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For several years, I moderated a private motorcycle forum dedicated to helping amateur mechanics better understand how/why professional mechanics approach work in different ways. Amongst the most misunderstood aspect of old motorcycle work are valve guides.
Much of this misunderstanding comes from old literature as well as Uncle Bob’s grey beard advice to “lap ‘em in.” In most cases, this information reflects a difference between making a motor run and making a motor last. Recently, I’ve read a few posts here on the AMCA forum which perpetuate some of this misunderstanding. The purpose of this thread is to give a 10,000 foot overview of guide replacement to help dispel some of the confusion. It is not a primer on how to replace guides, but rather an overview of general proceedures.
So, what’s the big deal?
Most people believe that valves shed heat through the seat. They do – but what most fail to realize is that the efficiency of that transfer is directly related to seat and guide condition. If the seat is not concentric to the guide of if the clearance from stem to guide is excessive – the valve will perish in an unreasonable amount of time. Similarly, if the guide is not a good fit in the bore it develops hot spots. If these hot spots coincide with thrust areas the result is rapid wear and often a bluing of the valve stem. The bluing is an indicator of the valve being overheated. Overheat a stem often enough and very bad things happen.
The biggest mistake is using any type of goop to try to make a guide fit. Whether loctite or scotch weld, goop should never be used to make up for a crappy guide to bore fit. As to why – the answer is super simple – the guide relies on its fit in the bore to transfer heat. Loctite is a poly and insulates. Scotch weld does transfer heat but has no holding ability. In short – you’re either insulating or wasting time, but you aren’t actually repairing anything if you rely on goop to deal with hogged out or buggered up valve guide bores. An equally poor idea is knurling guides or guide bores to make them fit. Again, "making" something fit by stretching and puckering the metal isn't the best idea here. The parts were precision machined for a reason.
Guide material also makes a difference. In addition to there being several types of “cast iron” there also are several types of bronze used for guides. All of these materials work – and it is almost always the best idea to fit guides to original pattern. And, unless you can find the exact same operating parameters as your machine -- don't be tempted to transfer knowledge from another make/model. Just because soft bronze guides and stainless valves at .001 work in one motor does not mean they will work well in another. If you really don't know -- just ask. The AMCA has a lot of knowledge.
Where no guides are available, it is advisable to make them from 40,000 – 60,000 psi grey cast iron. It is NOT difficult to make guides though it is best done in a collet vs a three jaw chuck to ensure the ID and OD are concentric. Again, if you screw up on your sizing, it is not advisable to knurl guides in an attempt to make them “fit.” Worn out is worn out. Too large a hole is too large a hole. Any type of Band-Aids only shorten the life of your repairs. However, if you don’t truly understand what I’ve laid down in this paragraph, chances are you shouldn’t be trying to make your own guides either.
Let’s start with guide removal. Before you do anything, make sure you take the time to clean the ports and make the guides clean, clean, and clean. Any carbon or broken off pieces can track mark your guide bore or even rip them oversize. For many old guides that have been in place for years or on sensitive alloy heads, it is a good idea to bore the old guides until they are within .010-.020 of the outside diameter. Then, it is time to heat your parts. Generally 250-300 F for cast iron, 300-350 for alloy does the trick. The very best way to remove a guide is to press it out vs beating it out. However, if you don’t have fixtures, this can be difficult.
For most DIY, make sure you have a large drift which fits squarely on the end of the guide and the heaviest hammer you can swing with authority. Momentum is more important than weight here. We want the guide to push out smoothly in a few strategic blows vs getting whacked a million times. Guides come out the opposite of their shoulder. Some guides also use clips on the port side – double check for this. Here's two shots of it being done on a cast iron harley head:
IMG_4399.JPG Notice that I media blasted the chamber and guide first . . that was to remove carbon.
IMG_4401.JPG
Once you have driven or pressed out the guide; stop to examine the guide bore. It MUST be free of deep gouges, scratches, or shiny/dull spots. It must be perfectly round. Top notch guide jobs include a honing of the guide bore on a sunnen or other rigid hone to ensure perfect heat transfer. Then, it’s time to fit guide to bore. For most cast iron parts a .001/.002 interference fit is good. For alloy, follow the manufacturer as each alloy is different. You may need to buy or make oversized guides and turn them to your exact needs. If you get lucky and your bores aren’t messed up – then standard guides will likely pop straight in. If you have cracks radiating from the valve guides . . .stop. Most cast iron heads can be repaired with silver solder (no, it won’t melt out), but alloys may need more specialized help. This work is for the specialist, not the DIY.
When it comes time to install your new guides; it is not a bad idea to chill them. Overnight in the freezer or a few hours in a bag in the ice tray often does it nicely. Reheat your head/cylinder parts and get ready to move with authority. Again, the best way is to press or draw the guides in – but a good drift and a steady hand will do it well. Many like scotch lock – I use a dab of threebond under the shoulder to ward off any seepage down the guide bore. It shouldn’t happen, but shouldn’t has a weird way of appearing in vintage motorcycles.
