E: Engine Half Time Pinion Sizes

LoneStar

Well Known and Active Forum User
VOC Member
Hello all,

My current ET49 half time pinion, combined with new cam gears, has backlash in its engagement with the idler gear. I know these pinions are available in various sizes to deal with this situation - but I'm not clear on what exactly "+.004" means, for instance.

Is there any way to determine which size is likely to be needed, based on some measure of the existing backlash?

Thanks,

Dave
 

Robert Watson

Well Known and Active Forum User
VOC Member
+.004 means that the gear is .004 bigger than standard for the gear. You measures gears by fitting specially ground pins between the teeth and measuring across the gear to give a dimension. If you do this then you can look up what standard is and then pick a slightly bigger one to try.

it means either a wait while suppliers ship gears back and forth, or finding someone in your neighbourhood who has a bucket full of them in stock!
 

LoneStar

Well Known and Active Forum User
VOC Member
Robert,

Thanks. My current, too-small, pinion is stamped +4. The requirement is to eliminate backlash, and the challenge would be to translate a diameter increase into backlash reduction. Someone much better at geometry than me could probably do it, and determine the gear oversize that would fit perfectly.

As it stands, Coventry Spares has three oversizes beyond .004, and a friendly return policy - so I expect I'll order all three and send back two.
 

BigEd

Well Known and Active Forum User
VOC Member
VOC Forum Moderator
Robert,

Thanks. My current, too-small, pinion is stamped +4. ............ Someone much better at geometry than me could probably do it, and determine the gear oversize that would fit perfectly. ........

As it stands, Coventry Spares has three oversizes beyond .004, and a friendly return policy - so I expect I'll order all three and send back two.
Take advantage of Coventry Spares return policy and save yourself a lot of time. If you think you might like to try measuring then there are a series of articles in WhitakerPedia (link below) written by Neville Higgins, "The Prof" and then think on. :)
Measuring timing pinions
 

Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
Our Ki Wi friends down under have a spares system where they keep a full set of HT pinions and send the set, then the owner will select the correct one he needs and send the rest back with spares holder ordering a replacement. I'm sure we could do this at the VOCSC.
 

John Cone

Well Known and Active Forum User
VOC Member
When i rebuilt my engine, Maughans sent me a box with all the various size half pinions in. You select the one you need and send them back and pay for it.
 

Cyborg

Well Known and Active Forum User
VOC Member
Having a selection of gears to choose from definitely makes life way easier. The plus .004 may no longer be plus .004 due to wear. It’s possible it never was plus .004 depending on who made it. When setting backlash, watch for cam pinions, large idler etc. being out of round or bushings that are not quite centralized.
You might find using thread wire gauges to measure the gears is a bit awkward if you’re not used to doing it. I haven’t spent a lot of time measuring them so when there is a discrepancy between the stamped size and the micrometer, I still wonder if it’s the gear or me.
You mention new cam gears, so hopefully they are round. If you pressed them on the cams yourself, then best to make sure the outboard cam bushings have sufficient clearance on the spindles.
 
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LoneStar

Well Known and Active Forum User
VOC Member
Thanks everyone. It seems there's quite a lot of information out there, and that measuring these gears isn't particularly simple (although the spindle-distance approach provided by VOCS is workable). Given the ease of ordering several sizes and keeping the best fit, that seems the best approach.

My cam gears are from Maughan, installed by Megacycle who also sized the bushes to new spindles. The idler is steel, some years old but likely from Maughan. So, reason to hope for a trouble-free setup.

I do have to clean up the mainshaft threads, as they're distorted due to the unfortunate practice of punch-locking the nut.
 
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