I'm wrapping up my first motor rebuild right now and just after I got the timing belt on, the amount of peak torque it takes to rotate my crank 1 full rotation ( measured by turning crank with my torque wrench ) jumped from 8# to around ~30#. This was done while timing belt adjuster pulley was loose, no spark plugs and camshaft lubed with new motor oil so I didn't expect the jump to be that high. I'm kind of worried something is wrong here.

From this thread ( https://www.miataturbo.net/engine-pe...ng-info-70450/ ) I see that the OEM valve spring is 40-43# and ST double advertise 74# but measured to 90#. That's obviously a huge difference and would cause a lot more resitance .. but still 30# seems like way more than my expectation ( based on my extensive experience with building zero motors previously ).


Could anyone with a ST double motor help me rotate their motor through one rotation with a torque wrench and LMK what numbers they have ?

Thnx ahead of time.

 

Sounds about right.

 

Tore everything back down tonight and have it a bit figured out so just dropping this here in case others run into this issue.

The cause was that the SD double valve spring is too strong and overpowered the adjuster tension spring. This meant that the tension wasn't consistent on the entire belt during rotation. For example, at one point, the belt tension between the crank and exhaust cam was taunt, but belt between the intake and exhaust cam was a loose because the adjuster gave too much slack and allowed the intake cam to 'snap' forward too much. To put it another way, different section of the belt was pogo'ing back / forth on rotation causing the higher peak torque required.

I manually set the adjuster's location to put the same tension throughout the belt and torqued it before measuring again. Peak torque now comes in at 22.x# which is closer to my expectation but more importantly the feel during rotation is much smoother.

 

Isn't that why there is a specific way to tighten the belt, because of the very problem you described?. The old 1-5/8 turn method.

Glad you got it sorted though. A loose TB is a bad thing.

 

There is. From what I have seen, the tensioner pulley is supposed to be just barely finger tight. Then you rotate 2x for the pulley to self adjust into position before final torque. It worked for me the few times I did it before so I had blamed the valve spring before looking everything over closer.

 

PSA - I like to fully remove that idler tensioner spring once everything is tight and in place. Just one experience of this thing popping off during a race and dropping down between the crank and belt (causing it to jump timing mid-race!) has been enough justification for me to pull them out now on every job I do.

 

Hmmm .. make sense since that spring does nothing after the pulley has in torqued. Thanks for the tip!