Injuneer

The GM Extreme Duty timing set does not include the DRIVEN gear (the gear included with the bearing/gear/shaft assembly). As BW_Hunter has already explained, the XD set has a new DRIVE gear, which is actually the rear part of the cam sprocket.

On the other hand, if the bearing can be removed from the assembly and replaced, maybe the shaft can be removed as well. Then maybe a hardened coating (like the moly coating used on piston skirts) could be applied to the shaft's sealing surface to restore it, after any groove has been polished out. I suspect an outfit like Swain could advise if this is feasible.

TA Dreaming

thanks shoebox!

Fred, Jegs says the LT4 extreme duty kit does come with the water pump drive gear. Is this new for the kit? Am I still misunderstanding and that small gear doesn't mesh with the cam gear and then drive the water pump?

Chevrolet Performance LT1/LT4 Timing Chain Set | Chevrolet Performance

Thanks!

bw_hunter

It does come with the drive gear, it is integral with the cam gear.,the driven gear, with the shaft that sticks out of the timing cover is not included.

All versions of the LT series timing sets come with the drive gear...

TA Dreaming

What's the third gear included with the kit used for then?

Injuneer

The XD set (I have one on my engine) includes the water pump DRIVE gear. That consists of teeth located ON THE REAR OF THE CRANK SPROCKET. The DRIVE gear is not the gear on the water pump drive shaft. It is the gear that engages and DRIVES the gear on the water pump drive shaft, which is the DRIVEN gear.

The XD setup is identical to the stock LT1 timing gear in that respect. But some people opt for something like a Cloyes double roller set, and that set does not have the drive gear teeth on the back of the cam sprocket. So they use an electric water pump. I suspect that is why JEG's states that the water pump drive gear is included. Not all timing sets include the gear (teeth) on the back of the cam sprocket.

Injuneer

I can't tell.

TA Dreaming

Ok I see. I'm sorry. I was using the wrong term. When I was saying water pump drive gear, I was referring to the driven gear. The part number shoebox posted is for the driven gear assembly. Looks like GM called it the water pump driveshaft assembly.

I'm fairly certain that third gear is the water pump driven gear. I don't have the kit in my hands to verify though. Looking at the driven gear assembly on the motor, the gear is clearly a press fit. My guess is press off the old one press on the new one.

Thanks!

bw_hunter

Maybe I should have asked this earlier, but is your gear damaged? If not, why are you intending to replace it. The only issue I've ever seen regarding the water pump drive system is a grooved shaft. I imagine it's possible to damage the gear, I've just never seen or heard of it...

TA Dreaming

because I'm building a new engine. That's really about it. I looked at the sealing surface and it's in great shape. I can polish it with a high grit sand paper and return it to new I'm sure. I have no idea how many miles are on the core engine I bought to rebuild. It looks great on the inside but that doesn't mean much.

When I switch to the LT4 ED kit, I intend to switch out the driven gear if that's what the third gear is in the kit. I'd be afraid of any minor pitch difference between the gears or hardness difference that might cause a wear issue. At least that's my thinking anyway

Injuneer

If that is the driven gear, what are the three holes for? Maybe there is a plate attached to the driveshaft, with bumps/pins that engage the holes. Being a press fit would seem hit-or-miss. Last place you would want any sort of slippage possibility.

If you opt for the XD set, the cam sprocket is extremely heavy. I believe that may have been due to an issue of the LT4 cam having some sort of torsional problem. Before installing, the shop that built my engine machined openings in the face of the sprocket to reduce mass.

TA Dreaming

I'm going from memory, but I'm fairly certain my current water pump driven gear has holes in it and it's the original unit. So three holes in that gear don't concern me.

I've seen some rumblings about failings with the LT4 ED kit compared to a stock LT1 kit. I figure I'll be spinning the engine to 6500 occasionally. It's a street car mainly and not something I'll be using for competition. What's your opinion on which timing kit to use?

Thanks

Injuneer

I have the LT4 Extreme Duty set, and never had a problem with it. Not a lot of miles on the engine over a period of 16 years. It was daily driven the first 2 years, then became a "track car" and has sat in the garage way too much over the last 5 years.

But... the cam is a solid roller, big spring pressure, 0.590 gross lift I & E. Redline is 7,200 RPM, with the 300-shot of nitrous cut off at 7,000 RPM. Because the aftermarket ECU has built in data logging, I have documented at least two instances of the engine turning 8,000 RPM when I missed 3rd gear (used to have a T56 manual). While the rev limiter is set for 7,400 RPM, it doesn't do any good if you hit the wrong gear.

This shows the lightened cam sprocket. Unfortunately when the photo was taken, the water pump drive assembly was not installed.

Injuneer

Link to Engine Builder magazine - "Rebuilding the Chevrolet LT1 Engine"

Rebuilding the Chevrolet LT1 Engine - Engine Builder Magazine

TA Dreaming

Thanks, Fred. I've got the How to rebuild book on its way here. I also have the FSM but I like to read instructions in plain English, too.

I'm going with AI's 200cc LT1 top end kit and their 234/242 108 cam. I'm looking more for durability and broad based power. Just a good driving engine. I'm not too worried about squeezing every last ounce of power from the motor. I'm ok going with a heavier timing set but don't mind listening to other's opinions. That's why I ask. I appreciate your help and insight.

Thanks

lt1-xjs

The surface where the seal rides can be repaired to better than new. Don't pitch them.