Alum vs steel pushrod thermal expansion final answer?

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harleyhappy
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Alum vs steel pushrod thermal expansion final answer?

#1

Post by harleyhappy » Tue Jul 10, 2018 12:14 pm

I spent hours last night reading every thread on the board about pushrods, but could not find a final conclusion about the difference between aluminium and steel pushrods (colony quality aside)

If aluminium alloy expands more than steel, and we're all running cast iron barrels with aluminium heads, then why do alloy pushrods need to be adjusted tighter than steel ones? I have found no answer on any thread, only that steel expansion is "better" "closer" etc to the rate of expansion of the rest of the engine, which does not sound very scientific (even if it is true) surely it should be the opposite (steel adjusted tighter than alloy due to steels lower rate of expansion)

I have a shovel with alloy pushrods and solid lifters, with the adjuster in the lifter, and my top end clatters, with a noticeable improvement if I keep the pushrods adjusted to be stiff to rotate at cold. I'm debating buying the S&S steel pushrods (still solid) in the hope of quieting down my top end, but would like to understand whats actually happening in my engine before I spend all that cash.

So can anyone explain whats really going on? I'm not doubting the group experience, which seems to be that steel are better, but why? I've listed some of my guesses/questions below.

Does the wall thickness/alloy mix of the push rod tubes mean they actually expand less than the steel?
Are the alloy rods flexing? (seems unlikely they'd flex enough to be audibly shorter)
Are the alloy rods losing heat faster, and thus running cooler (and shorter) than the steel ones?
Are the steel ones actually sloppier at running temp than the alloy ones, but somehow quieter anyway?
Has anyone measured valve lash between the rocker arm and valve stem hot and cold with alloy and then steel rods? (I cant accurately check this due to shovel rocker box design, but I'm guessing one of you guys might have whipped the front pan cover off whilst hot?)

Sorry, for the long post, hope I haven't implied disrespect to anybodys opinions, I'm just genuinely interested in whats happening in our engines (even if mine is a dirty shovelhead)



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Re: Alum vs steel pushrod thermal expansion final answer?

#2

Post by nmaineron » Tue Jul 10, 2018 12:55 pm

I have gone through this and have come to the conclusion that the steel grows closer to what the cylinders do than the aluminum does and remain tighter and quieter than the aluminum. JMHO :)

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Re: Alum vs steel pushrod thermal expansion final answer?

#3

Post by RooDog » Tue Jul 10, 2018 1:35 pm

What cam are you running? The Harley FLH cam, for instance, was designed to run with hydraulic lifters only and will never be quie with solids. On the other hand, some of the aftermarket cams have more sophisticate cam ramp designs with the idea of taking up the slop of solids more gently and thus tend to run quieter than any stock H-D cam can. Andrews claims that most of their street cams are designed to run either hydraulic or solid. Personally I would not over think the issue of heat expansion rates and run hydraulics if noise were my only concern, but on my hot rodded '68 Shovelhead, the stock hydros would pump-up at high RPM and cause an engine miss until they bled back down, so I trashed the OEM hydros and went back to the trouble free solids. Cost is always an issue, so "you pays yer money, you takes yer choices"....... RooDog

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Re: Alum vs steel pushrod thermal expansion final answer?

#4

Post by Panacea » Tue Jul 10, 2018 4:18 pm

Before you pull the colony rods, check the lash after the motor is hot, can you jiggle the rod? perhaps a tad tighter would quiet things down,,,

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Re: Alum vs steel pushrod thermal expansion final answer?

#5

Post by Doc37W » Tue Jul 10, 2018 6:05 pm

I'm with RooDog on this. Steel push rods seem to run quieter than aluminium push rods, partly because the heat transfered to the pushrods is not as great as the heat generated in the cylinders. Even though the heads are aluminium, they are finned better to lose the heat generated in the cumbustion chamber faster, so the steel push rods, once heated to max expansion, stay at a more consistant length than aluminium pushrods, which will cool faster, losing max length sooner. What he said about OE cams is also true, if you don't know what cam you have in the engine, assume it's stock. Save up, & buy a cam more suited for solids and your style of riding. One other thing to look at is the adjusters AND nuts themselves. One or more may have been over-tightened, where the threads on them are starting to weaken and backing off. Look for rounded corners on the nuts (overtightening and/or slopply fitting wrenches) and rolled/shiney threads on the adjusters. You may have to replace some or all to insure they stay locked into place. Doc

harleyhappy
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Re: Alum vs steel pushrod thermal expansion final answer?

