SRvsXT

The SR heads are the ones you want for HP. Larger intake valve is the main difference. SR 47mm , TT-XT 45mm.

Yes, the SR head is different to the early XT/TT heads although the later XT's used the same head as the SR, but I can't remember what model XT they used the SR heads on. Along with the larger inlet valve, the SR head has deeper fins and is supposed to have a larger diameter inlet port(although I have compared my XT and TT heads with my SR heads and not noticed anything but the diferent inlet valve size.

If you are intrested in having a head ported/gas flowed, I can recommend Harry Bell at Wideline Engineering (03 5983 9684) in Bittern VIC. The last time I spoke too him, a few month ago he was telling my all ablout his new computerised flowbench.

Harry has had many years of experience with 500 engines, having build a TT that won the VIPER series in VIC in 2000? and then converting it for roadracing and doing quite well in VIC club road racing over the past few years.

CJ

I have a series of SR500 hot-up articles from CYCLE magazine from the late 70's. In one of these articles they spent a lot of time with Jerry Branch (at the time one of the USA's top performance tuners) flow testing various changes to a SR head. After a lot of testing / trial and error, they were not able to make any real substaintial gain in flow throught the inlet port, (increase of just over 10% flow from memory). One big recommendation was NOT to remove any material from around the valve guide boss for exactly the reason you mention, the guide WILL come loose.

The bigest limitation to better / increased flow through a SR head is the position of he Carbie. This results in a long flat inlet port which is further conpromised by the need to keep the valve gear compact to limit the overall height of the engine (there is not a lot of excess metal to carve into). In fact a SR head that I have that has been ported (by an unkown) was found to be porous and leaked air when Harry bench tested it since so much metal had been removed and the wall / casting is now very thin, as a result it now has a layer of epoxy resin on the outside to stop the port sucking air through the walls.
I a conversation with Harry Bell several years ago we discussed milling out the existing inlet port area and welding in a complete new port to reposition the carbie and give a slight down-draft effect. This was never attempted as it is such a huge job and it would make the engine illegal for Period 5 classic racing.

I am not sure what progress Harry has made over the last few years with his porting and since he has got his new flow bench, but it is definitly a job for someone with a lot of experience with the SR head and good equipment.

CJ