Piston Slap: Why Replace Parts In Pairs?

piston slap why replace parts in pairs

David writes:

Over the last few years, I’ve had work done on my ’99 Ford F-150 at various places near my work. It seems that when a wear item goes (like ball joints), the mechanic wants to replace absolutely everything in the system — tie rods, pitman arm, trailing arm, etc. Or when the left side brake caliper goes bad, they want to replace the right one, too. Or give me all new hoses when I replace my radiator.

The reason the mechanic gives is always, “Well they are the same age, and if the left one is bad, the right one is not far behind.”

This gets really expensive really quick. Is this worth it? Why do mechanics always want to replace everything in the system, if only one part is bad? Is this strategy only to boost profit? Or is there some truth in their reasoning?

Sajeev answers:

Do you think that if one 17-year-old moving part goes bad, the other 17-year-old bit is not going to fail in the next few months? Or, being generous, the next year?

You didn’t mention a problem with tires being replaced in pairs. So apply the same logic to any other chassis bit (i.e. not seats, floormats, etc) and you’ll be set. A new tire can be shaved to match the tread of the other three, but a new shock/spring/brake item could upset the balance created by a matched set of worn items. It’s much like replacing only one headlight bulb only to realize the other is so dim you regret not replacing both at the same time.

Replacing parts for one side only works if you drive on two wheels and never let it drop back on all fours without cushioning the fall.

So what if a mechanic recommended replacing late-model, low mileage suspension bits in pairs? That might raise a red flag, unless a big pothole did a suspension component in.

Finally, with the radiator hoses in mind, why not get it all done while the mechanic is in there? You are saving labor (easier to reach with the radiator gone, you only add/burp coolant once) and rubber parts are notorious for weathering after 10+ years on the road. Such is the life of owning an older vehicle, my friend.

[Image: Shutterstock user FabrikaSimf]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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6 of 59 comments
  • DougD DougD on Apr 26, 2016

    Although I do most of my repair work myself these days, I've found that if you replace as much as possible while you're in there it saves hassle in the long run. When we got a new clutch put in the car I had them do the water pump, should have also had them do the rad hoses and alternator. We had a mechanic for a while who tried to save money by only replacing the part that was worn out. It probably saved a bit of money but the car had to go to the shop twice as often, which got irritating. I'd rather spend a bit more money for a lot less hassle.

    • See 1 previous
    • DougD DougD on Apr 28, 2016

      @turf3 Good point, actually yes I had them do the timing belt at the same time as well. These were jobs that were much easier with the engine out, and since it had over 100,000 miles on it I figured it would pay in the long run. The alternator was just hindsight and specific to my car (MK1 Ford Focus). It crapped out the next year and it was a miserable job to get it out from where it sits behind the engine.

  • Jacob_coulter Jacob_coulter on Apr 26, 2016

    Usually it makes sense to replace in pairs, also take in mind that when you replace many suspension parts it requires an alignment after everything is installed, so not having to do that twice is a money saver. I will say though some mechanics can over shoot on the "might as well" type repairs in order to simply drum up business, like trying to sell both front and rear brake jobs at the same time. That's a really common one, when the reality is for most cars, rear pads last twice as long as front ones.

    • See 1 previous
    • Jacob_coulter Jacob_coulter on Apr 26, 2016

      @bumpy ii It's still true for well over 90% of cars on the road, if a mechanic is trying to do all 4 at the same time, more times than not it's an upsell.

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