Setting up valve guide clearances
We have been modifying TR-4s for unleaded here in California
for going on to 20 years now. The guides that we use are a molybdenum
bronze material but we have also use phosphor bronze and aluminum-silicone
bronze. We recently did a 1956 AC ACE also. The ACE was set up
with the clearances that you mention in your posting and had valve
stem seals also. We have problems with "squeaking" from the inlet
valves and guides until we removed the seals! The coefficient
of expansion of the bronze alloys is so much higher than that
of the cast iron head that as the temperature goes up the valve
stem to guide clearance goes down. We have actually had exhaust
valve seizure problems on a hot engine when the valve stem clearance
is less than .002". What we normally use and have had good success
with is .0015" clearance on the inlet valve stem and .002 to .0025"
on the exhaust valve stem. These are run without valve stem seals
on TR-3s and TR-4s that have had the rear camshaft journal modified
to increase the amount of oil supplied to the cylinder head and
valve gear. With these clearances and using stainless valves with
hard chromed stem we have had neither oil consumption nor seizure
problems and have had great durability even under extreme racing
The Engine Room
Santa Cruz, Ca.
Valve Guides Revisited 2012
I run bronze guides. You just need enough valve to guide clearance that you don't stick a valve and they work fine. Don't use valve guide seals either. I had them put 0.003" clearance on my intakes and 0.0035" on my exhausts. I remember Greg having good recommendations in this regard too, so am interested in his view on the proper clearance. I was running 190 degrees at Road Atlanta and didn't stick a valve, so was pleased with that. The spring pressures should be specified by the cam builder. The Larry Young cams want about 100 lbs seat pressure. Ones with more dramatic opening events may want more, but that's largely to prevent valve float at higher RPM than you run. If you are having the seats ground, ask for a triple angle or radiused grind - I've been told that those can make a big difference in flow. My (newly prepared) head was milled 130 thou, I cleaned up the combustion chambers a little to remove any sharp edges primarily, they cut off the exhaust valve guide flush with the top of the port, and I had a triple angle grind done. NO porting whatsoever was done to it. I'm not sure I could tell the difference in power between that head and my old Comptune / Uncle Jack's porting services head.
In talking to 3 different machine shops it appears that the "standard" valve grind for car engines is the triple grind. As about the only engines that have valve jobs any more are race engines, I think this is the "standard" for race engines. Tractor and industrial engines I think still use the traditional single angle grind. The machinist I'm using now says he has 0.040, 0.060 and 0.080 width valve seat cutters. I think they all use carbide valve seat cutters now and they don't really "grind" the seats with stones anymore. Anyway, he says he uses the 0.060 on the exhausts for more heat transfer surface and 0.040 on the intake seats, for what it's worth.
Richard, I presume that by "intake" you mean total valve lift. .430" lift is quite a bit more than stock. I would use bronze guides and would try to obtain exhaust valves that have hard chromed or "plasma coated" stems. We use .0015" to .002" valve stem to guide clearance on the inlets & .002" to .0025" on the exhausts. Valve spring pressure with the valves closed should be 85 to 90 psi. You need to make sure that the spring you use do not go into "coil bind" at full valve lift. With the valve fully open, you must be able to stick a .040" feeler gauge in between the coils of the spring. We still "grind" all of our valve seats, and then lap them with fine compound to make sure we have a good seal. I guess we are "old school". We generally use seat widths of .040" on the exhaust & intake with hardened valve seats installed in the head in both intake and exhausts. I like to use 4 or more angles on the valve seats, 15, 30, 45, 60, & 75 degrees nicely blended to form as much of a radius as possible. The hardness of the seats prevents the seats from wearing or "pounding" and changing shape as the engine runs. The most critical area in a head's ports is the 1/4" or so as the seat "blends" into the "throat" of the port. That area critically effects the flow of the port and the valve seat. This is not to say that other areas of the ports are not important. But on an engine with a cam like yours, you could spend a lot of money and time for very little effect. The hotter the engine is, the more important the overall flow of the head and manifolds becomes. On a full race engine, ie cam with over 295 degrees duration, compression over 12: 1 and a power band going up to 7,000 rpm, a lot of power can be picked up by working on the exhaust port flow of the TR-4 .
