8 valve GS750 - high performance valve jobs and back cut valve specs

[Chuck78]

Finally I have found a very competent high performance local-ish machine shop that has some motorcycle experience and are very enthusiastic about high performance builds... They were very familiar with back cutting valves between 30 and 37 degrees, and said that they will charge me $160 for a high performance valve job including a Serdi radiused valve seat job (if they have the correct cutters for my engine, they think they did - otherwise 5 angle or better), lapping valves, and backcutting the valves to my spec. $80-100 to replace all valve guides if needed.

Now I was hoping to get some tips on what to as for in the technical specs of the job. I read that GS's love a 37 degree back cut on the valves the most, with great gains. Is this true beyond the 4 valve liter or bigger bikes? 37 degrees still a good number for an old 8 valver?

Also on the valve seat cutting, is it best for a 45 degree on the valve and 46 on the seat? Is a 45 seat cut more standard across most makes? Or is 45/45 very acceptable also? If Mike at M&M Cylinder Head Service has the correct size Serdi radiused seat cutter (said it depended more on valve stem diameter?) but it is a 45 cut, can the valve be easily ground to 44 to have the same effect?

Any tips much appreciated. I have to give Ray a call this week once I have read up some more. I HAVE to have my engine rebuilt by AMA Vintage Motorcycle Days and the WV GS rally in July... Thanks to all.

[Chuck78]

Well I just got dropped off my head, a spare 850 head (if valves or springs are needing replaced), and some 1979 GS850 cylinders to M&M Head Service north of Columbus today, and was very impressed with their enthusiasm and knowledge, as well as their counter full of motorcycle parts and office floor loaded with musclecar engines and parts...

He said he does a 37 degree back cut on the 4 valve suzuki heads, but was instructed that 30 degrees flows best on the 2 valve heads. He said he has 4 customers that bring in old Suzuki engines for high performance machine work, and has done extensive mods to their heads, and they have all been very pleased with the performance gains.

Any other input is welcome. They will be getting to my head and cylinders next week sometime.

[TeamNeal]

Whats the word Chuck? Have been thinking about having this done to my 650 head.

[Chuck78]

I actually just picked up all my parts from the machine shop yesterday, and had them inspecting/measuring my GS650 cylinders (background of photo) as well for a GS550->673cc build that's next on the list!



I was VERY VERY PLEASED with how the head turned out. Mike from M&M really put some nice work into the intake ports especially, giving it a really nice radiused entrance from the port up to the edge of the 45 degree valve seat. He said that on the 750 head at least, it was a bit tighter space than the bigger japanese inline 4 engines that he has worked with, which give you more room to make a long sweeping transition, but nonetheless I have a nice radiused approach to the valves now! Back cutting was no problem either. Luckily my springs and valve guides were all within spec, as I was worried about the delays getting valve guides from Suzuki...

I know the 650's combustion chamber, piston, and intake ports were the most advanced of all of the 8 valve suzuki 4's, so I don't know if the 30 degree angle is still the rule of thumb on those or not... I have a 650 head as well as some 750 and 850 heads here to compare if you really wanted a comparison of the chambers and the valve angles and all. I am not the one to ask for experienced advice on this mod for a 650 though, although i definitely plan on having the exact same treatment done on the 650 head at some point maybe late July or August. The wife always is complaining that her GS550 isn't fast enough (even though with pods/pipes/jetted, it SMOKES our friend's 82 KZ650..., and I think the 550 as is is plenty fast. speed demon! and I thought I was the fast one).

I am still waiting on some other rebuild parts from Suzuki, and still need to seriously clean up and repaint the head and block fins as well... Not to mention get my 4x8 bench back after the garage sale useage this weekend, so I can have a nice big space to tear down one or two engines on, if the second is needed for parts...