Here's one being inserted -- this fit was so good that I did not use any type of sealer under the shoulder. So far in 2500 miles the heads have shown no sign of either guide dripping.
IMG_4407.JPG
You can see reams for initial sizing in the above photo. I then take the guides and hone them to my final fits. I do like going for the middle range on factory specs. For most intakes this is around .0015-.002 (OHV) or .003-005 (flathead) and for exhausts .0025-.0035 (OHV) or .006-.008 on flatheads. I like to ream to within .0005 and then to finish on rigid hone whenever possible; brush hone for other times.
Once everything cools down, it’s time to address the seat. Do not, do not, do not just “lap ‘em in” This is the totally wrong approach for a fresh guide. Yes, the motor will start and run – but it will not have good power when hot and the valve will not last but 50-60% of its normal service life. This is because it is nearly impossible to have the valve index to the same spot with a new guide. One of the most important things is for the seat to be fully concentric with the guide. Any more than about .002 concentricity and you will have hot spots on the valve face. This is compounded by lapping. When you lap, the moment the motor fires the valve creeps up/out the valve seat because of expansion. Because you only lapped it in one spot for room temperature . . . your gas seal is cruddy when the valve expands. Therefore, we cut or grind new seats to proper width and then fit the valves. You can lap if you like . . .but it’s more to see pattern than to seal.
In the below pictures you can see the pilot for a neway cutter and the initial seat. This seat then has to be top and bottom cut for width and placement on the valve face. Improper placement also shortens the life of the valve.
IMG_4409.JPG
IMG_4410.JPG Note, the seat in the above picture is extra wide because I was cutting for an oversized head and dealing with multiple previous valve jobs. I often cut the 45 wide first. Some people start at 70/60 and work to 45 then 30/20/15. It all depends on how familiar you are with your equipment and how you were taught. All ways work and after a few dozen heads you get a pretty good feel for how much material you'll actually remove.
The final step is to check your stem protrusion. On flatheads, we often can relieve for sunk valves and simply back off the adjusters; but on OHV, it often means new valve seats or other trickery
Again, you sure can just knock out and knock in guides and lap the valves. The motor will run. Maybe not very well or for very long, but you’ll get back in the wind.
The question is why you’d want to do so when for a very little bit more work, you can do it professionally and be done for decades.
Much of this misunderstanding comes from old literature as well as Uncle Bob’s grey beard advice to “lap ‘em in.” In most cases, this information reflects a difference between making a motor run and making a motor last. Recently, I’ve read a few posts here on the AMCA forum which perpetuate some of this misunderstanding. The purpose of this thread is to give a 10,000 foot overview of guide replacement to help dispel some of the confusion. It is not a primer on how to replace guides, but rather an overview of general proceedures.
So, what’s the big deal?
Most people believe that valves shed heat through the seat. They do – but what most fail to realize is that the efficiency of that transfer is directly related to seat and guide condition. If the seat is not concentric to the guide of if the clearance from stem to guide is excessive – the valve will perish in an unreasonable amount of time. Similarly, if the guide is not a good fit in the bore it develops hot spots. If these hot spots coincide with thrust areas the result is rapid wear and often a bluing of the valve stem. The bluing is an indicator of the valve being overheated. Overheat a stem often enough and very bad things happen.
The biggest mistake is using any type of goop to try to make a guide fit. Whether loctite or scotch weld, goop should never be used to make up for a crappy guide to bore fit. As to why – the answer is super simple – the guide relies on its fit in the bore to transfer heat. Loctite is a poly and insulates. Scotch weld does transfer heat but has no holding ability. In short – you’re either insulating or wasting time, but you aren’t actually repairing anything if you rely on goop to deal with hogged out or buggered up valve guide bores. An equally poor idea is knurling guides or guide bores to make them fit. Again, "making" something fit by stretching and puckering the metal isn't the best idea here. The parts were precision machined for a reason.
Guide material also makes a difference. In addition to there being several types of “cast iron” there also are several types of bronze used for guides. All of these materials work – and it is almost always the best idea to fit guides to original pattern. And, unless you can find the exact same operating parameters as your machine -- don't be tempted to transfer knowledge from another make/model. Just because soft bronze guides and stainless valves at .001 work in one motor does not mean they will work well in another. If you really don't know -- just ask. The AMCA has a lot of knowledge.
