Old school port job what do you think?

[gggGary]

Picked up some XS parts from an old hill climber last summer and got this 72 head.
Someone had been in there pretty heavy. Short of bolting it on an engine to find out, what are the odds it would be an improvement? I also got a new 256 750 big bore kit with pistons rings and wrist pins from him. He claimed that under racing use the smaller larger early wrist pins would eventually distort the thin small end of the rod. They reduced the size of the wrist pin with the 447 motors leaving more meat around the pin on the rod end. I have a 72 bottom end to put this stuff on.

The last pic is an unported intake from an 81 for comparison.

 

[Dead Roman]

bolt it to an engine and let us know. easy fix if it runs like crap/ I reckon its gonna move a-lot more fuel.

 

[grizld1]

70-73 wrist pins are 2 mm. larger in diameter than later versions, leaving less meat at the small end of the rod, and the PO told you exactly right--when the motor is wound up tight on a regular basis, the combination of fat, heavy wrist pin and weak small end causes the small end to stretch, and the usual result is that the piston beats itself to death. There are competition rods for the 256 motor that will hold up to an aggressive hand on the throttle, but the stock rods won't.

The problem with enlarging ports is that velocity can be lost and you can wind up with worse flow than you started with. I can't tell anything from a photo, but maybe Jack will jump in here, put his good eye to work, and give you a highly educated opinion.

 

[HooliganCycles]

I hand port all of my heads on every bike i have and do them for customers also and have had no problems with it.... On the other hand tho there is a lot of material missing around those guides, i would say bolt em on and see what happens, but i bet its not very impressive. I hope im wrong tho, let us know how they turn out. Good luck.

 

[Lester]

Conserning the rods.... I am in a similar situation but I DON'T plan on racing it, just riding like its stolen every now and then... what are your thoughts on it holding up?

 

[bowser]

Flowtest the two heads and see what the difference is

 

[grizld1]

Lester, aggressive street riding is enough to cause trouble with the stock rods in early motors. There's history behind the grief. Yamaha brought out the first XS1 motors with needle bearing wristpins that didn't hold up, and made a mid-year change to plain bearing wrist pins of the same OD as the needle bearing setup--22 mm. After a history of rod failures had developed, for 1974 (TX650A) Yamaha did two things: they detuned the cam and reduced the wristpin OD to 20 mm., lightening the wristpin and leaving more meat in the top end of the rod. If you're going to ride it like you stole it, either plan on short engine life or install performance rods.

 

[gggGary]

On Grizld1 s advice I scrapped the head.

 

[5twins]

Well, I won't just scrap it. There's probably a few good parts in it (valves, valve springs, etc.). That hardened steel washer that the valve springs sit on in the head is a keeper. Yamaha seems to think it's made of gold or something and gets like $9 for one.

 

[gggGary]

Well I don't throw away much, two rocker arms are on the way to Australia.

I will probably try to heat and remove all the exhaust and valve cover studs.
Yup the valves are coming out.
It had two intake spigots to fit 32mm Mikuni VMs on it.

 

[grizld1]

Uh, gggGary, I don't think I advised scrapping that head, or any other action for that matter (though that intake does look pretty hogged out to me). If there's a shop with a flow bench in your neck of the woods you might see what numbers you get at a range of lifts and PM them to Jack for his take, or compare them to some of the specs he's posted over at XS650 Garage.

 

[Lester]

Lester, aggressive street riding is enough to cause trouble with the stock rods in early motors. There's history behind the grief. Yamaha brought out the first XS1 motors with needle bearing wristpins that didn't hold up, and made a mid-year change to plain bearing wrist pins of the same OD as the needle bearing setup--22 mm. After a history of rod failures had developed, for 1974 (TX650A) Yamaha did two things: they detuned the cam and reduced the wristpin OD to 20 mm., lightening the wristpin and leaving more meat in the top end of the rod. If you're going to ride it like you stole it, either plan on short engine life or install performance rods.

Sorry I wasn't reading very carfully... don't have an early xs, I have an 1980.... but same thinig applies? Any rods you could recommend... most bang for the buck will be good, I'm on a family mans budget for a few more years. I'm not raceing or setting speed records but I would like the rod to hold up.

Thanks

 

[grizld1]

The rods in later engines are pretty tough. As long as you're not winding the motor past redline on a daily basis, they should give you a lot of good use.