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Eh? GS500 Major Service says to LOOSEN the head nuts !!

Discussion in 'Maintenance and Servicing' started by lipstikpig, Apr 3, 2012.

  1. I am a mature (age) newbie, licenced 2 months, riding a Suzuki GS500 (Dec 2008) naked bought privately in Sep 2011. I've added about 4000km so it has now just over 30000km.

    I have the Haynes service manual plus reading as widely as I can to learn about my bike by doing my own scheduled maintenance. I am not an experienced mechanic but I have done plenty of basic car servicing and have tools, so I am carefully following the Haynes instructions.

    But there is something there that is freaking me out, so I want advice before I do anything stupid!!

    The official Suzuki maintenance schedule says to "tighten" the cylinder head nuts at 1000,6000,12000,18000,24000 km and I assume avery 6000km after that. But it does not give detailed instructions.

    The Haynes manual goes into more detail: "The cylinder head is secured by eight 10mm domed nuts .... Slacken the nuts evenly and a little at a time in a reverse of their numerical sequence until they are all slack. Using a torque wrench, tighten the domed nuts evenly and a little at a time in numerical sequence to 35 to 40 Nm."

    The Haynes manual clearly says to do this every 6000km/12months.

    Now this intuitively seems to go against what I know about heads and gaskets on cars, eg if you loosen the head on my car it must be checked and machined for straightness, and reassembled with new bolts and gasket.

    Fully loosening the head nuts seems to me to be just asking for trouble.

    What do folks here think of the Haynes instructions to fully slacken off the head nuts and re-tighten every 6000km? Would you do it? If I took it to a dealer service dept, would they do this?
  2. If your quote is complete... it doesn't say to fully loosen the nuts, just back them off a bit, in reverse of the tightening sequence. Then tighten them according to the sequence to 35 to 40nm.
  3. Follow the proceedure, I would do it, I am a mechanical engineer. Considering the trouble they have gone to, it must be a known issue and they need the re-torque.

    I dont trust dealers to do anything.

  4. Retorqueing the head in this way used to be routine with air-cooled engines. Once upon a time failing to do it was the one thing that would invalidate VW's warranty on their engines.

    In more recent times, it seems to have fallen by the wayside. My DR, for example, doesn't have an instruction in the manual to do it.

    However, the procedure that you quote as advised by Haynes sounds sane and reasonable to me. I would do it if I were servicing the bike myself. It will do no harm.
  5. Yeah, the quote is complete, ie I quoted everything relevant.

    I understand "a little at a time ... until they are all slack" to mean "until they are all fully loosened".
  6. That's interesting. My GS500 is also air-cooled, as you probably know.

    Thanks everyone for quick response and reassuring comments so far. Feel free to add anything more if you have them ... cheers!!
  7. My Clymer manual for the BMW says to do it too. Never done it though.
  8. I have a factory workshop manual and it says to do this every 6,000km.

    You need to remove the fuel tank to get to the nuts. Do it with the fuel as low as possible as you need to undo the fuel pipes and petrol will pour out. Also have somewhere set up so you can rest the tank on its side without damage. If you can drain the fuel it will make refitting easier. BTW my 2010 GS500 has three outlets; overflow towards the middle of the tank and on the left side the main and reserve outlets.

    A 30cm length of 2x4 is also handy for supporting the back of the tank off the frame while you r&r those pesky hoses.
  9. I don't think the instructions are asking to slacken off the nuts completely, but I would question unstressing/unseating the gasket tension and the benefit of doing that. What's the purpose of retensioning by backing off first? Interesting. I'm a mech eng too, and I'm struggling to understand the instructions. Is it to do with the temperature cycles? Why not just run over the bolts with a torque wrench to confirm they're tight to the required torque?

