Measuring the Barrels and Pistons of My Barn Find Triumph T140 Bonneville Dessert Sled | 05
Having removed the barrels and pistons from the engine of my 1975 Triumph Bonneville T140V I am now going to measure them to see how worn they are.
Here are some of the tools I use:
Bore Gauge: ebay.us/JuEuf9
Micrometer set: ebay.us/eorkHi
Imperial Combination Spanner Set: ebay.us/gYz85M
Imperial 1/2inch Socket set: ebay.us/jylH05
Imperial Allen Hex Bit Sockets: ebay.us/iFV0OQ
Although I can’t get the head off the barrels of my Triumph T140V Bonneville, now that they are not on the engine I can access the bores from the underside to measure them. Being able to access the bores I can still measure them to see if they are worn. To measure the bores you need to use a bore gauge. I have previously done a video which goes into this in more detail when I was working on my Cafe Racer engine, but most of what you need to know will be in this video.
I roughly measured the bore and I could see that they would be around 76mm so I chose an anvil for the bore gauge that could be set to 76mm as a base measurement. When measuring the bores of an engine the basics for each bore includes 6 measurements. 2 near the top where the combustion chamber is, 2 at the bottom of the piston stroke, and 2 in the middle. One set of 3 measurements should be in the axis that the conrod moves, so that you can get the thrust faces. These are the areas where there is likely to be the most wear, because the conrod is actually pushing slightly backwards and forwards as the piston goes up and down. The other three should be at 90 degrees to the first measurements so that you can see how these compare to the areas that are most likely to have the most wear.
You set the bore gauge using a micrometre of the right size. And it is important to make sure you calibrate the micrometre before you start, by measuring a test piece of a known size and making sure that the micrometre zeros correctly. It is a little fiddly to set the bore gauge, but with patience and a little practice it is quite achievable.
By comparing the measurements you can see if the bore is oval, tapered or just plain worn out. For oval the 90 degree measurement would be different, they are most likely to be different at the combustion chamber measurements. Tapered would be where the measurements at the top are different to the ones at the bottom. Worn out would be if the measurements vary a lot, and they don’t match the piston measurement.
When I measured these bores one seemed completely straight, and the other had a couple of very slight differences, but when I re-measured it also seemed to be pretty straight. I then went on to measure the pistons.
The piston measurement is fairly simple. You measure near the bottom of the piston skirt on the thrust faces. If you have pistons that you are concerned about you can take other measurements. But be aware the shape of a piston might not be completely round, and is likely to be narrower at the combustion end than at the other. This is because pistons do get a lot of heat and they are designed so that when they warm up to their operating temperature that they should be round.
The measurement from the bottom of the skirt is the one that you compare to the bore to see how much clearance you have. In this case the difference seems to be .09 of a mm which is around 3.5 thousandths of an inch. Depending on the source of your information the clearance that you require can be quite different. It is usually bigger in higher performance or turbo charged engines because the pistons may run hotter and expand more. For the Triumph T140v the most common recommendation that I can find is a clearance of 4 thousandths of an inch. My measurements giving what I think is a little on the higher side of 3.5 should be fine. Knowing this I will take a little care at first, in a running in way, to get them to bed together before giving the engine any real loads or high speeds. But what I think it shows is that these barrels were bored to match these pistons shortly before the bike stopped being used, which goes further to confirm my opinion that the engine was at least partially refurbished.
When I came to check my measurements against the ones in the Triumph workshop manual, it appears that my bores and pistons are a little under the size for a standard bore. This discrepancy isn’t really a big deal as long as the pistons and barrels match up with sufficient clearance.
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