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Old 04-12-2016, 11:43 AM   #1
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Default Larger diameter idler for shaved head

I've shaved my head (NB 1999) 0.080" and I've researched a ton on the timing belt tension issues with doing so. I planned on using a tensioner without the bracket on the idler side but am a little concerned about how it lays against the water pump and possible slippage.

Someone mentioned in another thread that they used a larger diameter Kia idler but didn't know the model. Anyone have any more info on doing a larger diameter idler pulley?
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Old 04-12-2016, 01:30 PM   #2
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Old 04-12-2016, 01:44 PM   #3
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Yeah, I saw that and was originally (and still might) do that but I don't like the way the modified tensioner sits on the water pump. By moving it inward to tension, it's not fully seated on the space on the water pump.
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Old 04-12-2016, 03:18 PM   #4
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Default Larger diameter idler for shaved head

Shaved/decked 0.8 mm and running stock tensioner without noticeable issues.
Edit: 0.08" is more than 2mm, that's kinda extraordinary... And reading more just make me crawl back under my rock.
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Old 04-12-2016, 04:12 PM   #5
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Yeah, it's definitely not for the faint of heart. Lots of clay and scoping to make sure of clearances.
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Old 04-12-2016, 08:41 PM   #6
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I have a seriously decked head and I've never been happy with the belt tension. It has skipped a couple of teeth a few times when the car backfires whilst cranking which is bad as Im only a couple of degrees from interference. Both times the battery was low and the engine stopped on the valves. It was me that mentioned the oversized idler which you can use from a Kia though I'm not sure on the model.

To solve the issue I'm currently building a replacement billet water pump plate to run an electric water pump. I've designed it to move the idler in by 2mm so that I can get a bit more belt tension. It will change the timing by a few degrees so I will need to adjust the cams back to counteact this advance.

I'll have some pics of the plate next week.
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Old 04-13-2016, 12:45 AM   #7
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Thanks for the reply and those are my fears. Did you ever try the modified tensioner in the place of the idler?
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Old 04-13-2016, 12:59 AM   #8
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Have you tried it yet? How loose is the belt?

The issue with my engine is the ridiculous cams which give can fire off an almighty backfire if the engine doesn't start up immediately. The backfire causes the cams to rotate backwards against the direction of the crank and that's where the belt can jump teeth. I now just keep the starter motor engaged whilst the engine is cranking until just after it fires and then I don't get the backfire... If it does the load goes against the starter and the engine stalls stifling the bang.

I think the slightly larger idler is probably fine for anyone with more reasonable cams... You'll just have to go to a few wreckers and find one that will fit and is a bit larger. I have no idea what model it came from.... as far as I can see there is no part number on it, maybe you could find out what model car it is so they can be purchased new.
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Old 04-13-2016, 01:13 AM   #9
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We just got it on about an hour ago with the stock idler. Using the straight-edge deflection method it's just within spec but I don't know if I trust it. Twisting the top we can get it past 45 degrees with moderate effort and if we really crank it can be 90 degrees. So it's ALMOST tight enough but we rev up to 8500+. We have 272/264 cams but we are naturally aspirated. Never had backfires and it turns so easy with our lightened crank and balanced engine. You can EASILY rotate the engine by hand. It's insane.
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Old 04-13-2016, 01:17 AM   #10
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It's probably fine then... you don't want too much belt tension anyway otherwise you'll risk pulling off a cam pulley.

I've never had an issue whilst the engine is running, even with the stock idler. The issues have only every been with the backfiring.
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Old 04-13-2016, 01:24 AM   #11
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Thanks for the input. It's so hard to find others with as shaved as we are so it's invaluable.
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Old 04-13-2016, 01:30 AM   #12
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Yeah I had no idea it was an issue until the first time I cranked the engine. The tune was bad so it was spluttering and backfiring everywhere. I was fortunate that the battery was low when the teeth skipped and I felt and heard the engine stop against the valves. It was only then that I realised the belt had been skipping and I was very lucky to have not bent anything.

I actually don't know how far my head is shaved. I'll measure it next time the engine is pulled down. I estimate it to be 2mm but it might be more, it's hard to measure when it's on the block.
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