When to replace a timing chain?

"Is a chain or a belt?"

Chains, normally, do not require replacement.

lynlynch, I'm sorry, but you are wrong. They DO last longer than belts, but most manufacturers usually recommend replacement around 90-100K miles. The chain stretches and the sprockets, which often have a stamped steel hub and are overmolded with nylon teeth, begin to lose teeth allowing the chain to skip a tooth.

Generally a belt should be replaced at about 60K and a chain around 90-100k.

OK, I'm wrong. Never had to replace a chain, though.

yes. this is the Miata FAQ site.

Hi,

bobc is one of our more knowledgeable auto guys will be along shortly.

The manufacturer recommends changing them, don't know about Mazdas interval.

If it breaks you may damage the valves depending on the design of the engine.

It will die instantly if it breaks. The pistons keep moving and the valves have stopped, some in the Open position which may then be hit by the pistons when they get to the top.

It's in the 2/5 hundred USD range, I think. Never had one changed. But I would, if I were you. A head repair will be more costly.

There's also a good chance that the Mazda may have a "safe" engine. One that has valves that cannot contact the pistons. Then your car would just die.

Change it. It might die at night on a lonely road.

The belt is less than $30 USD, and a nine year old with good instruction can do the job fine...Get a manual for your car, ascertain the tools required and wait for bobc to assist via CR4...

A quick Google came up with this ...

http://www.freeengineinfo.com/mazda-timing-belt-replacement.htm

Regards Woody

Whether it's you, or someone else in there, while your at it - pull the water pump and all the hoses you can see & replace them

Do it Now! ....deserved a GA

"interference engine"

Thanks, that term escaped me.

Lyn

From my experience of watching a fellow tech pay hundreds in valves, the Miatas have interference engines.

If you are observant and technical you could attempt it yourself with the risk of bending most of the valves in the engine head. Otherwise, send it to a shop.

The job mainly requires disassembly and assembly and lining up the exact tooth count of the existing belt to a new belt without moving the pulleys as well as following the tightening procedure.

What is meant is that the piston at TDC will interfere with the valves when fully open or at their lowest position inside the cylinder.

When the piston as moved off its TDC position there is no harm in having the valve below the piston crown when at TDC. The intake valve starts to open before TDC and is fully open only when the piston is already on its way down. The exhaust valve opens when the piston is far away but closes fully only after the piston has past its top position. This is called valve overlap and is needed to exhaust more of the burned gases during exhaust and intake.

Most modern high speed car engine are "interference" engines due to high compression ratios used. Most often the piston actually has a valve shaped depressions in its crown to allow a wider valve opening when the piston is near TDC.

If the belt brakes, the valves can be fully open where the piston will hit them when fully on top. This results in an expensive repair job.

Thanks. I kinda knew what the concept, but had forgotten the term.

Thanks DCaD Will defer to the Mazda dealer.

According to Alldata, the 1.8 DOHC Mazda engine is not an interference engine, and the recommended mileage intreval for relacing the timing belt is 60,000 miles.

I was looking in Alldata as well and could not find where it states that it is not an interference engine. Where did you find that information at?

However I did find "CAUTION: Incorrect removal or installation of the timing belt can result in damage to internal engine components."

I found that information in the Technician's Reference section under timing belt intrevals.

There is a rule of thumb, (and there are a number of exceptions! The likelihood of your vehicle being one of them seems to be directly proportional to how much it will cost to fix a valve crash for your vehicle.) but most of the time, when an engine is designed with a belt instead of a chain, most of the time it is NOT an interference design because designers know that belts have a tendency to break, whereas chains will usually stretch and maybe skip teeth, but they USUALLY don't break.

Don't just replace the chain/belt. replace the sprockets/cogs as well. and replace the water pump/hoses and the front main seal too while you have everything disassembled. Be careful removing the radiator in this vehicle, I have been told that the plastic tanks on this vehicle tend to be quite fragile, especially as they age.

Some good information here:

http://grassrootsmotorsports.com/forum/grm/guidance-on-first-timing-belt-change/11292/page1/

I broke a timing belt on my Eclipse and it destroyed the motor, so I recommend that you get it fixed.

