Scooterotica

If I change my crankshaft from a 58/107 to a 60/107 and want to keep my port timings the same, is it a simple matter of putting in a 1mm base packer and a 1 mm base packer or is it more complicated than that?

I meant to write 1 mm base packer and 1 mm head packer

Everyone knew what you meant

Anyway, if you use 1mm at either end your exhaust timing will barely change but your transfer timing will increase.

Use edens port timing calculator

60mm stroke means the piston is 1mm higher at TDC and 1mm lower at BDC, You only need 1mm of packing (not 2mm)

If you put a 1mm base packer the piston will be in the same position at tdc as with your 58mm stroke , your port timings will be differnt though , as

above use Edens port time calculator, to find out how differnt they are.

You can then , either change to a 1mm head gasket or 0.5mm base and 0.5 head packer , or any combinations making up 1mm, to get the timings you

want .

Also check there is enough clearance in the casing when the piston is at BDC.

For avoidance of doubt; Carlos is right you only need 1mm total packing.

You may find that the 60mm crank will not fit in your casing, many of the 60mm cranks for sale at the moment have large conrod big ends.

You may also find that the piston skirt bottoms out before BDC, as the 60mm crank will pull the piston 1mm closer. You can trim the piston skirt, but this will alos affect your inlet timing....

Increasing the stroke will increase the port durations if you add a 1mm base packer but you may run into problems with the piston skirt bottoming out.

If you run a longer rod this has the effect or reducing the timings, this is why a 60 stroke /110 rod combination works really well. The 110 rod also solves the problem of the piston bottoming out too. Typically this crank configuration will require a 3mm base packer and 1mm head gasket to return the cylinder/crank combination back to near identical timings as the 58/107 combination.

GT cranks are 60/100 and require no modification to fit into a standard casing, but you'll need the GT special SE bearing if you have a piston with a 16mm wrist pin.

In laymans terms what are the real effects of running a 60 - 100 crank as opposed to the standard 58 - 107?

OK, in terms of GT186 kits these are the influences
58/107 exhaust 173 transfer 124 (no head gasket, standard build)
60/107 exhaust 174.6 transfer 127.1 (1mm head gasket only)
60/110 exhaust 174.1 transfer 126.7 (3mm base gasket & 1mm head gasket

Thanks for all your replies. They have been most helpful.