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I see a lot of different base gasket thicknesses and I believe BDT actually sells a whole pack of base gaskets with a variety of different thicknesses . Without the general go to “ ask your builder response “ what exactly are we looking for with different base gaskets ? Are we trying to make the piston flush with the top of the cylinder? Or when the piston is bottomed out should it be flush with the transfer ports ? Just curious how a slight difference in thicknesses can make such a significant change in performance,
 

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Base gaskets thicknesses can be altered for changes in port timing.
Normally a standard base gasket are around 0.020" thick.
In my case changing my 1983 CR250R port timing from a mild 182°/126° to a strong 191°/130°. I need to raise the transfer ports 0.040"and the exhaust 0.120" plus mill the cylinder top deck 0.060".

Now not to disturb the transfers delicate roof angles it's better I raise the cylinder 0.040" with 2 extra 0.020" base gaskets. Then I only need to raise the exhaust ports 0.080" and lower all the ports floors 0.040".
Plus mill 0.100" off the cylinder top deck
 

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Base gaskets thicknesses can be altered for changes in port timing.
Normally a standard base gasket are around 0.020" thick.
In my case changing my 1983 CR250R port timing from a mild 182°/126° to a strong 191°/130°. I need to raise the transfer ports 0.040"and the exhaust 0.120" plus mill the cylinder top deck 0.060".

Now not to disturb the transfers delicate roof angles it's better I raise the cylinder 0.040" with 2 extra 0.020" base gaskets. Then I only need to raise the exhaust ports 0.080" and lower all the ports floors 0.040".
Plus mill 0.100" off the cylinder top deck
To the OP's original question; while this is an option to vary port timing considerably, it is not one that I recommend, nor was the intent of the BDT gasket kits of varying thickness to be used for major port timing changes. Nice thinking on how to get the timing you are looking for Willard.

I see a lot of different base gasket thicknesses and I believe BDT actually sells a whole pack of base gaskets with a variety of different thicknesses . Without the general go to “ ask your builder response “ what exactly are we looking for with different base gaskets ? Are we trying to make the piston flush with the top of the cylinder? Or when the piston is bottomed out should it be flush with the transfer ports ? Just curious how a slight difference in thicknesses can make such a significant change in performance,
The intent of the BDT "pack of base gaskets with a variety of different thicknesses" is to allow to fine tune the squish clearance in fine amounts. Since we are dealing with 250R engines from 85-89, their are slight differences between the years, as well as stack up tolerances...having base gaskets of minute variances in thickness allows you to quickly set the squish clearance during engine builds.

Also; IMO as long as you don't vary the cylinder up/down more than .020" or .5MM, you are within a range that port timing changes are not really noticeable to the end user.

Carlos
 

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Carlos, thanks for the advice on stacking base gaskets.
I was thinking about getting Cometic to make me a rubber coated metal spacer which would give me the extra .040" or 1mm cylinder lift. Or maybe try a .020" metal spacer with a base gasket on both sides of it.
Much better then glueing 3 base gaskets together and risking a leak or a distorted cylinder.
 

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Seems odd to use non standard base gaskets when the appropriate spacer plate can be used.
Wouldn't you say that the professional engine builder/porting guru should have the port timing spot on before the engine is shipped to the end user? It doesn't make sense to mess around say track side adjusting port timing with gasket swaps.
 

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Carlos, thanks for the advice on stacking base gaskets.
I was thinking about getting Cometic to make me a rubber coated metal spacer which would give me the extra .040" or 1mm cylinder lift. Or maybe try a .020" metal spacer with a base gasket on both sides of it.
Much better then glueing 3 base gaskets together and risking a leak or a distorted cylinder.
Or just make an aluminum spacer plate of the thickness you are looking for. You can get Aluminum sheets in all thickness in .010", .012", .016", .020", .025", on and on. At BDT Motorsports we make our own custom spacer plates as needed, many years ago, I have made them 100% on a vertical band saw, or the thinner sizes with a 'jig' saw, then finished with hand files and emery paper.

I think you clearly understand what different gasket thicknesses are used for and why the statement I posted above makes all the sense in the world; "having base gaskets of minute variances in thickness allows you to quickly set the squish clearance during engine builds"....the 'key' being "during engine builds"...so no need to explain further.

Carlos
 
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