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warning about rocker spacers

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trfourtune

Jedi Knight
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Hi all,
i am here to warn you all about rocker spacers. I have noticed that there are now delrin rocker spacers out there as well as steel and aluminum.
here is the warning: each of these spacers expand with temperature changes at different rates. Why is this a concern you ask? here's the beef, if you set your clearance gap with the steel ones at 10 thou cold,then run you motor, they heat up and expand and close this clearance to a lesser amount, lets say to 5 thou hot.Now here is the catch, if you set your aluminum spacers at the same 10 thou clearance, the hot clearance will be 0 thou. Aluminum has roughly double the expansion rate with temperature than steel. delrin is even more!
the coefficient numbers i have found are:
carbon steel-.00000633
aluminum-.00001244
delrin-.000045 to .000052
So the expansion is:
(inches/(inch of material length)) times the CHANGE in temperature degrees F

Delrin has 7 times the thermal expansion rate of steel!
IMHO this is a BAD choice of material for rocker spacers!

So be careful with the end gaps.
I have steel ones for my tr4 since the expansion rate is the lowest so there is less variation in the clearance required between hot and cold.

you've been warned.
Rob
 
Makes sense Rob, but wouldn't the Delrin continue working well with a smaller gap than the steel or alloy ones? Greater lubricity should mean it can push against the rockers pretty hard without actually keeping them from moving.

I'm also puzzled why the steel ones close up ... if the oil is that much hotter than the rocker shaft, perhaps it is hotter than it should be. IOW there shouldn't be much differential expansion between a steel shaft and a steel spacer, as I would think they stay reasonably close to the same temperature.
 
trfourtune said:
Hi all,
i am here to warn you all about rocker spacers. I have noticed that there are now delrin rocker spacers out there as well as steel and aluminum.
here is the warning: each of these spacers expand with temperature changes at different rates. Why is this a concern you ask? here's the beef, if you set your clearance gap with the steel ones at 10 thou cold,then run you motor, they heat up and expand and close this clearance to a lesser amount, lets say to 5 thou hot.Now here is the catch, if you set your aluminum spacers at the same 10 thou clearance, the hot clearance will be 0 thou. Aluminum has roughly double the expansion rate with temperature than steel. delrin is even more!
the coefficient numbers i have found are:
carbon steel-.00000633
aluminum-.00001244
delrin-.000045 to .000052
So the expansion is:
(inches/(inch of material length)) times the CHANGE in temperature degrees F
Delrin has 7 times the thermal expansion rate of steel!
IMHO this is a BAD choice of material for rocker spacers!
you've been warned.
Rob
Click to expand...
Hi Rob, I love talking in Millionth's of an inch, I used to make military flight hardware for Moog Inc.
I would agree Delrin is not a great choice for a long life rocker spacer. Racing maybe?, with constant checking/replacing.
Let's say it's 50 outside and the engine valve train gets to 250-275 degree's.
Not only does Delrin expand (.011") but it also looses it's strength and becomes softer. Not good for a locator!

I would use the steel ones like you said.
Weight is not an issue here, because the spacers are so close to the center of rotation.
No rotating mass!
They are stronger than aluminum, can be made thinner and they will only expand a thousand and a half! The same as the rest of the valve train.
Perfect!
My 2 cent's.
Lyle
 
You don't want the spacers "pushing" against the rockers at all(that is called binding)binding creates heat that gets out of control which causes problems. you just want to keep the rockers in position without binding. Because the delrin expands soo much, you should install them with really really large cold gaps.
the steel ones close up as do all because the mounts are relatively fixed positions and the hot oil flows through the shaft and transmitts the heat to the spacers.
it's the linear expansion between the mounts of the hot parts that closes the gaps.
Rob
 
trfourtune said:
because the mounts are relatively fixed positions
Click to expand...
Ah yes. I was thinking the mounts would deflect with the bar expansion, but of course it's only located to the mounts at one place and can move at the others. Thanks for clearing that up!
 
I put the delrin spacers in mine, thinking they would be better than the original springs. Am I hearing that I did a bad thing? Should I change them out? I hate redoos!

Tinkerman
 
Tinker,
no you don't have to, but you need to know what end gaps you installed them at and check this against the expansion rate to figure out what the end gap is when hot, if the suppier did not supply a specific end gap for the delrin. If the end gap supplied is the same as what steel ones require, i would do some calculations. say you installed them and set the end gap at 70 degrees F on your cold motor and the oil temperature at the top of your motor is 300 degrees F (max possible on a hot day running hard) then you have a 230 degree temp difference.do the calculation based on the length of the spacer,rate,and temp, and you now can figure out the hot clearance. I would want .001 clearance minimum at the hottest conditions possible.
rob
 
One more point, your rockers also expand,so you should calculate those as well for the totals between the towers. Use the appropriate rates, steel for steel rockers, etc.
Rob
 
Come to think of it, the head itself expands as well, spreading the mounts farther apart. Normally it will average slightly hotter than the coolant, so the temperature difference to the shaft & spacers should be quite minimal.

If the oil in the pan is getting more than 30-40F above the coolant, you've got worse problems than friction in the rocker spacers, IMO.
 
the point to remember is that yes, the space between the shaft mount expand with the head and block, but when you introduce parts that expand at 7 times the other parts, you can run into trouble.
the oil reaching the rockers is the end of the line when it comes to oil path and the oil supply to the top is hotter than the oil supply to the mains and cam, it gets heated up along the way (not much though).The head typically is the hottest part of the engine (near the exhaust ports and valves). The heat gets transmitted to everything in its vicinity.
Just trying to make people aware. hopfully save some failures or problems. Take it or leave it.
Rob
 
The best thing to do when designing tight tolerance parts that see a 250 degree temperature difference is to make them from the same material.
Everything expands at the same rate and the clearance won't change more than a couple thousands of an inch.

Aluminum is slightly better based on strength/ weight, but expands and wears faster than steel, unless you hardcoat Anodize it.
A couple grams of weight is not improtant on these parts.

Why would you ever use plastic for a rocker shaft spacer?
*Weight, no advantage.
*Cost, yes, plastic is cheap to machine, but has major problems for an internal engine part.
How does long term exposure to oil, gasoline and heat cycles effect the material?!
I know for a fact that Delrin will harden and become brittle over time.
*HP, no gain unless something is binding or clearanced wrong.
I see no advantage over thin wall, medium carbon steel tube spacer. It's light, cheap to make and will last the life of our TR3's.

I personally would never use Delrin as a rocker spacer, and I machine it all the time.
My 3 cents worth.
Lyle
 
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