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The description says they are "for 30 series tire," which would appear to be the later model Hot Laps tires, as opposed to the 870c-type tires, which are something like part number TR-8? Is the description inaccurate?

I copied the stock chassis exactly, front belt is a zc-651, front drive pulley is a zc-630, both diff pulleys were unbagged but looked identical to original.....why is my front belt so tight? Without a tensioner it's very snug, in fact I can't get the tensioner in place it's so tight. Did the 870c have a slightly longer chassis (and thus different belt...)than the yz10? Something seems off

ZC-651 on both

Well poop. I'm out of ideas. I'll count pulley teeth next

Bet you have 35 tooth pulleys installed-

pretty sure 870c should have 35 tooth pulleys.

The 870C, '92 works, and '93 Works all have 35T pulleys. The '94 and above have 33T pulleys. The '91 Works has 2mm belts/pulleys (not sure the tooth count on the pulley). I think the '93 WCS also has 2mm belts/pulleys.

Start countin' dem teeth!

They aren't 2mm, and they aren't the small diff ring versions....
Did they have a under/overdrive pulley?

everything appears correct. I suppose a new belt could stretch a little to break in? can you try the old belt? also, try the new belt on the old chassis.

the old chassis setup was pretty flexible, maybe with all the carbon it's really stiff and a bit on the high side of tolerance.

I am not aware of an under- or overdrive pulley for the YZ10...

Here's a thought: do you notice a difference in belt tension if you remove the top deck? The works models had top decks with slots for the rear screws, to allow you to flex the chassis to adjust belt tension (the '94 does this too, but more accurately with a turnbuckle attached between the top deck and rear gearbox case). It could be that your top deck is a bit long and flexing the chassis too much.

Counted a 35t pulley and 15t center, same as came on my parts car. Chassis and jammin upper are copied exactly from the originals. I will try the old belt next and see what's up. The new belt is stretching enough just in the chassis that it almost has the warps out of it from being packaged for so long lol

stickboy007 wrote:I am not aware of an under- or overdrive pulley for the YZ10...

Here's a thought: do you notice a difference in belt tension if you remove the top deck? The works models had top decks with slots for the rear screws, to allow you to flex the chassis to adjust belt tension (the '94 does this too, but more accurately with a turnbuckle attached between the top deck and rear gearbox case). It could be that your top deck is a bit long and flexing the chassis too much.

Just sitting without a upper, the chassis is bowing slightly with the belt. The upper causes the chassis to sit flat and takes a bit to get it installed.

Hmmm....so this is not an original lower deck, but a copied one? A perfectly exact copy? Something tells me the holes for the front bulkhead are just that little bit too far forward...