S3 head mod advice...
- Thread starter redrider
- Start date
Jakobi
Super Moderator
Ahh.. They are pretty heads.. but not so practical. The actual head inserts (all 3 of them) are exactly the same volume as the stock combustion chamber. What varies is the height of the insert, which is effectively the same as if you just had some of the mating surface machined off the stock head. It'll reduce squish and increase compression.
Take the time to measure the squish height before, and then after. You won't very good squish clearance without going excessive on the compression. Thats just the nature of the beast.
I had 13.5:1 or so with the high high head and 1.9mm squish on my 300. When I bought the 250 inserts, I had the high high machined to increase volume and reduce comp. Coins better spent on machining the stcker, but they do look sweet.
Take the time to measure the squish height before, and then after. You won't very good squish clearance without going excessive on the compression. Thats just the nature of the beast.
I had 13.5:1 or so with the high high head and 1.9mm squish on my 300. When I bought the 250 inserts, I had the high high machined to increase volume and reduce comp. Coins better spent on machining the stcker, but they do look sweet.
redrider
New member
Thanks for the input fellas.
Jakobi, I have had the RB designs work done on my stock head and carb but no appreciable change in power or fuel efficiency. Was hoping for something more from the S3. Just had my engine bottom rebuilt due to a shattered nylon crank stuffer (screw backed out), but when it's all back together I'll certainly take squish measurements before and after S3 installation.
Jakobi, I have had the RB designs work done on my stock head and carb but no appreciable change in power or fuel efficiency. Was hoping for something more from the S3. Just had my engine bottom rebuilt due to a shattered nylon crank stuffer (screw backed out), but when it's all back together I'll certainly take squish measurements before and after S3 installation.
Jakobi
Super Moderator
The RB head will be of much better tolerance than an S3. Its kind of like taking a step backwards.
Out of curiosity, what jetting are you running? Thats where the magic is. Setting the head up just gives more consistant results when dialling it in, and over a greater range of conditions.
Out of curiosity, what jetting are you running? Thats where the magic is. Setting the head up just gives more consistant results when dialling it in, and over a greater range of conditions.
redrider
New member
Just checked:
I'm about .5 mm shy of the exhaust port completely uncovered at bottom stroke (new wiseco piston installed, previous was the wosner).
With stock head my squish is 1.32mm at the cylinder wall and ramps to 1.7mm before the head volume shoulder due to the angle on the head different than the angle of the piston surface.
Jetting: 175 Main, 42 idle and mid postion on stock needle (I forget the taper code). Running 98 oct.
I only have one gasket under the cylinder (.5mm).
The RB designs head mod was done by the previous owner when the bike was new. When the S3 head arrives I'll check the squish again to see where I am. I understand the S3 inserts have the same volume cut as the stock so I'll evaluate some adjustments after I check the squish with the inserts. Hoping to end up about 1.1-1.2mm on squish with my piston flush with exhaust, then I'll start playing with the jetting again.
I'm about .5 mm shy of the exhaust port completely uncovered at bottom stroke (new wiseco piston installed, previous was the wosner).
With stock head my squish is 1.32mm at the cylinder wall and ramps to 1.7mm before the head volume shoulder due to the angle on the head different than the angle of the piston surface.
Jetting: 175 Main, 42 idle and mid postion on stock needle (I forget the taper code). Running 98 oct.
I only have one gasket under the cylinder (.5mm).
The RB designs head mod was done by the previous owner when the bike was new. When the S3 head arrives I'll check the squish again to see where I am. I understand the S3 inserts have the same volume cut as the stock so I'll evaluate some adjustments after I check the squish with the inserts. Hoping to end up about 1.1-1.2mm on squish with my piston flush with exhaust, then I'll start playing with the jetting again.
Jakobi
Super Moderator
You very likely won't get those clearances out of a stock head/S3 without dropping base gaskets and using the high high insert, I used a 1.3mm base gasket stack on mine, with the black insert and had 1.9mm clearance. Take out 0.6mm of gaskets would give me 1.3mm squish, but the compression would have been through the roof. I recall that even with 1.9mm I had a final UCCR of 12.5:1, and 13.5:1 is probably the absolute max you want to go on a 300.
When I set the 250 S3 head up, I used 1 x 0.5mm gasket and got 1.25mm squish using the black insert. Then had it machined to reduce volume and correct the compression ratio, still 14:1 after machining. Since then I've slipped another 0.1mm gasket in to time the ports for more top end, and also drop the comp some more for better over rev.
When I set the 250 S3 head up, I used 1 x 0.5mm gasket and got 1.25mm squish using the black insert. Then had it machined to reduce volume and correct the compression ratio, still 14:1 after machining. Since then I've slipped another 0.1mm gasket in to time the ports for more top end, and also drop the comp some more for better over rev.
