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Baseplate under cylinders
- Thread starter fishman
- Start date
Adding a plate under the cylinders is a common method of lowering the compression ratio. Perhaps the 840 kit raised it too high... could also be to stop the pistons and valve from shaking hands. This is an interference engine after all.
Another link or two in the cam chain?
Possibly a XS set up with XT500 pistons, the space is to compensate for the different wrist pin height. I don't think you need a plate with SP370 pistons.
If it's an 840cc, this would probably be a modified SP370 piston (85mm) on a 533 rod.
It was quite common in sidecar cross.
Another modification, often performed, would be an XT500 piston on a 533 rod, giving 880cc.
Both with 74mm standard stroke.
Neither of these need an extra baseplate.
For best performance, you would try to keep the squish distance close to original; say, 1.5mm or so.
If you add a baseplate to lower compression, you will lose the squish.
During the run of many years, I put together a spreadsheet with data of potentially useful pistons (either measured, or from published data), rods and strokes (74 standard, versus 84mm stroked) as I found them, as a guidance for my own projects.
To give you an idea, without any gaskets, the modified SP370 piston (dome volume 23.3cc) with 533 rod, as was popular in the 80's, has between 0 and -1mm (negative meaning, below deck) deck height. Depending on who did the work, and what their target was at the time, presumably.
With 0mm deck height, comp. ratio would be 11.0, with -1.5mm it would be 9.3.
So with base and head gaskets, you would be good.
The XT500 piston (dome volume 7cc) on a 533 rod would have similar deck height as the SP370 piston.
With 0mm deck height, comp. ratio would be 8.5, with -1.5mm it would be 7.5.
This is all assuming a standard head volume of 65.4cc.
Either 256 or 447 rods would give lower deck heights and compression ratios with standard 74mm stroke.
So they would certainly not need a baseplate.
A CR500 rod is 4mm longer than a 533 rod. To get the same numbers as above, with standard stroke, this WOULD call for a 4mm baseplate.
Of course, playing with base plate and gasket thicknesses, would also change your cam timing.
Baseplates were sometimes used with stroked crankshafts, although the shorter 256 rod & 84mm stroke would work without a baseplate for the SP370 and XT500 piston. As Jim says, it could be used to lower compression (same SP370 piston, 256 rod and 84mm stroke would give 12.3 compression ratio without gaskets), but at the cost of squish.
It's uncommon to find the longer 533 rod on a 84mm stroked crank, because of its larger BE diameter. This would put the BE hole in the crank really close to the edge.
Another, even older mod, uses the 85mm(?) Porsche (Carrera?) piston (840cc with standard stroke).
Could be that these have a larger deck height.
I have one somewhere, but would need to find and measure it to be sure.
There may certainly be variants I haven't seen yet.
Your baseplate may be the result of one.
There were a couple of engine wizards working on these blocks back in the days.
On the other hand, there were also people mixing bits, copying what they heard would work.
Curious as to what will appear, if you ever open up your engine.
It was quite common in sidecar cross.
Another modification, often performed, would be an XT500 piston on a 533 rod, giving 880cc.
Both with 74mm standard stroke.
Neither of these need an extra baseplate.
For best performance, you would try to keep the squish distance close to original; say, 1.5mm or so.
If you add a baseplate to lower compression, you will lose the squish.
During the run of many years, I put together a spreadsheet with data of potentially useful pistons (either measured, or from published data), rods and strokes (74 standard, versus 84mm stroked) as I found them, as a guidance for my own projects.
To give you an idea, without any gaskets, the modified SP370 piston (dome volume 23.3cc) with 533 rod, as was popular in the 80's, has between 0 and -1mm (negative meaning, below deck) deck height. Depending on who did the work, and what their target was at the time, presumably.
With 0mm deck height, comp. ratio would be 11.0, with -1.5mm it would be 9.3.
So with base and head gaskets, you would be good.
The XT500 piston (dome volume 7cc) on a 533 rod would have similar deck height as the SP370 piston.
With 0mm deck height, comp. ratio would be 8.5, with -1.5mm it would be 7.5.
This is all assuming a standard head volume of 65.4cc.
Either 256 or 447 rods would give lower deck heights and compression ratios with standard 74mm stroke.
So they would certainly not need a baseplate.
A CR500 rod is 4mm longer than a 533 rod. To get the same numbers as above, with standard stroke, this WOULD call for a 4mm baseplate.
Of course, playing with base plate and gasket thicknesses, would also change your cam timing.
