28-9-2015
Problem: I've got a problem with a Mercedes fuel injector 6mm tie down bolt hole that's been damaged drilling a broken bolt out and is too big for an ordinary Time-Sert repair.
Remedy: If the injector bolt hole is in good condition and not oversized, the special Time-Sert kit is the best repair but once it's gone bigger than this you'll need to use Loc-in-Serts. If the hole hasn't gone bigger than 8mm diameter in the bottom section where the thread used to be, you should be able to use a Loc-in-Sert. More than that it's bad news but it's probably a new head. Before you use Loc-in-Serts you would have to make sure that the hole you want to repair has at least 2mm of "meat" either side without penetrating a water or oil gallery.
7-9-2015 T.O. I'd like to pass on a tip about keeping swarf out of the cylinder when doing a sparkplug thread repair with the head on. I had an expensive new vehicle in for a sparkplug thread repair and was worried about possible swarf damage from the tapping operation if I did the job with the head on. And, of course, a head-off job would have been very expensive. I've always used grease on the tap previously for these repairs and that catches most of the swarf but I decided to pressurise the cylinder as well this time using a small flow of compressed air [note - small - not full pressure]. In this case I used a nipple in a manifold sensor hole to attach the air hose to. You may have to take another route to get air into the cylinder, depending on the particular vehicle. During tapping I wound the tap out several times and cleaned the swarf off with the gease. When I'd finished I did a thorough visual inspection and the cylinder didn't have a trace of swarf in it. Great stuff ! ! Obviously the air had kept all the swarf on the greased tap and I hope the hint is helpful.
26-8-2015 Q: What is a Whitworth thread? Is it the same as UNC?
A: Whitworth which would generally be labeled BSW [and stands for British Standard Whitworth] is an old standard - now pretty much outdated. It has been generally replaced by the newer worldwide standard called UNC [Unified National Coarse]. What's more even though people may call a thread "Whitworth" nowadays it will almost always be UNC. In practice it mostly doesn't matter whether a bolt is labeled BSW or UNC. They both generally interchange with each other. A 1/2" diameter thread is the only common thread where BSW and UNC will not interchange in practice. Also, sometimes in close tolerance or large threads you will have trouble screwing a mix of BSW and UNC male and female threads together. UNC generally has the same number of Threads Per Inch on a given diameter as the old BSW. The differences are that the angle of the "V" on a BSW thread is 55 degrees and on a UNC thread 60 degrees and also the shape of the peak and the trough is slightly different from one to the other.
History is - back in the early 1800's when engineering and metal machining was starting to be more common, every engineering shop just made threads to whatever number of Threads per Inch [abbreviated "TPI"] that suited them. So there was a horrible mixup, You couldn't go and buy say a 1/4" bolt and know it would screw into a threaded hole in something you needed to repair. Then about the mid 1800's a fellow called Joseph Whitworth came up with a standard system so that at any diameter the number of TPI would be standardised. This became the BSW system. Then, about 100 years later when there was much more interchange between nations the UNC system grew out of an amalgamation of British and US engineering standards and became the preferred standard for classifying coarse threads.
14-8-2015 Q: I am doing up an old aircooled VW engine and the threads in the head bolts won't hold any tension, they keep stripping. Can you help?
A: Firstly, although many people think this engine block is aluminium, you are dealing with a magnesium alloy casting, which is far weaker than aluminium alloys. Threaded holes in magnesium alloys have to be deeper than in aluminium. (See blog 17-7-2015 below). If you know you have a magnesium alloy casting we would recommend any thread repair should preferably be 3 times the diameter in length or at very least 2-1/2 times. So for the 10mm diameter bolt in a VW engine your repaired hole should have a thread insert 25m to 30mm long when using the original hi-tensile bolts. If in some application you were using a higher grade of hi-tensile bolt and higher torques/tensions you may even have to increase the engagement length of the bolt to more than 3 times. We hear over and over of thread inserts stripping in either magnesium or old (fatigued and softened) aluminium castings. If you are doing this kind of repair it is always good insurance to make the thread insert longer rather than use standard length inserts which are normally 1-1/2 D [diameters] long. The usual cause of this kind of stripped thread is precisely that someone has used a standard 1-1/2D insert where they should have used longer.
29-7-2015 Q: How do I remove a Helicoil?
A: There is a special tool available which is a triangular shaped piece of hardened steel with 2 sides sharpened and a handle attached. You could make one yourself with a piece of hardened steel like a power hacksaw blade ground to a "V" and sharpened. This tool is tapped into the hole with a small hammer and the sharp sides bite into the coil. You simply then keep downward pressure on the tool as you wind counter-clockwise to get the coil out.
A2: You can also pry the top wire coil away from the tapped hole with a small screwdriver or similar tool and bend it across the diameter of the hole to make a tang. Then simply use the fitting tool for the correct size and wind the coil out counter-clockwise.
17-7-2015 Q: I want to replace the threads in an old cast iron engine block. What length of insert will I need to hold 65 ft. lb. torque on the bolts.
A: As to tension and torque and pull out strength it is not easy to get a good relationship between the 3 because torque is a measure of axial force [force/distance] and tension in the case of a bolt is purely a load applied and doesn't involve the "distance" factor i.e. The 2 units are for "incompatible" forces and cannot be simply related to each other. For example for any given torque wrench reading you can get a range of tensions in a bolt. This could vary as much as 35% which is huge. The reason for this big variation is that the amount of friction in different fastener assemblies varies widely . Some of the factors that make a difference are, the condition of the male and female threads, the fastener material itself, the type of finish under the head of a bolt or on a nut and whether there is any lubrication used during assembly. So nothing is precise. If you take a look at the University of Bristol study on our website, the general conclusion at the end of the paper is that when 1-1/2D of thread length is used in aluminium or cast iron with grade 5 hi-tensile bolts, the bolts will break before the female thread will strip. Obviously, you are not going to tension bolts to breaking point so you should not strip a thread at 1-1/2 times the diameter in length. Also, when any kind of thread insert is used it will increase this pullout strength by at least 10% and helicoil type inserts will do better at perhaps 20-25% increase in strength.