#6

Post by harleyhappy » Tue Jul 10, 2018 7:01 pm

Thanks for all of the replys so far

I have completely rebuilt the engine form the crankpin out, so there are no surprises in there, the cam is a new Andrews J grind, the lifter adjusters, their threads and nuts are all in good condition. The adjustment is not slackening off, its just noisier than I think is reasonable (though as I live in the UK, I've never heard another shovel to compare it to) its noisier than my Ironhead. The pushrods are fine, no idea if they are Colony or not. I'm only considering changing them if the steel rods will make it quieter, and adjusting the alloy rods so they are tight as you can get away with when cold does not fill me with joy.
I can't get the adjustment any tighter without affecting cold starting. I do understand that my alloy rods are probably fine, and that steel ones would be a slight improvement, I just wanted to understand why that might be.

Surely the way/rate the head and barrel expands is near irrelevant, as its not effected by the pushrod material, and is a near constant (eg not a variable) as the weight of experience on here (all users with near identical [at least in this context] engines) is that steel rods are quieter, even with an initial adjustment that is slacker than alloy. So either the steel rods must be lengthening more than the alloy ones, or they are running looser and are only coincidentally quieter.

I appreciate your input Doc, but I get on my bike and ride till the tanks empty, and theres no traffic for a very long way around me, so the temperature of the motor does not fluctuate much. I've stopped every mile or so away from home and it takes about 10 miles for it to start clattering (a bit, its not awful) and then stays the same for the rest of the ride. So I cant imagine the rod temperature having an initial peak.

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Re: Alum vs steel pushrod thermal expansion final answer?

#7

Post by RUBONE » Wed Jul 11, 2018 12:39 am

Many times top end noise has zero to do with pushrods, and lots to do with rocker shaft endplay. Every time the valve is lifted the rocker arm will slide to the left as the pushrod pushes up, and back to the right as it goes down. That will create a click in the top end if it is more than a few thousandths. Too often I see folks chasing a gremlin without seeing all their holes...

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Re: Alum vs steel pushrod thermal expansion final answer?

#8

Post by RooDog » Wed Jul 11, 2018 3:15 pm

You have never had another shovel to compare it to? Then I take it this current concern is with a Shovelhead? This is a Panhead discussion group, but no biggie.
Having check my official Harley-Davidson Shovelhead service manual it calls for Rocker Arm Clearance spec of .004" to .025", and that seems to be quite a wide variance, so it must not be really critical, at least to 1978 standards. I have used both shim washers and the dreaded wave/spring washer with no noticeable effect on noise.....
I once pre-delivery prepped a new Iron Head Sporty fresh off the truck and out of the H-D packing crate in 1977. I thought this was going to be a sweet ride before anybody else got a chance to F it up. Boy, was I mistaken! That bike sounded more like a piece of neglected farm equipment rather than an example of Milwaukee's finest. But then, I have never much cared for Sportsters anyway......RooDog

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Re: Alum vs steel pushrod thermal expansion final answer?

#9

Post by Doc37W » Mon Jul 16, 2018 7:09 pm

Rubone is right about rockers clicking. The rocker boxes are bored at a 1 degree slope to the left side, allowing the rocker to slide (and click with solid lifters) against the rocker spacer (hydrualics maintain pressure to keep them there, centering the rockers over the valve stem). There are different size shims to take up the space (should be .004" on all for quiet running). This can be done without taking the boxes and heads off the bike. You have to remove the pushrods and reach up into the pushrod opening with rounded rod to feel how much back and forth clearance you have. .004" is very, very little. If it seems excessive, make sure you have a .550" X 10" rod to push the rocker shaft out of the right side of the box, while assuring the spacer and rocker stay in place. Pull off the O-ring seal for ease of inserting the shaft (yes, you should replace them later) Put the shims on the shaft next to the big end and reinstall the shaft (use a plain nut to tighten the shaft, rather than wearing out the flexnut with several tightenings). Check clearance and add or take off shims as needed, ALWAYS using the rod to push out the rocker shaft. If you put in a .004 shim and have no end clearance, take it out and you'll have the proper end clearance. Go on to the next one, doing the same, untill you've done all 4 shafts. Yes, it will take a couple of hours to do, but when you're done, you will eliminated one more point of noise in your engine and possibly the source of the clicking. Doc

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