Base Packing

[phil23fair]

I am just starting a new build. 58/107 crank. GP 200 casing and a GP225 Barrel that has been ported. With a 0.7mm base gasket I get port timings of Exhaust 171, Transfers 126 and Inlet 155. The deck height is 1.1mm. The problem with this is that the BGM meteor piston protrudes 0.7 mm into the exhaust and transfer ports at BDC. The head has been machined such that deck height is near as dammit squish. If I add a 0.4mm base packer to reduce the exhaust step my timings go to 172 128 and 153 degrees and deck height (and squish) is now 1.5mm, I intend using a 25mm carb and clubman exhaust.
The question is should I leave it with the 0.7mm gasket and 171,126 and 155 port timings and ignore the exhaust step of 0.7mm, reduce the exhaust step by 0.4mm or finally add a base packer to correct the step and get the head top end machined back to give me the 171, 126, 155 port timings and 1.1mm deck height?

[dscscotty]

There's lots of other things to take into account, particularly pipe being used, which will have a significant influence on timings best suited, also what sort of characteristics/usage, touring, gearing etc.

[phil23fair]

Scotty, I'll be using a clubman pipe 25mm carb LI150 box with 47/16 sprockets. I'm after something that rides like a standard Lambretta but with a bit more poke. I'm pretty happy with the 171,126, 155 port timings. My main concern is the exhaust step and what effect this will have on performance or potential damage to the piston. I'm sure I have read somewhere that if the piston protrudes into the exhaust port that you run the risk of melting the edge of the piston (but I may be and am most likely wrong)

[hullygully]

I recently had a larger step into a TS1's exhaust port, but Ben @ AF's reassured me that it wouldn't be a problem, after showing me some info on his laptop.

[dscscotty]

The step won't be an issue, I'd go with the slightly lower inlet with higher transfer/exhaust durations, the exhaust port duration, depending on width/area is still probably a little to low, also because of the nature of the timings, cc and use of the clubman probably the gearing will be a little to high, go for around 4.7@ 47x17

[phil23fair]

Thanks both Scotty and hullygully for your confirmation that the step won't hurt (I dread to think what Ben showed you on his laptop that convinced you hullygully )
Scotty so if I add a 0.4mm base packer to my base gasket the deck height and squish without a head gasket goes to 1.5mm and port timings are 172 128 and 153 degrees. Is this now more in line with your thoughts or is the transfer too high?

[dscscotty]

the transfers are in the ball park as is the inlet, although maybe a little lower would be better(inlet), exhaust is probably a little low, 175-178 would be a good figure

[phil23fair]

Scotty Thanks very much