do you know what type of guide you have? if it is really thin-wall, it's called a guide liner. If it is like 3/32" thick or so wall thickness it is a replacement guide. They can be iron or bronze. Have you felt around the spring retainer and spring seat, while the engine is running? Perhaps, if it comes to that you can remove the keepers, retainer, v. spring, v. stem seal and look for any anomalies. But you have to be able to keep the valve from dropping in the cylinder. It can be done, but you have to be careful.
