Complete vented Optispark replacement guide
By John Reed

Way back in early '94 when I first picked up my Impala SS I was fairly disappointed when I opened the hood. You see I had read that the Impala engine compartment was supposed to contain one of the most powerful small block Chevy engines ever, the LT1. Unfortunately it did not look like an LT1, at least not like the one in the Corvette.

  • 1992 - 1996 LT1/LT4 Corvettes (Y-Body)
  • 1993 - 1997 LT1/LT4 Camaro's, Trans Ams (F-Body)
  • 1994 - 1996 Fleetwood (D-Body)
  • 1994 - 1996 Impala, Caprice, Roadmaster (B-Body)

    After studying both the Corvette and the Impala engine bays, I determined that the dramatically different appearance between these two nearly identical engines was limited mainly to the dress items attached to the top of the respective engines. After studying both the 'vette and Impala parts catalogs I determined that it would be very easy to duplicate the look of the Corvette LT1 in the Impala by first removing the "home plate" induction resonator and it's bracketry, and then installing the fuel injector rail covers and valve covers from the Corvette. This was surprisingly easy, and so dramatically changed the look of the engine, other Impala owners thought I had changed the engine itself!

    Although these items can very easily be installed, there are a few modifications and adjustments that must be made, and several different parts are necessary to complete the job. The major appearance change comes from adding the Corvette fuel injector rail covers, and you can optionally also install the plastic Corvette valve covers as well. Since the valve covers are a direct bolt-on, there are no instructions necessary for them, other than what is already contained in the service manual. This text will focus on installing the fuel injector rail covers, which are a little more complicated.

    Finally, from 1992 - 1994 the Optispark was what is known in the gearhead world as "unvented". The reason it is called this is because it does not have the vacuum hoses connected like the newer 1995+ LT1/LT4 engines have. What the vacuum hoses do, is they pull the moisture out of the Optispark.

    What a lot of people don't realize is the unvented Optisparks are actually vented!! Yes, they have 3 weep holes at the base of the Optispark to allow moisture to escape. Well, all this does is pull moisture back in. Put cold water on a hot Optispark, it's going to suck the water in! This is the BIGGEST problem with the design.

    Before you go out and buy a brand new 1995+ Optispark, be forewarned. You CANNOT bolt a 1995+ Optispark up to a 1992-1994 LT1. The CAM is different. You could change the CAM, timing cover, CAM gear, etc but this gets expensive, and is very labor intensive.

    So, what is the solution?

    You can do 1 of 2 things :

    1. Change the Optispark out and be up and running again. However you risk damaging it again. There have been people who have had to replace a brand new Optispark because they got them wet.

    2. Change the Optispark out with a modified unit.

    The conversion process is simple!

    Buy a older style Optispark (the one you're supposed to buy), and simply change the cap! Of course you will need to do a few additional things.

    First let's start with the parts list:

    GM Parts needed:

  • Optispark for 1992-1994 LT1's PN - 10457702
  • Cap and Rotor kit PN - 10457735 or
  • 1995+ Distr. rebuild kit (Pep Boys or NAPA)
  • 1996 Vacuum Hose Kit PN - 12556174 or
  • 1995 Vacuum Hose Kit PN - 12555323

    Order GM parts from or from your local GM dealer (yuk!).

    Other Parts:

  • Inverse Torx Bit - E4, deep well!! SnapOn part number STLE40 - I stopped a truck, or you can order online at
  • 2-claw puller - Autozone
  • 2 water pump gaskets
  • Gasket Sealant
  • Vacuum T or a brass T that will screw into the manifold
  • Long Breaker bar, or torque wrench
  • Brass fitting for the base of the Optispark. I got mine from the local parts house that carries various hose/pipe fittings.
  • 3/16" Drill Bit (slightly bigger than the hole your drilling out)
  • Bandages

    Recommended :