We like to improve the oil flow to the valve gear, ie, the top of the cylinder head. The best way to do that is to grind a passage that connects the two oil holes that go into the rear cam bearing housing when the rear cam bearing not in the block. Then the bearing is pressed into place as normal. this allows additional oil to go directly from the drilled hole coming from the rear main bearing up into the drilling that goes up into the head. No external oil lines needed, and the amount of flow allowed is just right. This it even a good thing to do on a street engine. Another way to do this is to grind additional flats like the two original ones on the rear cam journal. These flats connect the spiral grove on the rear cam journal and allow more oil "metering" up to the head. Tony is also right about valve stem seals. Even with the added oil flow gained by the above modifications, you do not need or want valve stem seals on these engines. They will just cause trouble like sticking valves!
As a general rule, most all of the guides will be some alloy containing aluminum and phosphorus. The higher the aluminum content, the more likely they are to grow, so one could probably do a comparison by weight, assuming the dimensions were the same from different suppliers. There's one more trick that can be done to bronze guides that isn't useful with the cast-iron variety--knurling. Aluminum bronze guides are more malleable than cast iron. Trying to knurl the cast iron simply causes it to chip, while the bronze guide will deform, making a ridge in the guide bore. Once that ridge has been reamed to the correct size, there are two advantages over using a stock bore. First, the guide tends to retain oil longer, rather than pumping it straight down the guide bore, and second, the contact area of the guide bore is reduced, making the valve less likely to stick. And, even with the reduced area, the wear characteristics are generally better than cast iron, and a good bronze guide can be knurled a couple of times before requiring replacement. Worth some experimentation, maybe. The business about valves sticking, I think, has much to do with improper clearances, and not the material itself, since the natural lubricity of phosphor bronzes is pretty good compared to cast iron, something demonstrated by the ability to use valve stem seals with bronze guides. Once the clearances are set up properly, and good stem seals found, bronze guides should contribute to a more consistently performing engine simply because the ability to use stem seals will reduce the amount of carbon accumulation on the valve heads and stems, keeping flow closer to optimum.
Having been familiar with knurling from 50+ years ago, I quietly mentioned knurling recently and got a positive response. Intuitively, knurling valve guides makes sense to me. I inquired quietly because it had not been mentioned on the list that I was aware of, and didnt want to start a useless thread of controversy. However, there seems to be a consensus that any kind of a seal on our wet sleeve engines is not a good thing.
At one time in the long ago past we used Phosphor, Aluminum- bronze. We found it very hard, and also very abrasive. It had a tendency to wear through the chrome surface of the valve stems very quickly. For the past 15 years or so we have been using a Manganese- bronze alloy. We had our guides made by Precision Engine Parts to our specifications. The inlet valve guides are tapered where they hang out into the inlet port & the taper actually goes back up into the head so that the contact point between the head and the guide is a straight line around the guide, which it will not be if the guide has a straight od. (because it comes out of the hole in the head into the port through the wall of the port where it is at an angle to the guide. As the guide gets hot and tries to expand, it only touches the head on one side as it projects into the port. This will cause the guide to bend slightly to one side, away from the long side of the port. The exhaust guides we have made with an internal counter-bore on the bottom end. This is copied from the air cooled VW exhaust valve guides. A certain amount of head carbon will accumulate on the part of the exhaust valve stem that hangs out into the port when the exhaust valve is open. When the valve closes, this carbon is pulled back up into the space at the bottom of the guide and will eventually abrade the valve stem and the end of the guide at this point. The counter-bore in