Different Boiler Tube Sizes
I have been asked by Luke, a fellow builder of PYRTE, after he had gone through the construction details whether he could use a different boiler tube size rather than the one that is specified.
After asking around at his local suppliers he told me he could get hold of a 108mm external diameter boiler tube and was concerned whether this would cause any problems later on in the build.
My reply was that the 108mm is barely over 1/4 inch larger than the specified boiler diameter and for that reason was acceptable.
As the top of the firebox is set at a fixed height because of the crankshaft, then raising the top up would mean altering the heights of the firebox panels to accomodate the strengthening rods sitting above the boiler barrel, along with the crankshaft bearings, so the easiest option is to sit the boiler barrel itself a little lower through the firebox.
By dropping the centre height of the boiler by 4mm and scribing the108mm diameter for the boiler holes, along with marking from that same point at 114mm diameter for the supporting boiler bolt centres, this allows the boiler to be sitting with its top at the correct height and the bottom a little closer to the burner flames, plus the gap at the sides of the boiler will be closer, allowing better heat transfer.
The only other considerations are the steering fork will need to have the main body reduced in length by 8mm to make sure the boiler barrel sits level with the floor, and obviously this will need to be ascertained once the rear wheels are made up and mounted on the rear axle, plus the steering rod needs to be kept as horizontal as possible which means the steering control arm attached to the firebox side needs its pivot point to be lowered to match.
All in all, using a slightly different boiler tube size is not such a big problem as PYRTE is not a copy of a specific production engine requiring spot-on dimensions, but a flexible freelance model which can be modified by yourself as the build goes on.
It goes without saying that as the boiler is a slightly larger diameter, then it will hold more water, meaning it may take a little longer to warm up, but will run for longer between fillings.
Hope this helps you out.
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