  • Plugs and Plugwires

    Converting the new Optispark

    Remove your cap from your new Optispark. Nothing like taking apart something brand new eh?
    Remove the rotor from the Optispark. This is a torx T15 I believe. Carefully remove the rotor, and the metal disc. Do NOT remove the slotted one! It's spot welded on for a reason. Only the disc directly behind the rotor should be removed.
    Remove the middle spacer. This is the center portion of the Optispark body. Set aside everything, but the back plate. BE careful when handling this. Try not to get any grease, dirt or anything on the disc.
    You will notice 3 holes on the base. Fill the center one, and the one closest to the long pointed portion of the base in with JB Weld or something similar. I drilled out the remaining hole. Try not to get the metal shavings in or on the Optispark. Next I tapped in a brass fitting. Do not use the center hole for this, as it won't clear the balancer hub. IF you can find a brass L fitting, this would be your best bet. I wasn't able to find one small enough, so I used a straight brass fitting from a local parts house.
    Vacuum the metal shavings from the previous step. MAKE sure it's clean before you put it back together.
    Reassembly is the same. MAKE SURE you don't forget to put the metal disc back in place. If you leave it out, the gap for the rotor and cap will be WAY off. The rotor and disc are keyed, but still make sure you put them in correctly.
    Remove the old RTV Sealant, and put some new RTV on the cap at the electrical connection. Put it back together with the newer style cap. Lightly blow into one of the hose connections while closing off the other. You should hear no hissing.
    You aren't supposed to turn the shaft on the Optispark (why I don't know), but I would still recommend slightly turning it (1/4 turn is enough) to make sure everything spins smoothly.

    The Nitty Gritty

    First disconnect your battery.
    Drain your coolant. Be sure to open the bleed screws. This will help it drain much quicker. Remove the rubber intake bellow.
    Remove the upper radiator hoses.
    Disconnect the bottom radiator hose from the water pump.
    Remove the coil wire, and the electrical connections from the Optispark and water pump.
    Pull the belt tensioner out of the way, and loosen the accessory belt.
    Remove the water pump. There are 6 bolts holding it in place. Additional coolant will dump out when you remove the drivers side lower bolt. Clean the old gasket material and set the water pump aside for now. You MIGHT need to remove the coil, but I didn't need to. Check the weep hole on the water pump to make sure it's not damp.
    This is the fun part (for me at least). Remove the 3 bolts from the balancer. The Haynes manual says you can simply pull it off without a puller. I was not able to. I had to use a 2-clawed puller. I got mine at Autozone. I believe the one I got was the 6". Somebody is borrowing it as we speak for this repair! MAKE SURE you make a note of the position of the balancer in relation to the hub. Both the hub and balancer should be marked, but not keyed.
    Remove the remaining plug wires. If they are old, you may want to replace them. Now is the time to do it!
    Remove the 3 bolts that hold the Optispark in place. Pull it straight out. There is a shaft that connects the Optispark to the Cam gear. This may or may not come out with the Optispark.
    At this point, you should look at the old Optispark and make a mark of where the notch (spline) is. This will assist you later.
    This would be a good time to put some gasket sealant on one side of the gaskets, and place them on the water pump so they can dry. This will make reinstalling the water pump much easier.
    With the old Optispark to compare, rotate the shaft on the new one until they match up.
    I found it easier to insert the shaft on the cam shaft, and then sliding the Optispark on top of it.
    IF there is any gap on any of the 3 mounting screws, you probably haven't lined the Optispark up correctly with the spline on the shaft. It should slide all the way on with no gaps on any of the mounting areas. Tighten the bolts, but do not over tighten.
    Run your vacuum lines. The clip goes on the alternator bracket. MAKE SURE the hoses are going to clear the belt.
    The hose that has the filter and regulator is the vacuum supply.
    Poke a hole in the rubber bellow (on the top) and push the L into it.
    Put a T or something similar on the drivers side intake manifold, and hook the vacuum hose up.
    Reconnect the plug wires.
    Reinstall the 3 bolts on the balancer. Tighten the bolts to 60 Ft-lbs. Make sure you line it up properly (see step above where you removed it)
    Put gasket sealant on the remaining gasket material, and reinstall the water pump. Tighten to 30 Ft-lbs.
    Reinstall the belt. It is much easier to do it now before you do the next step!
    Reconnect all the hoses, the coil wire and the other misc electrical connections (temp sensor, Optispark connector, etc).
    Lightly tighten the bleed screws.
    Refill the cooling system until it is full. Place a rag around the bleed screws, and bleed off the excess air. Don't let any coolant get on your new Optispark!
    Check and recheck everything.
    Fire it up.
    If everything looks good, your done!
    You should also pull the hose out of the bellow, and check for vacuum. If it gets clogged, or a hose gets pinched, it will cause the cap to cave in, and burn up the rotor.