to do a quick search for something just to prove it to ourselves

lyeinyoureye · Post by **lyeinyoureye** » Sun Feb 10, 2008 4:43 pm

I searched for Head AND shaving, but my problem was I read the thread with shaving in the title, didn't get the info I wanted, and posted w/o searching further. I suppose Quantum-man was somewhat warranted in chastising me for sloth.

In any event, here's the head in question. The guy who gave it to me said that he thought there was a crack in it somewhere since it ran really hot. Could I seal the water holes on the bottom with putty and put kerosene in from the top and see if any leaks out via the valves like Quantum-man said?

libbybapa · Post by **libbybapa** » Sun Feb 10, 2008 9:02 pm

woodrat wrote:It also states that you should not mill the head any more than .004".

What Bentley manual states that regarding diesel cylinder heads and which page?

As Quantum-Man said, the head is flat. Please do a CC calc and let me know how many thou need to be milled to go up 1/2 of a point in the compression ratio. On second thought, let's work through it right here.

1.6 liters = 1600 cubic cm.
1600/4=400cm^3 (displacement of one cylinder)
400/23=17.4 (size of combustion space with the stock 23 to 1 ratio)
400/23.5=17.0 (size of combustion space with a 23.5 to 1 ratio)
17.4-17.0=.4 cm^3 (volume of change in combustion chamber size to raise compression 1/2 point)
7.7/3 = 2.6 cm^2 (area of combustion chamber that will be eliminated by shaving - I am assuming that the valves are 1/3 of the combustion face - being generous)
.4/2.6=.15cm (amount shaved to equal 1/2 pt in compression)

In other words, if no valve work is done, the machinist would have to shave a 1.6 head over 60 thousandths of an inch to affect the compression 1/2 of a point. The maximum warpage allowed by the Bentley is 4 thousandths which is 1/15th the amount of material necessary to raise compression 1/2 pt. Considering that the compression ratio on the VW IDI diesels range 1 full compression point on the various motors, I can't think that 1/2 pt change is significant and 60 thousandths is a big shave especially when you need to consider that the calculation is for 60 thousandths being removed from the entire combustion face. Also, considering that the heads always mushroom to the edges, it is rare that any significant amount is removed from the combustion face itself. If 60 thou was needed to be removed from the entire combustion face to make the head flat, then it is unlikely that the cam would turn. It is also unlikely that there weren't other significant problems with the head due to it having gotten mind-bogglingly cooked to warp that much. That's my opinion, YMMV.

Andrew

woodrat · Post by **woodrat** » Sun Feb 10, 2008 9:34 pm

I misread the manual, it was the maximum warpage that I read. The manual actually says "gasket surface, do not reface, maximum warpage .004" section 3, page 25 of the 1985-1992 manual. So I guess if I read the Bentley correctly, you're not supposed to surface a diesel head at all.

My brand new head had to be faced by .004 to take out a scratch from shipping and with the deck height that my engine had, my parts guy said that I already needed the thickest gasket available. I got the impression from him that I should not mill the head any further. And according to Bentley, it shouldn't have been milled at all.

I did say that I couldn't imagine that you could mill the head enough to cause overheating without bashing the valves first.

Sorry for the Bentley misquote.

libbybapa · Post by **libbybapa** » Sun Feb 10, 2008 9:42 pm

The gasket needed is determined by piston protrusion alone and has nothing at all to do with the head and whether or not it has been shaved.

Yes, Bentley says the diesel heads should not be shaved. That has not stopped knowledgeable individuals from shaving countless diesel heads and having them run without problem when done correctly.

Andrew

Quantum-man · Post by **Quantum-man** » Sun Feb 10, 2008 9:51 pm

woodrat wrote:I misread the manual, it was the maximum warpage that I read. The manual actually says "gasket surface, do not reface, maximum warpage .004" section 3, page 25 of the 1985-1992 manual. So I guess if I read the Bentley correctly, you're not supposed to surface a diesel head at all.

My brand new head had to be faced by .004 to take out a scratch from shipping and with the deck height that my engine had, my parts guy said that I already needed the thickest gasket available. I got the impression from him that I should not mill the head any further. And according to Bentley, it shouldn't have been milled at all.

I did say that I couldn't imagine that you could mill the head enough to cause overheating with bashing the valves first.

Sorry for the Bentley misquote.

The thing is, do you want to believe The Bentley Bible which may have cost owners $100's of thousands in unneccesary replacement parts ... or do you feel safer with the huge accumulated IQ of the forum 'brain' which on average is a non-profitmaking conglomerate

Here you have lots of differing views but often a pathway appears out of the mist whilst the Bentley/Haynes/Mitchell/VW group all seem to copy each other...

woodrat · Post by **woodrat** » Sun Feb 10, 2008 9:51 pm

The gasket that was on my engine was a two notch, but when I measured the piston protrusion with a dial indicator and used the chart in bentley, it told me I needed three.

If compression ratio is not a factor in the gasket thickness, why have three different thickness and a chart to tell you which one to use? If it is only about making sure you keep the valves off the pistons, then the thickest one should be fine for everyone, right?

Yet I have read at least a couple of posts here telling me to NOT just go for the thickest one, that my CR will be adversely affected and the car will not be as efficient.

What is the gasket thickness issue all about? Seems to me that if they are telling me to watch out for differences in a few thousandths of an inch in piston clearance, that shaving .030" off the head would figure into how thick the gasket needs to be. What am I misunderstanding here?

Quantum-man · Post by **Quantum-man** » Sun Feb 10, 2008 10:01 pm

It's to make up for the fact that the pistons don't lie flush with the top of the block but protrude by varying amounts which is only not hitting the head because of the gaskets. I might be wrong there

Quantum-man · Post by **Quantum-man** » Sun Feb 10, 2008 10:04 pm

Quantum-man wrote:It's to make up for the fact that the pistons don't lie flush with the top of the block but protrude by varying amounts which is only not hitting the head because of the gaskets. I might be wrong there

In fact why not trim the piston crown to below the block and throw away the gasket??