cylinder head/cylinder sealing issue

[trond]

rebuilding the engine with new 98 mm cylinders I did a leak down test before install and found 25-30% on all 6 and air hissing from all cylinder head/cylinder interface. I took it all apart again and checked with bluing colour. What I found was poor contact. I tried with an old cylinder and found perfect contact. Cylinders are new all of them. Heads have been resurfaced. How can this be dealt with ?

[john walker's workshop]

I would use a micrometer and measure between the cam tower surface on the spring side of the head and the flycut surface to see if any area is higher/lower. All the heads need to mic the same. Check at 2,4, 8 and 10 o'clock,

[stownsen914]

One thing to look at is that you're not getting contact between the registers on the cylinders with the registers on the heads. It's important that only the top of the cylinder barrel touches the head sealing surface. With the cylinder mated to the head, there should be a few thou air gap between the register on the head and the register on the cylinder. You can measure this with a feeler gauge at the cylinder head stud holes. It would be a bit surprising if a new cylinder doesn't have the registers machined correctly, but who knows. Or possibly the heads are machined too deep.

I had exactly this problem one my cylinders when I got everything back from the shop. Luckily I noticed before assembling, and sent the cylinders back to have the registers shaved a bit. Apparently, at some point in the past, someone had machined something incorrectly.

[911MANN]

Interesting this should come up just now, as I had the same question just 2 weeks ago. My original 95mm ROW cylinders have a slightly longer (0.15mm) spigot end inserting into the cylinder heads than my new Mahle 98mm Max Moritz cylinders do. My engine builder is accustomed to seeing a larger gap between the shoulders when assembling, and he also noticed the Prussian Blue compound wasn't leaving a strong impression of a seal when fitting by hand, which raised the question. The fitment of the chamfered edge of the 98mm cylinders is probably a contributing factor there, as it's not a flat seal all the way across the spigot. I'm unfamiliar with the term 'registers', but that may be the same thing I am calling spigot (not at the case end).

After head machining, my available gap between the shoulders of the cylinder and head is 0.3mm, which is reportedly good enough to still leave clearance when torquing and the chamfered edge gets seated. But mine is not yet torqued- soon, I hope. Hope this might help.

[trond]

and a few hours later I no longer think it has to do with the cylinders, but rather how the heads were machined. Will continue tmrw but confident it will be OK. Should have done leakdown testing earlier, not just before lifting engine off stand to install in car....
EDIT there is clearance between spigots and nothing preventing contact sealing surfaces that I can see. Also interesting that ONE cylinder/head is sealing perfect. The 5 others leak with air, regardless of torquing pattern or anything I can do assemblywise

[911MANN]

Here is a sketch I made describing my condition when I was investigating. The numbers shown here are not quite as exact as they measured at the machinist when I took it to them for verification. The cylinder spigot height is only a tiny bit more than the head's socket depth, almost too small to see.

I am calling the H2 & C2 surfaces the shoulders, and that distance when more carefully measured is actually 0.3mm as measured with a feeler gauge. Almost too small to see, but it's there. My original ROW cylinder spigot height measured 0.15mm taller, which leaves a gap more easily seen. And that larger gap is what my builder said normally sees.

I'm not sure we are describing the exact same concerns, but if anything might be gained here, I wanted to share my details.

[Zuffenwerker]

I got a set of brand new 104mm nickies with basically the same issue. Got it assembled, did a leakdown and all 6 leaking. I sent one back to LN for inspection and they determined there was a machining problem so they made me some new cylinders

I say "basically" because my cylinders and heads for that project used flame rings. You need to return your cylinders if the old cylinders seal up no problem and you have a gap between the “shoulders"

[Neil Harvey]

If I can add some advice here. This may not be applicable to this actual situation.

First, make sure the Cylinder decks on the case are flat and parallel. This can be measured without the Cylinders fitted. It does all start with the case. Any difference here will show up as you get higher with cylinders and heads.

Measure the cylinder heights before fitting to the engine. You can fit the cylinders to the case without the pistons and hold own with the "nuts" used to hold the cylinders during assembly. With a straight edge check the heights. Assuming the cylinders are all the same height, any difference will be in the case decks. If you find any difference when measuring the cylinders, take care of these first. Then fit to the case and make sure the case decks are good. If the cylinders need any adjustment in height, remove material from the face that contacts the case.

Now measure the head heights. These should have been machined the same height during any recon work. If different, adjust to the smallest height. Remember this can change your P/V and CR too. A good practice when assembling an engine is to dry assemble parts and check measurements. Fit the cylinders and do the check suggested above. Then fit the heads, tighten the nuts to snug and check the head heights fitted. Again, a straight edge across the heads is a good way of checking the heads for having the same heights on the engine.

Lastly, check the cam housing for flatness. The straight edge is useful here as well.

If you want to make sure you have no chance of leakage under cylinder pressure, this is what we do and offer as a solution. One ring seals the combustion gasses and the other acts as a crush limiter and stops the head from "banging" on the cylinder top.




Also, we just had a one piece base gasket made for the later 964/993 engine. So far we have 0.030" and 0.040" thickness in stock and will soon have other thicknesses. Looking at our tooling to see if the earlier engines with the narrower head stud spacings can also have a one piece gasket made.

[trond]

Thanks Neil. Your continued support is appreciated, as is the quality of your services. Good to have a solution apart from replacing expensive and hard to find major components. Careful measurements first to understand what is the cause

[cgarr]

Make sure when they cut the heads the outer surface was also surfaced the same amount or you can get interference between that and the cylinder fins, the shoulder should be about .100 thousands. also if the head has to much radius on the sealing surface outer edge the sharp edge on the cylinder can hang up on that to and not fit flat on the cylinders.

[trond]

And bingo that was exactly the issue. I carefully used a fine file to make a tiny bevel on the outer radius of the cylinder sealing surface, and with that they all seal. Thanks a lot for the advice !!

Originally Posted by cgarr

Make sure when they cut the heads the outer surface was also surfaced the same amount or you can get interference between that and the cylinder fins, the shoulder should be about .100 thousands. also if the head has to much radius on the sealing surface outer edge the sharp edge on the cylinder can hang up on that to and not fit flat on the cylinders.