Yanmar and too tight head bolts

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Ana

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Hi!
Bought a cheap torque wrench. Result: it got stuck and I have tightened the cylinder head bolts too much!

So, what should I do?

Engine: 1-cylinder Yanmar YSM12 y 1980, the cylinder head is massive cast iron piece of art. I bet it is not the most sensitive cylinder head.

I had a short 1 minute test run with the engine, it runs all right. But what happens when it gets warm? (The engine is converted to fresh water cooling)

As I was tightening the bolts (to 134 Nm or 13,6 kg/m), I noticed that the bolts got tighter and tighter and the wrench did not tick. I did not tighten with all I could give, but I realised they were really tight (the wrench is 45 cm long).

So, should I let it be and believe that the tolerances in that primitive machine can handle that. I bet that over the years many mechanics all over the world have tightened head bolts with the gut feeling.

Or should I loosen the bolts? Then I probably should change the head gasket too...
 
If it aint broke dont fix it!

I am familiar with your engine-I had one in a narrowboat.

The worst that might happen is that the head bolts might be stretched and might allow the head gasket to leak. If so, new bolts and a new gasket.

I think you will be fine.

As a precaution you might slacken each bolt in turn and re-tighten to the correct-or seat of your pants-torque. It is often the case that when re-torquing a cylinder head you are advised in the manual to release each bolt and bring back to the specified torque. The analogy of pushing a car-hard to get it moving, but once moving it is easier to keep it moving. Not releasing can give a false reading.

The torque does seem high-13.6 kilograms at a 1 metre fulcrum is high in my experience for a cylinder head.

Simple mental and probably not very accurate conversion brings that to around 85 lbs feet.

Your call............................
 
134 NM is 98 lb.ft, which seems very high for cylinder head bolts on a small engine. The worst that can happen is that the bolts have stretched plastically, which you might have noticed. As you continued to tighten, did the force required go on increasing or did it reach a plateau or even begin to reduce? In this case it would be best to replace them.

Head bolt tightening on modern engines continues just to the yield point to reduce the possibility of fatigue, so if that is as far as yours went you have done no harm and possibly have done good.
 
I am not familiar with this particular engine. Are you sure the head is bolted on, not sitting on studs?

The torque isn't that critical but if you seriously over-tighten the nuts, the studs fail. They may not fail on the bench but when the engine is running, which is not good.are you sure about the torque figure for the head? My 1GM10 manual gives a figure of 75Nm for the cylinder Head nuts. The studs have to provide the force to seal the gasket AND oppose the force of the burning fuel as it drives the engine ( the "Bang" part of the 4-stroke cycle) which is why it is inadvisable to over tighten these fixings.

The first step is to check just how tight they are. This may involve borrowing another torque wrench. You start setting the wrench to the design torque of the nuts, then increase in increments until the nut just starts to move. If you back off as soon as you detect movement, you're unlikely to do any more harm. You might actually check the wrench you have by setting the torque to a much lower figure, say 40Nm, and seeing if it does indeed "click".

It's not the head you need to worry about, as there are no threads in it and Cast Iron rarely fails under compression. It is normal for the head studs to have coarser threads into the cylinder block so hopefully the nut end will strip first.

It is unwise to turn back the load on the head bolts as the torque compresses the head gasket and this may need to be replaced.
 
I have got the original workshop manual, and the torque is really 13,6 kg/m (98,37 ft/lb).

The cylinder head has two bolts (lower bolts, horisontal cylinder) and two studs with nuts.

As I tightened, the bolts/nuts got firmer, they did not become "soft", so I think the bolts and threads are ok (at the moment).

I am leaning towards letting it be and hope for the best...
 
Topcat47 said:
It is unwise to turn back the load on the head bolts as the torque compresses the head gasket and this may need to be replaced.
Click to expand...


Not so. Read my earlier post. Several engine manufacturers recomend slackening each bolt/nut in turn-in the reverse sequence of tightening-and re-torqueing after running in or rebuild. Of course the gasket will have been compressed by the first tightening, but the subsequent tightening, carried out correctly will compress it a fraction more-which it can cope with-or make no difference at all if the bolts/nuts go back to where they were. One at a time and in the correct reverse order of tightening is the trick.

As a retired small engine specialist of over 55 years experience I have carried this out hundreds of times without any problems.
 
Quite a dilemma. I'm sure that very many people have tightened head bolts to unknown torque and lived happily ever after. The problem is that you have some factual information that suggests you've over tightened the bolts. Can you live with the doubt or will it eventually get to you? Personally, I'd take remedial action. It sounds like you have ready access to the head so why not address your concerns. Typically, a bolted joint is tightened to about 80% of the material proof load (max load before plastic deformation). Do you think that you could have over-tightened the bolt by 20%?
 
Rotrax,

Your advice makes sense to me! It might be a good idea to loosen and then tighten to the required torque.

Cheers!
 
I had to change a valve on a yse12 once on a cruise. No Torque wrench, no manual, no internet. I just tightened them up as tight as I could. No problem.
 
Bluenote2 said:
I had to change a valve on a yse12 once on a cruise. No Torque wrench, no manual, no internet. I just tightened them up as tight as I could. No problem.
Click to expand...

That's exactly what I would do. We're not dealing with high tech here. Get out and enjoy.
 
Bluenote2 said:
I had to change a valve on a yse12 once on a cruise. No Torque wrench, no manual, no internet. I just tightened them up as tight as I could. No problem.
Click to expand...
Not a cylinder head bolt, but the big nut holding the wheels of a VW camper van on! I didn't have a torque wrench, but I computed that my weight on the end of the long Tommy bar would be just about the right torque. The wheel never came off, so it must have been good enough!
 
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