(NA8) Timing belt gear interference fit???

[theo]

TLDR
Is the timing belt gear supposed to be an interference fit with the crank nose? Or is is supposed to just slide on there?

The long version...

Doing some work on my MX5. Had a bit of an oil leak, so decided I would change the front crank seal while doing the timing belt.

Had a hell of a time getting the original timing belt pulley off.

Had watched a couple of videos online covering the process, and both times, the gear just slides off the crank. No such luck with mine.

Got the key out with a pair of vice grips, but the gear stayed stuck in place. Some gentle tapping on the gear. Some slightly-less-gentle tapping on the gear. Penetrating oil. Cautious levering behind the gear (only to chip the edges). Blow torch and more gentle tapping. No movement...

Ended up drilling and tapping two holes in the gear and getting it off with a puller. Even then, it complained the whole way.

Once it was off, I was expecting to see some sort of corrosion, or adhesive residue - but nothing. The inside of the gear and the crank surface (and keyway) are perfect. It was just REALLY tight.

So ordered a new gear, tried to fit it - same problem but in reverse. The inner diameter of the gear is smaller than the outer diameter of the crankshaft nose. Measured as accurately as I can (vernier calipers/micrometer) and the difference is 0.12mm.

Checked the demensions between the two parts - and they're identical. I'm guessing whoever put the gear on last time used some kind of press, or a BFH.

Anyone have any experience with this? What is the standard procedure for installing the timing belt gear? Is it supposed to get slammed on there? Or should it just slide on?


I have the new gear sitting at a machine shop with the measurements, but the operator wanted me to confirm this before going ahead with the job.

[theo]

OK - have answered my own question.

Paid a visit the Service Dept at the local Mazda dealership, has a chat to the Foreman. He said the timing belt gear should just slide on there - NOT an interference fit. He said it should spin freely on the crankshaft without the key in the keyway.

His suggestion for my problem was to linish the nose of the crank. I'm thinking I'll just machine the gear instead. Much cheaper/easier than removing the crankshaft...

[Mr Morlock]

I cannot help but think you need a little more informed advice. You should not have to be machining anything if the mating / matching parts are to spec.

[theo]

I cannot help but think you need a little more informed advice. You should not have to be machining anything if the mating / matching parts are to spec.

I agree, but doesn't seem to be the case on my car.

Looked back through the service history just now, it appears the crank has been out for a repair to the keyway at 140K kms. That would probably explain why its out of spec.

[greenMachine]

So, how did the pulley come off? Did it need a puller? BFH?

Machining may be too much, try a bit of wet and dry on the bore of the pulley, but whatever you do, do it in small steps.

[theo]

So, how did the pulley come off? Did it need a puller? BFH?

Machining may be too much, try a bit of wet and dry on the bore of the pulley, but whatever you do, do it in small steps.

Drilled and tapped two holes and used a puller.

Actually hit it with some wet and dry already. Just to make sure there wasn't a higher lip making it difficult to start.

0.12mm doesn't sound like much, but it'd be very slow going removing it with wet and dry.

[StanTheMan]

with interference fits....the H1 or H2 is the size of the hammer used to assemble. The bigger the number behind the H (for Hammer) the heavier the Hammer needs to be.....

[noobee]

anything more than a very light tap not recommended,theres only half a thrust bearing in these motors.they don't look like they are made for bashing
Mick

[StanTheMan]

sarcasm meter........may need repair or calibrating....