Technical Nuts
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Technical Nuts
...to deflect oil from tracking down the stud threads?
smart51
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I wondered why you might have 4 plain nuts and 4 domed nuts. Domed nuts are usually used for appearance, to prevent contact with the threads or to prevent ingress of dirt or anything corrosive. I can't see that this would apply. So the top culprit is oil, but surely thick motor oil wouldn't get in anyway. Penetrating oil is thin oil thinned further with thinners, and that doesn't always get as far in as you'd like. But perhaps an oily thread is more likely to come loose than a dry thread, even if it is slightly oily.
The Haynes manual says that the purpose is, "to prevent oil leaks", (Page 28, para. 13)
On a regular engine with a cylinder block, the studs will generally be shorter, and where they enter the block they will be bounded by the head gasket. On this engine the studs are secured in the crankcase externally.
I often find myself telling people to properly read the good old Haynes Manual because there are little gems of knowledge written within. Peter has proved that I should practice what I preach as that is one fact I missed.
Having said that I did think that the oil thing was the most likely explanation but asked the question in case I had missed something.
The question arose because I am in the process of an engine build that I got as a basket case last year. All the important parts have been cleaned up and the assembly is under way. When I cleaned all the small parts and sorted through them I was surprised not to find any dome nuts. I discovered that because the cylinder head had undergone quite a heavy skim that the cylinder head studs protruded so far above the head that even with two thick washers the dome nuts would bottom out. So I have to decide, do I take an angle grinder to the studs or do I go for standard nuts and some sort of sealant on assembly?
Having said that I did think that the oil thing was the most likely explanation but asked the question in case I had missed something.
The question arose because I am in the process of an engine build that I got as a basket case last year. All the important parts have been cleaned up and the assembly is under way. When I cleaned all the small parts and sorted through them I was surprised not to find any dome nuts. I discovered that because the cylinder head had undergone quite a heavy skim that the cylinder head studs protruded so far above the head that even with two thick washers the dome nuts would bottom out. So I have to decide, do I take an angle grinder to the studs or do I go for standard nuts and some sort of sealant on assembly?
After all that I've said and quoted, I strongly suspect that there is no need for the dome nuts. I can't really see how oil, that is apparently unable to seep beneath the washers, can possibly slither down a torqued head-nut.
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the hobbler
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Having built 1 or 2 extra engines in my time, I must admit to only fitting 'dome' nuts on the outer cyinder-head studs, and only for appeareance sake. On the head-studs that are covered by the rocker-cover, I used 'normal' nuts. As Peter has pointed out, if the head has been skimmed, one must take care that the 'domed' nut has not 'locked' on to the top of the stud. This problem is more noticeable with 'pattern' dome-nuts---they are not as deep internally as the genuine Fiat dome nuts. To get round this problem, I usually take just a couple of threads off the top of the head-studs prior to re-fitting them to the engine. Having 'listened' to all the comments with regard to 'oil-seepage', I must say that have never noticed this problem, and if the 'seepage' is inside the rocker-cover,one would not notice it anyway.
If it did get down the threads, the oil wouldn't safely stay under the cover, it would track down the studs to the top of the crankcase where it would be blown through the fins on the cylinder barrels and pollute the heater air.
Whilst this may not usually be an actual problem that is commonly recorded as having happened, Mr. Lampredi must have considered there to be the potential for an issue and specified accordingly. There seems to be nothing to gain by not using the dome nuts under the cover, and possibly to be a little perverse to only use them outside of the cover.
Thank you my friends for the input, I have now torqued down the head and this is what I did.
I shied away from cutting down the studs inside the rocker cover and did not want to use big spacers to facilitate dome nuts. Instead I used the original thick washers and normal M10 x 1.25 nuts but used a semi fluid silicon sealant when fitting to hopefully prevent any oil seepage.
For the corner nuts outside the rocker cover I cut about 8mm from the studs so I could fit thick stainless washers and dome nuts. As with other engines I have built I try to use stainless fasteners where they are exposed.
I shied away from cutting down the studs inside the rocker cover and did not want to use big spacers to facilitate dome nuts. Instead I used the original thick washers and normal M10 x 1.25 nuts but used a semi fluid silicon sealant when fitting to hopefully prevent any oil seepage.
For the corner nuts outside the rocker cover I cut about 8mm from the studs so I could fit thick stainless washers and dome nuts. As with other engines I have built I try to use stainless fasteners where they are exposed.
Well done Peter, go to the top of the class . I have just gone through the same problem tearing my hair (what's left of it) out. Imagine the scene - engine rebuilt with modded pushrod tubes and bags of sealant all bottoned up purring away on the stand. Ha !! it's not leaking thank the lord for that - but wait what's that oozing from within the cylinders - OIL!!!! where the bl**dy hell is that coming from????? After much cursing and removal of parts the oil is weeping down two of the head studs !!!! WHAT !!! never heard of this before. So I've fitted very small 'O' rings under the thick flat washers as I don't think domed nuts will fit without stud surgery - what a faff . I'm almost at the stage of thinking 'If it leaks now leave it and monitor the loss'
Just when you think these engines cannot surprise you any more - they do. Long term I will have to carry out stud surgery and refit domed nuts!!!!
Ian.
. I have just gone through the same problem tearing my hair (what's left of it) out. Imagine the scene - engine rebuilt with modded pushrod tubes and bags of sealant all bottoned up purring away on the stand. Ha !! it's not leaking thank the lord for that - but wait what's that oozing from within the cylinders - OIL!!!! where the bl**dy hell is that coming from????? After much cursing and removal of parts the oil is weeping down two of the head studs !!!! WHAT !!! never heard of this before. So I've fitted very small 'O' rings under the thick flat washers as I don't think domed nuts will fit without stud surgery - what a faff . I'm almost at the stage of thinking 'If it leaks now leave it and monitor the loss'Just when you think these engines cannot surprise you any more - they do. Long term I will have to carry out stud surgery and refit domed nuts!!!!
Ian.
I hope that your oil leak is cured by that action Ian......stud surgery? that sounds painful!Well done Peter, go to the top of the class
Just when you think these engines cannot surprise you any more - they do. Long term I will have to carry out stud surgery and refit domed nuts!!!!
Ian.
Thank you for putting me up there; I am always top of the class for pedantry and contradiction.
A previous, water-cooled classic that I owned had some head-studs that penetrated the water-jacket in the block. The guidance was that the studs should be properly chamfered at the end of the thread; the water seal was only achieved by its interference fit with the block. Clearly, for thread fixing components to be able to move against each other, there has to be a slight undersize on each thread dimension. As the nut or bolt is tightened, the thrust is applied only to one side of the theads leaving a tiny void on the other. This will create a route that could be subject to capillary action. One solution might be to use a low shear-strength, threadlocking compound on the head-nuts. But as these will be subject to re-torqueing after a number of hours' use of the engine, that benefit might be lost.
I've bitten the bullet and sourced some new dome nuts (had to buy a full set of studs and nuts???) too much work has gone into getting the engine oil tight to allow loss down the head studs - even if it would be a limited amount. So job done even if it's getting harder to find some parts at reasonable cost
. pic shows new dome nuts under the DMD roller rockers.Ian.