Where no guides are available, it is advisable to make them from 40,000 – 60,000 psi grey cast iron. It is NOT difficult to make guides though it is best done in a collet vs a three jaw chuck to ensure the ID and OD are concentric. Again, if you screw up on your sizing, it is not advisable to knurl guides in an attempt to make them “fit.” Worn out is worn out. Too large a hole is too large a hole. Any type of Band-Aids only shorten the life of your repairs. However, if you don’t truly understand what I’ve laid down in this paragraph, chances are you shouldn’t be trying to make your own guides either.
Let’s start with guide removal. Before you do anything, make sure you take the time to clean the ports and make the guides clean, clean, and clean. Any carbon or broken off pieces can track mark your guide bore or even rip them oversize. For many old guides that have been in place for years or on sensitive alloy heads, it is a good idea to bore the old guides until they are within .010-.020 of the outside diameter. Then, it is time to heat your parts. Generally 250-300 F for cast iron, 300-350 for alloy does the trick. The very best way to remove a guide is to press it out vs beating it out. However, if you don’t have fixtures, this can be difficult.
For most DIY, make sure you have a large drift which fits squarely on the end of the guide and the heaviest hammer you can swing with authority. Momentum is more important than weight here. We want the guide to push out smoothly in a few strategic blows vs getting whacked a million times. Guides come out the opposite of their shoulder. Some guides also use clips on the port side – double check for this. Here's two shots of it being done on a cast iron harley head:
IMG_4399.JPG Notice that I media blasted the chamber and guide first . . that was to remove carbon.
IMG_4401.JPG
Once you have driven or pressed out the guide; stop to examine the guide bore. It MUST be free of deep gouges, scratches, or shiny/dull spots. It must be perfectly round. Top notch guide jobs include a honing of the guide bore on a sunnen or other rigid hone to ensure perfect heat transfer. Then, it’s time to fit guide to bore. For most cast iron parts a .001/.002 interference fit is good. For alloy, follow the manufacturer as each alloy is different. You may need to buy or make oversized guides and turn them to your exact needs. If you get lucky and your bores aren’t messed up – then standard guides will likely pop straight in. If you have cracks radiating from the valve guides . . .stop. Most cast iron heads can be repaired with silver solder (no, it won’t melt out), but alloys may need more specialized help. This work is for the specialist, not the DIY.
When it comes time to install your new guides; it is not a bad idea to chill them. Overnight in the freezer or a few hours in a bag in the ice tray often does it nicely. Reheat your head/cylinder parts and get ready to move with authority. Again, the best way is to press or draw the guides in – but a good drift and a steady hand will do it well. Many like scotch lock – I use a dab of threebond under the shoulder to ward off any seepage down the guide bore. It shouldn’t happen, but shouldn’t has a weird way of appearing in vintage motorcycles.
Here's one being inserted -- this fit was so good that I did not use any type of sealer under the shoulder. So far in 2500 miles the heads have shown no sign of either guide dripping.
IMG_4407.JPG
You can see reams for initial sizing in the above photo. I then take the guides and hone them to my final fits. I do like going for the middle range on factory specs. For most intakes this is around .0015-.002 (OHV) or .003-005 (flathead) and for exhausts .0025-.0035 (OHV) or .006-.008 on flatheads. I like to ream to within .0005 and then to finish on rigid hone whenever possible; brush hone for other times.
Once everything cools down, it’s time to address the seat. Do not, do not, do not just “lap ‘em in” This is the totally wrong approach for a fresh guide. Yes, the motor will start and run – but it will not have good power when hot and the valve will not last but 50-60% of its normal service life. This is because it is nearly impossible to have the valve index to the same spot with a new guide. One of the most important things is for the seat to be fully concentric with the guide. Any more than about .002 concentricity and you will have hot spots on the valve face. This is compounded by lapping. When you lap, the moment the motor fires the valve creeps up/out the valve seat because of expansion. Because you only lapped it in one spot for room temperature . . . your gas seal is cruddy when the valve expands. Therefore, we cut or grind new seats to proper width and then fit the valves. You can lap if you like . . .but it’s more to see pattern than to seal.
In the below pictures you can see the pilot for a neway cutter and the initial seat. This seat then has to be top and bottom cut for width and placement on the valve face. Improper placement also shortens the life of the valve.
IMG_4409.JPG
IMG_4410.JPG Note, the seat in the above picture is extra wide because I was cutting for an oversized head and dealing with multiple previous valve jobs. I often cut the 45 wide first. Some people start at 70/60 and work to 45 then 30/20/15. It all depends on how familiar you are with your equipment and how you were taught. All ways work and after a few dozen heads you get a pretty good feel for how much material you'll actually remove.
The final step is to check your stem protrusion. On flatheads, we often can relieve for sunk valves and simply back off the adjusters; but on OHV, it often means new valve seats or other trickery
Again, you sure can just knock out and knock in guides and lap the valves. The motor will run. Maybe not very well or for very long, but you’ll get back in the wind.
The question is why you’d want to do so when for a very little bit more work, you can do it professionally and be done for decades.
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