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On Interference Engines


So… ugh, timing belt broke. It’s my sister’s KIA Rio. The timing belt went. Here’s my question.

The local (trusted) shop said that figuring out whether or not the valves and pistons had a knock-down, drag-out, fist-fight would cost $400. That’s just to assess whether or not it’s trashed. I would guess it is.

How involved would I have to get in order to figure this out for myself and spare my sister the $400?

If you have to ask…

Look here:

I simply cannot comprehend why a shop has to replace a timing belt or amass a large bill to determine if there’s engine damage or not.

Remove the spark plugs, bring a few cylinders up one at a time to TDC of the compression stroke, and apply a little compressed air to each cylinder. If it hisses out the intake manifold the intake valves are bent. (exhaust valves seldom bend)

Another option (more involved) is to remove a valve cover and check for excessive valve lash (clearance) on the intake valves.

Reading about shops having to resort to something like this to determine whether damage exists is an embarassment to the mechanic profession in my opinion.

I could give the shop a bit more of a break, the reason why is those engines that are listed as “non-interference” but still have some slight contact when things are just right (or more correctly wrong)

The Gates site shows the engine is a piston smasher; assuming the listing is correct. However, I do agree that it’s possible for a free-wheeler to make contact in certain situations.

My point was that a number of people have posted on this site about a shop charging, or wanting to charge, xxx dollars to replace a timing belt to find out if there’s engine damage or not. To me, this is totally unnecessary and absolutely ludicrous when the answer can easily be determined without replacing anything or running up a bill.

That test isn’t going to help any since the timing belt is still broken.
If you turn the crankshaft, the camshafts are going to stay right where they are, unless you open up the engine, and manually turn the camshafts at the same time as you turn the crankshaft.

If you just start spinning the crankshaft with a broken timing belt, you’re going to damage the valves if they aren’t already.


The crankshaft and camshaft(s) must be in time and turned simultaneously or doing a compression test is is virtually impossible…

Maybe I was not clear enough. The camshaft timing mark will have to be lined up of course. With the piston on a particular cylinder up (No. 1 for example) air is applied and the cam rotated as needed to test other cylinders. Usually it doesn’t involve more than testing one cylinder and No. 1 is a good place to start. It’s a valid test and bone simple to do.

“Another option (more involved) is to remove a valve cover and check for excessive valve lash (clearance) on the intake valves.”

I prefer this option since it only requires the tools to remove the cover.
Once I checked out an Accord engine with a belt stripped of its teeth due to oil soaking.
The valve lashes were so grossly bad I didn’t need to use feeler gauges.
Took about 15 minutes to determine the bad news.

I think the test OK4450 recommended is this:

  1. in each cylinder, turn the cam until both valves are closed. I’d probably remove the valve covers to do this and watch the rocker arms.
  2. through the spark plug holes, using a leakdown kit, pump some air in the cylinder and see what happens. If it won’t take air, a valve is bent.

It’s unnecessary to rotate the crank to do this.

The only side note on this test would be to make sure none of the pistons are at TDC before turning the camshaft(s), and trying to modulate the air enough to prevent the piston on the cylinder you are testing from flying down, pushing two (in a four cylinder) of the others up to TDC, possibly causing further damage to valves. The possibility of this happening is remote, but is there. Checking valve lash is another good, simple method of determining damage.

Good points. And it won’t take much air pressure to find out if a stem is bent. A cylinder with a bent valve stem won’t take any pressure at all. Nada. Zip. Zero.

I suppose I wasn’t clear enough about performing this test and I apologize for being hazy.

The fact remains that a shop does not have to replace a belt to determine whether the engine is damaged or not.
There has been more than one post in the past where someone is asking why after spending XXX dollars or whatever the amount may be to replace the goodies under the belt cover they’re now stuck with an engine that is junk or borderline junk.