Wrist pin height is a number I've been using to compare cylinder interchangeability and can be used to determine requisite spacer height.
It is designed to help line up timing based on aligning the bottom of the exhaust with the top of the piston at BDC. It's also useful considering alternate pistons in cylinders when the piston has a different compression height. In addition to the exhaust being fully open at BDC, the intake should also be fully open at TDC and the transfers have good access through the piston, but these can be much more easily modified by cutting the piston skirt appropriately. Also the deck height, or height of piston crown to head at TDC is important for compression but can be modified by decking the cylinder (or inversely adding thicker head gasket) and/or adjusting Squish band.
Other considerations for cylinder swaps that should be considered first are:
- Stroke. see Moped and scooter strokes for a list. It should be close within 1-2mm, but some variation can still run acceptably. Remember 1mm stroke difference is 0.5mm higher on bottom and 0.5mm shorter on top. this would lower compression and prevent full exhaust opening. You could deck the top by 1/2 the stroke difference to make up for the compression. the piston may also be modified removing material around the exhaust on the piston to allow the exhaust to open fully at BDC. Alternately you could add in 1/2 the stroke difference to the spacer and deck the head by the full stroke difference for much closer functionality. timing is also affected but much less severely and could still run well or be improved with porting.
- THE ONLY WAY TO CHANGE STROKE IS CHANGE THE POSITION OF THE BIG END BEARING ON THE CRANK. A longer conrod only moves the location of the wristpin farther up or down, essentially the distance from piston to crank, but the distance it travels (which is stroke) remains the same.
- Wrist pin size. The wrist pin fits a specific size on the conrod. without swapping the conrod, you need to keep the wristpin the same size. 10mm and 12mm are most commong, also 14mm sometimes. Usually the 10mm with a roller bearing measures 14 on the conrod, so 14mm and 10mm are sometimes interchangeable with a 2mm thick rollerbearing.
- Stud spacing. The spacing between studs is often not square, and varies in size. Close stud spacing could bolt right on, or bolt on just by widening the stud holes through the cylinder to get more room. Far stud spacing requires blanking off the old holes and cutting/tapping new stud holes to match your new cylinder.
- Transfers. As with any new cylinder you need to case match. Sometimes this isn't possible without adding material to the cases because your new cylinder's transfers may overlap the base on the engine case. If you're adding a spacer it can be a good place to match and ramp to get the different transfers to meet up.
- Cooling Orientation. For air cooled cylinders the cylinder is designed for maximum flow over the fins in it's intended orientation. When putting a horizontal cylinder in a vertical orientation however, the fins have very poor airflow and will overheat. One work around would be to add in a fan system similar to the v1 or vespa, ducted from a fan attached to the flywheel. Otherwise don't use vertical oriented fins on a horizontal engine.
- Ring location. In some cases when mismatching piston and cylinder, the retaining pin for the rings might be in a port, in which case the ring can snag the port and break. It's possible to drill out and move the retaining pin if you need to use a mismatched piston. Also be very sure to carefully and PROPERLY measure bore and ring gap when swapping pistons.
- Cylinder skirt OD The outer diameter of the skirt should be able to fit into the inner diameter of the cases. If not you can bore the cases larger similar to putting a Gilardoni on a Puch.
Wrist pin @ BDC
OK here is my wrist pin @ BDC logic.
1st measure the piston's compression height (D). This is the height from the center of the wrist pin to the lip of the piston. This is NOT to the top of the crown because the lip is what affects the port timing and opening of the exhaust at BDC. Also the ring location is less critical than the lip height. The easiest way to measure is to measure with calipers from the top of the wrist pin hole to edge of the crown, and add 1/2 the wrist pin diameter to get compression height D. Other measurements are A) piston total height, B) wrist pin diameter, C) dome height (affects squish) and E) the piston diameter, should be slightly smaller than the cylinder bore.
On the cylinder we need to know the location of the bottom of the exhaust port relative to the base, what I'll call exhaust height. The easiest way to measure is a depth gauge (back of most calipers) to get the distance from end of the cylinder skirt to the bottom of the exhaust, and subtract the height of the skirt from the base. (Y-X)
This gives the height of the wrist pin required at BDC relative to the base. It's simply the exhaust height minus the compression height of the piston. For example on my Flandria the exhaust height is 25mm above the base and the compression height is 31mm. Then the wrist pin at BDC = 25 -31 = -6mm. meaning the center of the wristpin is 6mm below the base at BDC for the stock engine.
On a Chinese bicycle motor cylinder (already matches the initial prerequisites on stroke, stud spacing etc.) the exhaust height is 29.5 and piston compression height is 26mm, then the wrist pin @ BDC = 29.5 - 26 = +3.5mm, or 3.5 above the base.
Then to put the bicycle cylinder on the flandria cases, the base would have to be offset by -9.5mm (-6 - 3.5 = -9.5). That is I'd need a -9.5mm spacer. a negative spacer would be removing material to drop the base, in this case milling the base on the cases down 9.5mm or milling the cylinder's base off by 9.5mm.
The number is also useful considering alternate pistons or changing conrods. For example instead of a spacer it could be possible (as long as it doesn't get so long it impinges on the skirt or cases) to get a conrod 9.5 mm longer. Or to find a piston with the same bore as the bicycle cylinder but with a 9.5mm taller compression height. Compatible parts may not exist, but if they do there's another option. Likewise combinations of conrod length piston height and spacers can easily be compared using the wristpin @ BDC number.