Porting - how much is too much?

I have the head off my SR500 and it has a loose exhaust valve guide. A lot of material was removed during porting and I'm wondering if this is the cause of the guide's failure.

Anyone else here had this problem?

The engine was said to be pretty fast, so I'm leaning toward putting in an oversized guide and hoping for the best.

 

How about some details.
How long did the old guide last?
Oversize guides usually work well.
Any photos available?

 

I'm not sure why someone would want to remove that much material from the roof of the intake tract, i.e., so much that there would be a lack of support for the valve guide. Usually, the diameter of the tract, before the guide, is where removal of material makes the most sense. In any event, comparison with a stock head will tell you where material was removed, to what extent, and whether it might potentially jeopardize valve guide support.

Nonetheless, you have a valve guide support issue. Assuming adequate base material to simply support a guide, oversize or otherwise, it seems you've two options: (1) weld up and rebore for a standard size guide; or (2) bore to oversize. Option (2) makes the most sense, if source such a guide. Welding on old-ish heads can be iffy at best. The old British stuff I have snaps, crackles and pops when welding is attempted. The Al is porous and has soaked in petroleum products for years resulting in really bad base metal for welding. maybe your newer japanese stuff is better.

GLWP . . . . .

Cheers,

Dave

 

options

There are times when the need to take material off of the roof of the intake tract, especially on old english heads, and also when you're increasing the downdraft angle of the port, reduces the material to support the guide. If you're going to try to keep this head, I'd suggest only welding the head downstream of the guide in the port, as that's usually where the support is at a minimum. Weld into the guide area, but don't fill the hole completely. Also, this area can be faired in so that it has minimal impact on flow. If you don't fill the hole, then you can bore the guide hole to the next larger size for a quide and still use the old hole to index off, that way you don't have a whole job reseating the valve into the old seat.

hth

 

I don't know how long the old guide lasted, since it was installed by the previous owner. I do know that an attempt was made to glue it back in with JB Weld.

My concern was the lack of surface area that grips the guide and transfers heat. I haven't talked to anyone who has had this specific problem so I'm putting in an oversized guide and hoping for the best.

I already bought a stock head for 45 bucks so I'm covered if this one doesn't work. It'd be a bummer to lose a ported head.

 

My grandfather had a Sunbeam S7 that leaked oil straight through a pinhole in the engine case.

 

porting

If I remember correctly the exhaust flowed pretty good on them stock and just needed an alignment with the pipe and a general clean up and polish . the intake was where most of the work was done and got the best results . the sr heads run pretty hot. you already got the oiler line running to the exhaust rocker I suspect? if you don't you should . b bill

ps mr 83 BSA . how are the srx girls doing? I'm almost over missing them now.( sigh ) . but am working on getting some 17's on my track SR b bill