the bottom of the guide makes a space for this carbon to go where it will do no damage! Some of the best guides we have used were a process from Winona-Van Norman, an automotive machine shop machinery and tool supplies. They called it "Bronze-wall" It was a bronze alloy "heli-coil" like insert that was installed into a standard iron guide, then "broached" into place and honed to size. When the insert wore out, it could be removed and a new one installed without changing the centerline of the guide. They were very durable and held oil in between the coils of the bronze liner. Kas is familiar with this setup and used it at the Comp Dept., I believe. This only issue is finding someone with the equipment to do the installation. There are no machine shops in my area that can do it. i do not know if Winona Van Norman still supplies the tooling and parts. The higher the cam lift, the faster the lift rates, the more important the guide material, clearance and installation becomes. The same of true of having the correct rocker arm geometry. If this is not correct, there is to much side load placed on the top of the valve stem and the guides will not last, no matter what the material they are made of. There is a Web site called www.mid-lift.com that talks about geometry in great detail. We have found that with the top of the valves at the stock hight above the head, the rocker shaft needs to be lowered about .090" from its stock hight to maintain correct geometry when using a cam that gives around .465" actual lift at the valve. As the lift goes up higher, the rocker shaft needs to be lowered, (or lash caps put on the tips of the valve stems to raise the contact point between the rocker arm and the valve tip.
in our engine shop, we avoid bronze guides unless we really have to use them such as with some specially coated titanium valves cast iron guides last for a very long time under very arduous conditions about the only OE manufacturer that I've seen that routinely uses a bronze guide is M Benz, and they are a very special product and only used in some of their engines not all bronze guides are equal. we have encountered numerous grades. all of them are notoriously difficult to size. we have special tooling to deal with them. there is an argument that some types of bronze guides can run lower clearances and therfore improve heat transfer from valve stem to guide. i have never seen any documented or visual proof of this . IMHO, especially with older cars like TR's, they are mostly a marketing fad, with unscrupulous vendors appealing to the 'go faster" set
there has been some mention in the posts over the last week of bronze guide liners. these are still very common with budget priced rebuilds , with the american K-line brand being very common, and especially good for some american V8's where the guides are not removable. whilst we use them occasionally, we use cast iron "sleeves" (called thin wall guides) where possible to repair these types of heads. if we have a metallurgist amongst us, then some real advice on bronze guide composition would be good. my challenge to all the readers of this subject is for someone to demonstrate some real world benefits in old engines, over an above a properly fitted & reamed cast iron guide .
Enquiries Road & Track wrote:
> i think you guys are missing the point. bronze guides were intended to be
> able to run *smaller* clearances for better heat transfer. there
> is absolutely no point in increasing clearances unless you have chromed
> stem valves (very rare nowadays). you want the absolute minimum for best
> heat transfer.
Umm, the issue is not the cold clearance, but the hot running clearance. Since the thermal coefficient of expansion of bronze is, depending upon type, about 2-1/2 to 3 times that of cast iron, their use in cast iron heads requires additional cold clearance, so that the hot running clearance is adequate. When the guide grows with heat, the actual running bore gets smaller, because the mechanical strength of the cast iron is higher and the coefficient of expansion is lower than that of the bronze. This is a matter rarely of concern when using bronze guides in an aluminum head, since the coefficients of expansion are closer to the same (a ratio of perhaps 3 to 4), but with cast iron, a much bigger deal because the ratio is more like 3 to 1. This is why so many racers in this country have experienced valves sticking when using the recommended factory cold clearance (appropriate only for cast-iron guides).
