Member postings for Dominic Collings 1

That's great thanks. That's more than accurate enough for that I need.

That's great David thanks. I can't see a quoted tolerance on their website +/- ???. Does it say in the catalogue?

That's exactly what I'll do Bowber and yes they check both sides as do I. I have had a situation where it's been 1.18 on one side and 1.24 on the other so ok but not by much. Makes you very nervous if they take the low reading first.

I get what you're saying about using a micrometer over a calliper but for several reasons a micrometer isn't the tool for the job in this instance. The top of piston is slightly convex while the combustion chamber is concave so when you squish the solder it goes to a slight taper. Scrutineers measure just in from the end in an attempt to avoid a false reading from any snipping at the end of the solder. I've tried all sorts to get the cleanest cut possible from pliers to tin snips, I've even tried a cigar snipper and all leave a slight snipped section at the end. The best thing I've found is a pair of wallpapering scissors. This seems to leave the cleanest, squarest and least deformed cut. However they use the external calliper prongs where they taper to a point to try and get as "exact" a measurement as possible. If they used the flatter thicker section further up it would just show the thickest part of the tapered section. This would be the same with a micrometer. Obviously you can get micrometer with a radius end but it would be very difficult to measure a specific point on the solder. The solder is very soft and if you do press too hard with a calliper to get a falsely low reading it tend to leave a slight whiteness mark which is a good reason to request a re-test. I doubt a micrometer would leave such a mark. Ultimately it's how the scrutineers are trainer to measure the squish so it pretty pointless measuring it any other way despite the methods apparent failings. Initially I think I'll buy a 100mm standard. At least that way I can test how accurate my calliper is. I didn't realise they're only £20.

Edited By Dominic Collings 1 on 24/05/2015 20:18:22

Interesting read Michael, thanks for the link. It's also interesting that you have a couple of measuring devices that read low Peter because I think that's the problem I'm having. So to put the problem in context I'll explain the issue. My son races go karts in a class with several technical rules and regulations. One such rule relates to the size of the "squish". That's the volume that remains between the top of the piston when at top dead centre and the top of the combustion chamber. This measurement cannot be less than 1.2mm. The engines are two strokes so burn oil as they run and you're left with a deposit of carbon on the top of the piston over time as a result. This lowers the squish gap and when the measurement gets down to 1.22mm it's time to take some action. This basically requires the removal of the top of the combustion chamber, the carbon is either scraped off or extra head gaskets are added to restore the squish gap. Unfortunately the engines are sealed buy a builder with a licence to work on them to prevent cheating. The squish gap is critical to the engines performance so typically engine builder will set the squish to 1.26mm so you've only got 0.04 of a mm before taking action or 0.06mm before you're failing a squish test. Since the engines are sealed to test the squish we bend a peice of 2mm solder at right angles, feed it through the spark plug hole and rotate the engine. As the piston comes up it squishes the solder and we take a measurement with a vernier to see where we're at with the squish gap. We check this measurement all the time.

That should be simple enough but I had a cheap digital vernier £35 and it always seems to read high at 1.28-1.30mm. However I noticed that when the scrutineers checked the gap with their vernier it always showed much lower. So to make sure I'm not running too tight I acquired a 150mm Mitutoyo. Since these are around £150 I figured they must be more accurate. However I sent an engine back to the builder for a squish adjustment and when it returned I checked the squish gap with the Mitutoyo to find it already reading a non legal 1.18mm. My old cheap calliper was reading 1.26mm. That's a significant difference. So on a race day I'd borrowed a few calipers (all of the cheap £35 ones) and all read between 1.24 and 1.28 so I figured it must be my Mitutoyo that was reading low. I reasoned that it was doubtful the engine builder would be using a cheap or uncalibrated vernier so we went off racing. At the end in scrutineering the guy tests the squish gap with solder and pulls out a Mitutoyo calliper and gets a reading of a barely legal 1.2mm. As a consequence I'm a bit lost now as to what to do.

Despite their price are Mitutoyo's known for reading low? Is there another brand that's renowned for accuracy?

Edited By Dominic Collings 1 on 24/05/2015 07:41:41

To be honest the digital one is a pain. I use it a lot and sure enough at the most critical moment the battery's flat. A dial calliper would probably be better unless they're less accurate of course.

I guess a slip gauge would be best. Is there anyone on this forum with a set in the Dunfermline/Rosyth area with a set, or even one for that matter I could pop round and try?

Sorry I mean a vernier calliper not a gauge but I guess testing it would be the same. What about feeler gauges? How accurate are they usually?

Doea anyone know of an easy way to test the accuracy of a digital vernier gauge?

Yes it's on new brass to try and get it closer to the original. Is new brass tube likely to be lacquered? Doesn't look like it although I guess it won't hurt to give it a once over with a fine grade scouring pad before trying the ammonia trick. Failing that I'll just try dirtying it up with some dark brown wax. Obviously it would be much easier just to cut down the original tube and re-tap the end but the owner was to keep the original. Still I've got to wait for the die to turn up first, just thinking ahead.

Great thanks that's am M10 1.0p ordered so hopefully that'll work. Any ideas how I would artificially age brass? I'm thinking of perhaps a chemical just to add a tarnish. Or do I just leave it in the garden for a week or so?

No I'm not counting correctly then. That makes it 10 teeth per 10mm so assuming that 1.0p on a metric die means one tooth per mm that's 10 teeth in 10mm which makes fine M10 1.0p the most likely? I'm completely lost on the 1/8" die bit. Surely 1/8" is about 3.2mm ish, how does that cut a thread on a 10mm ish bar?

Ok thanks for all your help so far but I'm still no further forward. I've done as suggested and have measured the following.

I'm counting 27.5 tpi and 11 teeth over 10mm. The bar definitely is not plated, is the same colour from inside to out and is not magnetic. Outside diameter is 9.62mm on the vernier, 8.29mm internal diameter so the walls are only 0.68 thick hence the need for a very fine thread I guess. I'm calculating 3/8" as 9.52 so it's a size in between 10mm and 3/8" although the "10mm" brass bar I bought for the job measured in at 9.85mm so I'm not sure that measurement means much. There are sections of the lamp that are not threaded and are a slip fit which tightens with a grub screw and the 10mm stock fits through these holes fine. Is this any more help for an thread ID?

Ok thanks I can measure the diameter no problem with a vernier but how do I measure the pitch?

Hi I'm mainly a hobby cabinet maker and woodworker taking on the odd project for friends and family. A guy at work has asked me to help with a dining table relocation and modification. They've a lot of antique European furniture and in moving the table to another room they've been left with a ceiling hung lamp that used to go over the table but now the tables gone, the lamp sits too low in the room. The lamp is pretty simple consisting of a brass rod 24" long with around an inch of thread on one end and half an inch of thread on the other to connect the decorative ceiling rose on one end and the bulb brackets on the other. They like the look/style of it so want to keep it but make it shorter. Simple I thought since it's apparently Italian so I was just going to get some 10mm brass tube, cut it down to 12" to save the original antique shaft if they ever want to reinstate the complete original lamp somewhere else and add an M10 thread on both ends. However despite being Italian/European and the rod being about 10mm in diameter the thread is much finer than a standard M10 thread. So my first question is what standard is the thread likely to be?

Cheers

Dom