    Just as an aside, torque values are very rubbery. If the bolt and nut and nut bearing surface aren't lubed, much of the torque could go into overcoming the friction. If the bolts were done correctly in the first place, I'd be inclined to leave well enough alone.
  10. Rob, I figure they wouldn't tell you to do this if the bolts were correctly tensioned so I assume that over time the heat cylce, some other physical phenomenom or aliens may cause the bolts tension to change. To counter this you retension and I assume you slacken slightly first in case the bolts stick or otherwise give a false reading.

    But what the hell would I know as I am no engineer, although as an economics student I have studied the one true science :LOL:
  11. I believe it's down to the fact that an air-cooled, largely aluminium engine, is extremely "mobile" due to the large temperature variations it sees and the high coefficient of thermal expansion of its materials of manufacture.

    As a result, the tension in the head studs is constantly changing. Head studs are steel, which expands much less with heat. A head nut torqued to 18 ftlbs cold can easily see a tension equivalent to 70-80 ftlbs (IIRC, can't be arsed to work it out from scratch) at working temperature and even more in exceptionally hot conditions. Under the circumstances it would be something of a minor miracle or a major engineering feat if head bolt tensions remained uniform over time.
  12. I thought most modern cylinder head bolts were torqued to yield these days aren't they? or is that just cars?
  13. Out of interest is it only the air cooled motors where retnsioning is recommended or liqid cooled as well. I am assuming that liquid coling may give a more uniform tempreature to the motor and less hot spots

    But as I said earlier, what the hell would I know?
  14. Once upon a time, it was routine to retorque a car's head bolts about 500 miles after any head removal but not beyond that. I assume that's gone by the wayside with modern gasket and bolt designs.

    My K100 didn't have a routine retorque in the service schedule that I can recall. Beyond that, I don't know 'cos my wrenching experience on watercooled bikes is strictly limited.
  15. Aluminium alloy heads are notorious for going soft, although not as often now as they once did; there's better metallurgy now, for the most part, and far more capable cooling systems (on water cooled engines, at least).

    The long studs on air-cooled engines will anneal at different rates to the case, heads, barrels, and each other depending on their composition, lengths, and locations etc.

    Backing the tension off ensures there is no stored/latent energy in the joint; that would cause distortion of the case/barrel/head and mating surfaces.

    In some cases there might be breaking of the gasket seal, but only as a result of pre-existing uneven loading. Most gaskets will reseal adequately, and most engines I've seen don't use gaskets on these faces anyway. Slight gasket weeping is more desirable than a stud being pulled out of the case... ask any old air-cooled VW mechanic. :)
  16. Pat if the Ks didn't rquire it and the Rs do I suspect it is an air cooled issue.
  17. I did this job a few days ago, without any problems that I'm aware of.

    I didn't measure at the time, but the torque required to release the cylinder head nuts was relatively high, maybe around 110 Nm (80 ft-lb). I progressively loosened them in reverse sequence until they all turned easily, but weren't loose. Then I progressively tightened them in sequence from 20 Nm (15 ft-lb) up to the final spec of 40 Nm (30 ft-lb). Per the manual, this is done after the block is allowed to cool overnight.

    Interesting that to undo the cylinder head nuts required more than double the cold torque spec.

    Thanks to everyone who commented in the thread, reading everyone's comments gave me the confidence to do the job. It wasn't difficult. (y)
    • Like Like x 1
  18. Its probably a case of backing them off to release any binding (galling / corrosion etc) to aid in getting a more accurate torque reading.
  19. As others have said - it's to ensure the studs haven't seized. It's not an air cooled vs. liquid cooled instruction, and it should be a standard practice. I've seen similar instructions on everything from diesel generators to big marine diesels.
  20. Hey Mate.
    Sorry that I didn't see this beforehand.
    The last two posts are the main reason that you have to loosen the nuts/studs. You would have found that out as you said when you backed them off.
    You'll have found that if you backed them of 1/4 turn, you may have had to tighten them up 1/2 turn to get them to the correct tension.

    Another important note is to never, ever use a torque wrench in the reverse it was designed for, they lose their calibration and will never be accurate until re-calibrated.