Just a note of caution: With much higher grades of hi-tensile bolts being available currently it is possible to get to get much more tension in a fastener than for the grade 5 hi-tensile example given above. Because of this, you will need longer inserts [longer than 1-1/2D] in many applications using modern materials. Also note the example above is for a relatively weak type of cast iron - cast iron is available in a wide range of grades and shear strengths.
13-7-2015 Answer to Teaser in Newsletter #13 "What are the 2 solids/dry films that have the lowest coefficients of friction?" - This must have been a hard one because we only received one correct answer.
The 2 compounds are:
1. Tungsten Disulfide (Disulphide - alternative spelling) with a coefficient of friction 0.03*.
2. Teflon (Registered Trade mark). The generic name is Polyterafluoroethylene [PTFE] and coefficient of friction 0.04*.
Tungsten Disulphide is classed as a dry film lubricant i.e. probably technically a liquid and Teflon as most would know is a solid. To give some idea of a comparison the coefficient of friction of graphite is about 0.1 i.e. nowhere near the 2 substances named above.
* Coefficients of friction are a bit difficult to define absolutely and possibly this teaser was a little ambiguous but however you want to define a coefficient of friction, I’m pretty sure the 2 substances above would come out as 1st and 2nd.
9-7-2015 Q: Will a flex-hone remove much metal and will it remove scratches in a bore?
A: Flex-Hones are NOT a metal removal tool. They are a finishing tool and at most will remove a few microns when used as directed and yes, they will either remove a scratch or remove the sharp edges that cause damage to rings or hydraulic seals. All metal removal processes such as machining and rigid stone honing e.g. with a Sunnen hone leave minute sharp peaks spread over the entire machined surface. Flex-Hones take the sharp peaks off this machined surface so that rings or seals are not damaged by the mating surface and also seal better against long term leakage. Here's a true life story. Someone who had never had to do with Flex-Hones before and thought they would remove metal set out to prove they would do just that. The Flex-Hone tool was used in an experimental bore in an aluminium block. The Flex-Hone was plunged up and down in the block for maybe 5 minutes and many hundreds of cycles. And . . . the end result . . . about 1/2 a thou. or 12 microns [0.012] was removed from the bore and both ends of the hole were bell-mouthed ! !
7-7-2014 RS: Hint for Drilling Stainless Steel.
Use very slow speeds and keep a constant flow of ordinary tap water on the drilled hole to make sure the drill bit stays cool. A laundry spray bottle with a large water jet can be used to apply water.
Cross Tools Co comment: We haven't tried this ourselves yet but RS "swears " by it. Please can the community give us feedback.
30-6-2016 Q: We want to use special bronze inserts in aluminium with stainless steel bolts What problems are we going to have?
A: Corrosion due to electrolysis will be a real problem with all those materials in contact with one another and you'll likely have bad problems when you come to undo the bolts again. Bronze [or brass] with aluminium has a score of about 19 on a scale of 34 for corrosive activity and stainless steel with aluminium has a score of 26 on the same scale and stainless steel with bronze scores 6 to 8. A good score which is neutral is zero (0) and an "acceptable" score 1 to 4
26-6-2015 Q: A helicoil has come out. Can I use a Time-Sert to replace it?
A: Probably not because a Time-Sert is not bigger than a helicoil. It is identical to a helicoil in outside diameter for any given thread and it uses the same tap to prepare the hole. This tap is known as a Screw Thread Insert tap abbreviated "STI". You will probably need to use either a Big-Sert or a Loc-in-Sert (you can click on the word for a link to each insert type). The only time it would be possible to replace a helicoil type insert with a Time-Sert is where the helicoil has simply wound out of the hole and the thread is still in good condition i.e. not stripped.
24-6-2015 Q: What lubricant should I use with Flex-Hones?
A: Flex-Hone offers a special oil which may have some slight advantages and is available from Cross Tools but it is expensive for what it is because of dangerous goods shipping requirements. Frankly, you can use any oil such as ordinary engine oil soluble oils etc. - preferably with some "body" to it rather than things like WD40. Never use solvent type liquids not even Kerosene because they attack the resins that bind the abrasive particles and consequently greatly shorten the life of the tool. Grease can also be used - smear it on the bore and it will help give a lighter cutting action and finer finish if required. In heat exchanger tubes water is often used but it is not as good as oils.
22-6-2015 Q: I have tried double stacking helicoils without success. I Always have trouble with the tang in the middle dislodging.
A: Try a neat fitting punch to break off the tang, or use needle nose pliers and gently agitate the tang up and down [not sideways]. This has to be very gentle and patience is needed because it may take some time to break off the tang. Too vigorous an up and down movement will probably dislodge the tang. If the hole is large enough to allow you to inspect the threads and you suspect that a coil is dislodged, you can often flick it back into its thread track with a small screw driver or similar tool.
26-6-2015 JLH:Re the problem when breaking off the tang of a helicoil . To avoid dislodging the last coil so that the bolt then jams on it (especially problematic when stacking two inserts in series) - I use a pin punch with a square end that is a loose fit inside the coil. Hold the punch lightly against the tang and give it a smart whack with a small hammer - can't remember the last time this failed. I have punches made from bits of rod, nails etc and keep one, in each kit.
