Valve Guide - Head Fit

Pushrod Twin

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Is there a recommended maximum oversize dimension for valve guide outside diameters? Or do we just keep boring the head until the damage is machined out?
I have a front head which leaks oil down the outside of the exhaust guide!
 

greg brillus

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Roy, there are oversized guides available, but your probably best off just cleaning up the hole nice and true again and making your own guide to suit. They should be an interference fit and installed once the head is heated first. Cheers...........Greg.
 

Pushrod Twin

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Thanks Greg, I am aware that there are oversize guides available, I guess I should check out the sizes, but my gut feeling is that they wont fill the cavern that's been created here!
The engine ran about an hour on the dyno, then another hour on the road before I crashed. I felt the need to strip it because there were oil leaks. I could also feel loss of compression on the front cylinder. I thought that maybe the valves had tangled when over-revving during the crash. However, there was no visual evidence of valves or pistons having touched, but there was an excessive amount of oil in the front combustion chamber and on the valve seat, clearly preventing the valve sealing. On removal of the valves there was also a large amount of oil in the exhaust port. There was so much oil around the root of the exhaust guide that it had not caked hard but was a pasty coagulation with shrinkage cracks through it. That all washed out easily and I thought I could see a gap between the guide & the head. I set the head on a fixture in the vice with the guide vertical & the locking collar removed then filled the collar recess with solvent, see pic. A couple of hours later the solvent was well run though into the port. While blowing the solvent out with a duster gun from the collar end I could see bubbles forming in the port! You can just see them in the left side of the port in the next pic. I expect that when the guide is removed that there will be a very large score mark.
 

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greg brillus

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That's a shame. It must have picked up when someone removed the guide in an earlier life, probably didn't heat the head first, unless there was a burr on it. This is common on oil pump sleeves too, and the scores in the housing cause wet sumping. Oh the joys of old machinery.
 

Pushrod Twin

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Yes, the head has had a huge amount of work done to it.The previous owner procured this engine minus a front head. He purchased the head from someone in the UK in the period before new heads were available & it appears to have been reclaimed from someones scrap heap. It may have dropped a valve once, the combustion chamber has been welded & re-machined. There are porosities all around the sealing ring area. I lapped it to the barrel & used Loctite 515 in the hope of filling some voids. It appeared to work. The stainless seats have been set too deep into the chamber, they are .020"-.030"below the chamber surface. I bored the badly worn rocker tunnels but they failed to clean up. The inlet guide has been changed as well. There is a half a 1mm dia hole showing both at the top & the bottom of the guide bore in the head, but it doesnt appear to connect, it doesnt leak solvent & I cant make blow bubbles like the exhaust. The dilemma is; do I risk dragging them both out & creating a bigger nightmare, or do I pour Loctite 290 Wick In around them both & just ride it?
 

timetraveller

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Just catching up with this. There is another alternative. One of our German members used to make a lower valve guide which was machined from solid and consisted of the lower valve guide shape and a hollow cylinder which was a decent fit into the hollow between the top of the lower guide and the bottom of the upper guide. I assume that it had a small flange which would go into the crescent shaped cut out to stop it rotating. There was a wide slot down one side for the rocker fork to enter and as well as being a decent fit into the hole for the lower guide it was also made the correct length to be trapped by the base of the upper guide. It could not move up and down unless the top guide did as well and it could not rotate. I have never seen one but it seemed to be a nice mod. I designed something similar for a local Comet owner who's lower valve guide had become loose. It was only like the upper part of the German one but a new oversized lower guide never moved after he fitted the modification as this part trapped the top of the lower guide and was itself trapped by the upper guide.
 

clevtrev

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Just catching up with this. There is another alternative. One of our German members used to make a lower valve guide which was machined from solid and consisted of the lower valve guide shape and a hollow cylinder which was a decent fit into the hollow between the top of the lower guide and the bottom of the upper guide. I assume that it had a small flange which would go into the crescent shaped cut out to stop it rotating. There was a wide slot down one side for the rocker fork to enter and as well as being a decent fit into the hole for the lower guide it was also made the correct length to be trapped by the base of the upper guide. It could not move up and down unless the top guide did as well and it could not rotate. I have never seen one but it seemed to be a nice mod. I designed something similar for a local Comet owner who's lower valve guide had become loose. It was only like the upper part of the German one but a new oversized lower guide never moved after he fitted the modification as this part trapped the top of the lower guide and was itself trapped by the upper guide.
Nothing new, saw that one forty years ago,.
 
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