Baseplates were sometimes used with stroked crankshafts, although the shorter 256 rod & 84mm stroke would work without a baseplate for the SP370 and XT500 piston. As Jim says, it could be used to lower compression (same SP370 piston, 256 rod and 84mm stroke would give 12.3 compression ratio without gaskets), but at the cost of squish.
It's uncommon to find the longer 533 rod on a 84mm stroked crank, because of its larger BE diameter. This would put the BE hole in the crank really close to the edge.
Another, even older mod, uses the 85mm(?) Porsche (Carrera?) piston (840cc with standard stroke).
Could be that these have a larger deck height.
I have one somewhere, but would need to find and measure it to be sure.
There may certainly be variants I haven't seen yet.
Your baseplate may be the result of one.
There were a couple of engine wizards working on these blocks back in the days.
On the other hand, there were also people mixing bits, copying what they heard would work.
Curious as to what will appear, if you ever open up your engine.
Hey fishman, been chewing on your engine a bit more.
Did you have a closer look in the mean time?
There are some more costly options to get what you seem to have, using once 'commonly available' components.
Not impossible, as the sky was the limit in the XS's competitive sidecar cross days.
Possibly some previous owner preferred reliability over extreme engine displacement (not overboring the cylinder until there's almost no wall left).
In that case, you could have a long stroke (84mm) crankshaft combined with 80mm pistons (844cc).
Halfway the 70's, there was a factory kit with 80mm pistons, which would result in 750cc with a standard (74mm) stroke crankshaft.
Those pistons had 22mm pins, which would limit the choice of con rods to the 256 type.
The long stroke combination calls for a 4.5mm thick baseplate (+ gaskets) to achieve zero deck height.
In 750 form, this kit would already have a quite decent compression ratio; 10.9 at 0mm deck height, 9.3 at -1.5mm.
My first (very much abused by POs) XS, a daily rider XS2 bought around 1980, was set up like that.
It was quite temperamentful, and quickly taught me the right and wrong ways to kickstart, as well as putting carbs back into their rubbers along the side of the road. When we were still in the 'getting acquainted' phase, it used to kick back viciously and once made me go limp for several weeks.
If this is the combination you have, the compression ratio would be high, somewhere between 10.4 and 12.3.
Did you ever try to kickstart the engine? It would be quite a beast to kick over.
Do a compression test?
If you take off the bottom engine cover holding the oil suction filter, you can look at the position of the con rod big ends relative to the crankshaft wheels.
Compare them to (pics of) a standard crankshaft. If the big ends 'stick out' more, you have a stroker crankshaft.
Instead of the factory 80mm piston kit, someone could also have put in TX750 pistons, which have identical compression height etc.
Only, these are flat and have zero dome volume. Leading to very low compression ratio, like 6.9 to 7.5.
At least it would kick over easily!
Did you have a closer look in the mean time?
There are some more costly options to get what you seem to have, using once 'commonly available' components.
Not impossible, as the sky was the limit in the XS's competitive sidecar cross days.
Possibly some previous owner preferred reliability over extreme engine displacement (not overboring the cylinder until there's almost no wall left).
In that case, you could have a long stroke (84mm) crankshaft combined with 80mm pistons (844cc).
Halfway the 70's, there was a factory kit with 80mm pistons, which would result in 750cc with a standard (74mm) stroke crankshaft.
Those pistons had 22mm pins, which would limit the choice of con rods to the 256 type.
The long stroke combination calls for a 4.5mm thick baseplate (+ gaskets) to achieve zero deck height.
In 750 form, this kit would already have a quite decent compression ratio; 10.9 at 0mm deck height, 9.3 at -1.5mm.
My first (very much abused by POs) XS, a daily rider XS2 bought around 1980, was set up like that.
It was quite temperamentful, and quickly taught me the right and wrong ways to kickstart, as well as putting carbs back into their rubbers along the side of the road. When we were still in the 'getting acquainted' phase, it used to kick back viciously and once made me go limp for several weeks.
If this is the combination you have, the compression ratio would be high, somewhere between 10.4 and 12.3.
Did you ever try to kickstart the engine? It would be quite a beast to kick over.
Do a compression test?
If you take off the bottom engine cover holding the oil suction filter, you can look at the position of the con rod big ends relative to the crankshaft wheels.
Compare them to (pics of) a standard crankshaft. If the big ends 'stick out' more, you have a stroker crankshaft.
Instead of the factory 80mm piston kit, someone could also have put in TX750 pistons, which have identical compression height etc.
Only, these are flat and have zero dome volume. Leading to very low compression ratio, like 6.9 to 7.5.
At least it would kick over easily!
Haven't looked yet, but the former owner sold it because he couldn't kickstart it. So what you are saying could be true.
