Cracks in XN6 head: repair or not?
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  1. #1
    Fellow Frogger!
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    Default Cracks in XN6 head: repair or not?

    After five years of rallying, the Berline engine finely filed its first protest; fair to say, it was forced into it! By way of background: I found an XN2 engine new in its crate in the US and bought it along with an XN6 head which was used but in very good condition. Brian Holm, very knowledgeable and a reputable seller sold me the head at a premium because it had no cracks and this made it a rare find. The XN6 head is supposed to flow better than the XN2 head and that’s why I bought it. We flow improve all the heads and I later found out that after we do that the XN2 and the XN6 heads behave identically. The new engine received Wiseco 11:1 CR forged pistons, a copper gasket and a Camtech cam designed with the guidance of the late Owen Wuillemin. At the time I was also considering oversized valves but I opted out to retain good midrange response. Fitted with 45 mm DCOE’s the engine developed 170 bhp at the flywheel in its latest iteration, but it was not considered fully optimized: The 275 degree cam was better than bigger cams from other manufacturers, but the engine would rev with good power to around 6,200 rpm, the original plan being for just under 7,000 rpm. The valve spring pressures were also low: an estimated 55 lbs at the seat and 175 at full lift, our target was 80 and 200. The head porter forgot to add 2 mm washers to increase spring tension and found the washers and called us long after the engine had been assembled. We made the decision to leave as is rather than take the head off; there would be a rebuild soon enough! Well, there wasn’t. The engine soldiered along with the entire car through five years of abuse, I finally missed a shift and over-revved it to valve bounce. I finished the event and we took cylinder pressures: 240, 240, 240, 150 psi. Added oil to the low cylinder, no change, adjusted valves no change, the verdict was for a bent exhaust valve. A trip to the dyno showed a loss of 15 bhp at the wheel but except for rougher idle the car still pulled very strongly so we kept using it for more than six months. We kept monitoring cylinder pressures but they remained steady so I was not too concerned. At the same time we asked Camtech to develop a bigger cam, as close to 300 degrees and as much lift as they could get. We got the 293 degree cam last week (many thanks to Graham Wallis for all his help) and took the head off to repair the valve and install the new cam. As expected the exhaust valve in cylinder 1 is bent, you can actually see light through it. What was unexpected? The cracks we saw running from the intake and exhaust valves to the spark plug holes of cylinders 2 and 3. Running temperatures were normal and we were not losing coolant which would indicate that combustion gases were escaping though the cracks. I recall reading many posts as to how common these cracks are but had never seen them before. Most posts also indicated that they are not a big concern. As an aside we have installed big valves in all the other heads, this should make it even easier for cracks to form and propagate to the spark plug hole. I am fairly determined to replace the bent valve and reinstall the head without repairing the cracks. What is my downside risk on that? I think the downside risk in repairing the cracks is significant; at the very least the head will have to be skimmed because of the heat and the extra compression will make the engine very vulnerable to low quality fuel. Not to mention the risks of a botched repair.

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  2. #2
    1000+ Posts Mike Tippett's Avatar
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    I have a KF2 404 Injection head with such a crack, but it propogated up the spark plug hole of one of the middle cylinders and then the threading was dislocated to the point that one of the plugs was nearly impossible to screw out (I broke an OEM Peugeot spark plug wrench getting it out).

    This was after a severe overheating episode in traffic when the coolant froze solid on a long mountain descent in -10 degrees temp, followied by a sharp climb up a major bridge with traffic and thus no chance to stop or slow down....so I never did hold a grudge against the engine - my fault for a dilute coolant mix......

    The other central cylinder (2 or 3, I forget which) has a hairline crack between the valve seats but that cylinder is fine and there appeared to be no issue there.

    These cracks are well known to Le Club 404 and the advice is usually to ignore them, except where you can't, as in the above example (but a heli-coil might fix that).

    The head on my Coupe has never been off but just in case, a number of years ago I bought a new-in-box 404 KF2 head from Peugeot, which comforts me.

    I think you could probably re-use that head with no issues.
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  3. #3
    Fellow Frogger! callipygous's Avatar
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    Good info, thanks for sharing Thanos.

    From what I've heard from everyone is don't worry about the cracks. Who knows how long they've been there and they're likely to cause a lot less problems than the valve being bent. If it does fail, we'll know that the cracks are only a problem if you race!

    You could repair them, I would only bother if I was in the business of modifying the squish to a different profile and the head had to be welded/machined anyway, or it's something you would like to try as you're hopeful that you might get a decent gain from a squish change.

    I noticed mine were cracked too and was told not to bother doing anything. I did spend a lot of time smoothing what I think were rough castings with sandpaper to get a reasonable finish so I had time to examine them, and well, they don't appear to go too deep.

    So, you mentioned 45DCOEs, is that one or two? I've heard a lot of Peugeot people say two 45s are too big, so most opt for two 40s. Heard of a singe 45 being done, too.

  4. #4
    mlb
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    I've got them in just about every combustion chamber on mine, exactly as you describe them with no noticeable problems. I have experienced the tightness in the sparkplug thread same as Mike - although not to the same extent.

    But then again - I'm a bit of a cowboy apparently
    Fight for your opinions, but do not believe that they contain the whole truth, or the only truth.

  5. #5
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    Quote Originally Posted by mlb View Post
    ... I'm a bit of a cowboy apparently
    This way you don't have to cowboy up for special occasions ( like cutting springs), it comes naturally.

    The head is reconditioned with a light skim (0.1 mm), 5-angle cuts and new intake valve guides. My head porter called the craks a "misdemeanor" and said to "forget they are there". So we have a consensus, a rare thing in our days.

  6. #6
    Fellow Frogger! Rally's Avatar
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    After 25 years running a Peugeot dealership service deptment I have seen many XN heads with crakes between valve and spark plug, none of them has ever caused a problem.
    Many machine shops wanted to repair the cracks but I always said not to do so , and just refitted the head, with no problems.
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  7. #7
    1000+ Posts Wildebeest's Avatar
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    Cracks in Peugeot heads? I've always called them "occupational cracks". I like Thanos' misdemeanor cracks !

    So that I didn't have to look at them and to distract others I would gently peen the cracks closed using a small engineers [toffee] hammer.

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