The advice at the link recommends replacing a lot of stuff while you are in there, like the water pump and crank seal. Replace everything that you can afford, so you don't have to do the job again for another 90,000 miles. Be sure you get an estimate, though. Some dealers will charge you the shop rate for each part being replaced, even though 90% of the work is the same--that is, they have to do essentially the same procedures to replace the water pump as the timing belt, and they will try to charge you the shop rate for both.

MItsubishi dealers are notorious for this double and triple charging (it's why we call them "stealers"), so just make sure your mechanic/shop/dealer gives you an estimate.

As some have already pointed out, on some engines there isn't enough room between the open valve head and the piston crown at top-dead-center and if the valves stop closing while the engine is running. You'll do extensive damage. If the car is old enough, the cost of repairs can exceed the worth of the car.

Twenty years ago, my ex-spouse ignored the advice of her mechanic and when the belt broke on her Honda Prelude she was stranded 125 miles from home. Worse yet the $500 repair was done by an inept mechanic. The car was barely drivable but she got it home after which she had to spend an additional $1100 (one valve head broke off pounding the piston to oblivion and severely damaging the combustion chamber. Clearly that engine was a zero tolerance engine.

My Subaru however, suffered no consequential damage when one of the timing belts broke. It was easily replaced. The suggestion to replace the idler pulley(s) has a lot of merit. If the ball or roller bearings are worn and there is play, not only is timing affected but a failed pulley that has frozen can fail the new belt too.

Some belts will warn you when they are about to fail. A tear allows a small piece of the belt to fling outwards under centrifugal force. Each time that portion of the belt wraps around the top of the engine, it slaps the valve cover and makes a sound reminiscent of a sticking valve lifter.

Please don't depend on hearing that sound as a precursor to failure. Better to do as suggested: replace it.

L.J.

I personally would not wait for 90,000 miles. There is usually a "factor of safety" in manufacturer recommended maintenance intervals. On the other hand, when it breaks, you will have bent internal engine parts to deal with. I did not say if it breaks. I said when it breaks. When you do the work, or have the work done, replace the belt, idler and water pump. It will save a lot of labor cost to have it all done at one time. Some shops will try to charge you "flat rate" for each part. That is not fair. Pay the flat rate for the timing belt work, and agree on an hourly rate for the rest. Even if you pay for the mechanic's time when he has to pick up the wrench he just dropped, you will save money, and probably get a more carefully done job. The mechanic will not be in such a hurry trying to beat the flat rate book time.

Daily Visitor Number 1,999,999

Don't ask...Just get it done. The side of the road is the alternative. At 10K a year average it sounds like it hasn't been driven much. If you have tools you probably can do it yourself. If you don't have the tools forget it and call around for quotes....It isn't rocket science and would be a good "learner project". Patience and a good manual and TOOLS........The hillclimber

Yes, as 'Slow old Poop' said, they can destroy your engine. My pops had an older miata and the timing chain snapped. I forget the exact damage it did, but a new engine was required. He later bought a new Miata, and we changed it out ourselves. It was a whole day task (we're both electrical engineers ;) ), but we got it done and things are now well.

Per a little investigation a Miata has a timing belt which requires maintenance replacement. If it breaks before you replace it there will be considerable more money involved in repairs, head and most likely pistons.

I had an '89 and had the belt done early milage wise owing to the age of the car. It is not an easy job I was told and it comes with considerabe risk of a broken crankshaft if not correctly torqued. Plus while you are at it the advice was to do the water pump.

I agree with all those who said get it done soonest and if you have it done by a proper place and they get it wrong then you have a comeback. Do it yourself and you are up for the further expenses.

I consider myself to be a reasonable mechanic for many jobs, but I would only tackle a timing belt if I were guaranteed the time it takes to do it, with no "Bill!! Sarah needs to be taken to school!!" interruptions, time to go to the auto parts store... etc. I usually let the professionals do this one. That way, if there is a screw-up, it is someone elses nickel to fix it. I usually avoid going to the dealer if I can avoid it though. They typically charge an arm and a leg, but any competent mechanic could do it.

Bill

You may have discovered one of the well kept secrets of successful auto repairs.

Do not walk away from something until it it finished. As an example, if there are six bolts holding a bracket on, don't walk away from the job until all six bolts are tight. Another thing. If you start a bolt in the threaded part, don't walk away from the job until that bolt is tight.

When you are working on something at home the distractions never seem to stop, but that is how I have learned to not have the "comeback" problems. It is just a mental exercise.