lower the cylinder and ports?

[rd400canuck]

Hi guys,

I'm about to start to disassemble the RD400 for the winter the go through the motor and make sure I did everything right during my rebuild last winter and also to send out cyls and head for squish correction.

From quick older check Im getting 1.3 ~ 1.4 mm of squish. I am just wondering if I use a thinner base gasket to drop the cylinder say .4mm would i notice any difference in low and mid range power? Id have to watch my compression of course.... i think it was 140psi the last time i checked.

Thoughts?

[Yamaha 179]

I sell three thicknesses of base gaskets: .22mm, .35mm, and .56mm.  The .56mm gasket is stock thickness.  The thinnest gasket is made from sturdy material and works well, but it is for adjusting squish clearances much smaller than yours.  You need to send your heads or cylinders to a quality machine shop to alter the sealing surfaces to close up that gap.  If you are going to modify the engine
in other ways in the future you should probably wait and do everything together.  BTW, just closing up the squish clearance won't give
you a noticeable increase in power.  It will however, reduce the chance of detonation and engine destruction. 
Lyn Garland

[rd400canuck]

Thanks Lynn,

Im wondering now... are the aftermarket base gaskets from say those Vesrah kits thicker? I dont have one in my hand but thinking back to my rebuild It seemed like it was 1mm or something like that.

[Yamaha 179]

I've never measured a Vesrah base gasket but I doubt they would be that thick.  That is more in the size of a spacer than a gasket.
Lyn

[pdxjim]

Maybe sticking my neck out and showing more ignorance than knowledge, but ... in the quest for performance, wouldn't one generally want to raise (instead of lower) the cylinders with thicker base gaskets and deck/skim to regain proper compression /squish?

[Yamaha 179]

Maybe sticking my neck out and showing more ignorance than knowledge, but ... in the quest for performance, wouldn't one generally want to raise (instead of lower) the cylinders with thicker base gaskets and deck/skim to regain proper compression /squish?

That was quite a common way to increase power simply, easily, and cheaply years ago and it works really well with stock exhaust systems.  To be rather crude, you don't even have to modify or clean the ports; just raise the cylinder two millimeters with a shim and adjust the top side by milling the top of the cylinder or milling the cylinder and turning a bit off the head to eliminate dead spot at the edge of the combustion chamber where detonation is known to start.  Can be done easily with a Bridgeport and a Logan.  It ain't necessarily pretty, from the inside, but it looks stock and they will run really good for a "stock" bike.  Been there done that.
Lyn Garland

[rd400canuck]

Hi pdxjim...   my quest for more power is basically more grunt. I like a stock bike for this reason but I find my Rd400 could always use more low and mid power. I was hoping that lowering the cylinders by .5mm Id be producing more torque. However... I dont think .5mm will be too noticeable but combined with a .9mm squish it would help out. I ride my bike around town mostly ... I am rarely above 5500 and at cruise im 3000 to 4000 rom max so higher ports and aftermarket pipes arent something that would benefit me.

Plus I LOVE the look and sound of stock pipes.

[Organicjedi]

Hi pdxjim...   my quest for more power is basically more grunt. I like a stock bike for this reason but I find my Rd400 could always use more low and mid power. I was hoping that lowering the cylinders by .5mm Id be producing more torque. However... I dont think .5mm will be too noticeable but combined with a .9mm squish it would help out. I ride my bike around town mostly ... I am rarely above 5500 and at cruise im 3000 to 4000 rom max so higher ports and aftermarket pipes arent something that would benefit me.

Plus I LOVE the look and sound of stock pipes.

You could just lower the final drive gearing and install a lighter 520 chain while you're at it. Probably the best way to go to increase low end power unless you plan to do more extensive engine work.

[m in sc]

get a modern igntion with programmable curve. easier way to get what you want IMHO.