Actually, I don't think I'm missing anything. You should only be concerned with the clearances AT TEMPERATURE. Cast iron has a Coefficient of Thermal Expansion of around 6 micro inches/inch per degree F. Most bronze alloys are around 10, so the guides will grow quite a bit more and cause the bore to shrink more compared to cast iron guides. Now couple this with a change to 300 series stainless steel valves (CTE of 14), and you have a real mess compared to the stock set up. You have to take these differences into consideration when establishing the room temperature clearances using materials other than stock. I use bronze guides because those are readily available for my custom application. I also use the recommended clearance provided by the guide manufacturer for the valve material. Sure, I can make them a little tighter to get better heat transfer, but I'm guessing the gain will be very small for the potential tradeoff in reliability. I can get more performance out of my car by eating salad for a few months (after all, salad does equal speed). To be honest, I've used bronze guides now for over 15 years and couldn't really tell you if the room temperature clearance is larger or smaller compared to cast iron. My educated guess is that it is larger, simply because the materials used have greater CTEs. But feel free to tighten up the room temperature clearance with bronze guides and stainless steel valves. I can almost guarantee you will get them to stick.
One of the FOT shared some calculations with me concerning the shrinking of clearances when hot...
With Bronze guides with 0.004" clearance and Stainless Steel valves with 5/16" stems, when the exhaust valve hits 1000 degrees there will be 0" of clearance.
I have no way to check the math and it's way above my pay grade, but what Michael is describing is exactly the issue in my mind. You want to run a clearance just big enough to avoid sticking the valves.
I can't argue that Bronze is better than Iron for the guides, but a number of have found that if you don't run "enough" clearance, bad things can occur - and they get infinitely worse if you have enough lift that you're running an interference valve train.
The conventional wisdom seemed to be that the iron guides had higher wear or were somehow inferior to Bronze but I'm really unclear where that notion came from.
I had sent Tony the spread sheet some time ago analyzing the clearance between the guide and the valve at various temperatures. This is based on the coefficient of expansion of cast iron (cylinder head, 6.0X10b;b6 In / In / Deg. F.), manganese bronze (guides, 11.8X10b;b6 In / In / Deg. F.) and valves (21-4N stainless steel 9.9X10b;b6 In / In / Deg. F.). I think, based on an internet search, that 21-4N is the common stainless steel used for exhaust valves.
The model is based on the bore in the cylinder head being smaller than the O.D. of the guide at operating temperature due to the differences in their coefficient of expansion, just as others have recently mentioned. This makes the bore or I.D. of the guide smaller at operating temperature. The biggest factor is the coefficient of expansion of the valve being almost double that of the cast iron, which controls the I.D. of the guide. I've since refined the model to incorporate different rates of temperature rise in the head, guide and valve stem. The next thing to do was to get better data on the operating temperatures, particularly in a race environment.
I've been searching the internet trying to get an idea on temperatures of the exhaust valve, guide and cylinder head. So far I've found that the surface of the combustion chamber is in the neighborhood of 266 to 496 degrees F. Then I found that exhaust valves run in the area of 1000 degrees F. The interesting thing was that the point was made that the valve is only "exposed" to the exhaust gases for 1 out of 4 of the cycles, more or less. The point being that there is a lot of time to transfer the heat to the seat (which transfers the most heat) and the guide.
Question: I am aware of the "downside" of not having enough clearance, i.e. a stuck valve, but what is the "downside" of too much clearance? I think we are talking about a clearance of 0.004 versus 0.002 on the diameter of the exhaust valve, cold. That's only a difference of 0.001 on a side, cold! And at operating temperatures the difference will be quite a bit less.
A stuck valve can cost a race weekend and more, what can a "loose" guide cost?
Certain of the stock type of guides that are installed in TR heads will heat and cause sticking of the valve, not so much on the street but mostly in race motors. If it happens at the wrong time you can bend a valve or damage a piston. Been there, done that. Most guides run .001 to .0015 of clearance, but those TR guides often need much more clearance than that so they don't stick. The result is that the valve rocks and the life of the guides and your valve job is short. I switched to a valve guide liner that is made for a Chevy race motor. We run a tight clearance, a valve seal and there is no sticking at high RPM. Find someone that builds Chevy race motors and talk to them about getting this done. Since I switched over I have not had any problems.
Hope this helps.
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