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HOW TO: Installing a Fuel Pump Lash Cap in an LT1 and Checking for Proper Clearances

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HOW TO: Installing a Fuel Pump Lash Cap in an LT1 and Checking for Proper Clearances

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Old 06-21-2017, 10:29 AM
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FYREANT
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Default HOW TO: Installing a Fuel Pump Lash Cap in an LT1 and Checking for Proper Clearances

NOTE: This How-To guide is neither endorsed by or property of Corvette Forum or myself in any way/shape/form. All liability stemming from any actions taken in relation to this guide is solely placed upon the end user. (This means you!)

So you need extra fuel in your LT1 for whatever reason, and have decided to do one of the following:

• Install a fuel pump lash cap to boost fuel delivery ~25%
• Install the LT4 high-pressure fuel pump to boost fuel delivery ~30% (this does not NEED a lash cap but can be used for additional fuel supply)
• Install both the lash cap and the HPFP because you needed more than each of those can give you on their own
• Installed a cam with a 32% fuel lobe (requires a lash cap be used with this configuration)
• Install a cam with a 32% fuel lobe, AND a LT4 HPFP, (again requires a lash cap still)

Whatever your reason, this will serve as your guide to making sure you don’t get into a hairy situation with your fueling upgrades. So what’s a lash cap and where does it go?! A lash cap is basically a “shim” for the fuel pump that overdrives the stroke of the pump to produce more fuel delivery. It looks like the below picture. The cap is flat on the side touching my hand:



Great! Where do I put that thing? On the end of the pump? Or on the fuel pump lifter under the valley cover in the engine? You would think it actually goes on the pump itself, but actually it goes on the lifter in the engine below the valley cover as shown here:



Here’s what a lash cap looks like installed (and yes, my valley cover is off the engine in this pic):



Awesome. You know what a lash cap is and where it is supposed to go! But, did you know there are actually multiple different lash caps available in different thicknesses? YUP! So how do we pick the correct one? We’ll go over that shortly. So why can’t I just get a cam with a larger fuel pump lobe on the cam so I don’t need to do a lash cap?! Great question. The reason is that we cannot make the lobe for the HPFP larger than the bearing journals, or else we wouldn’t be able to get the cam physically into the block!

Ok so how does a 32% fuel lobe work then? Well, it actually works by making the base of circle smaller, to allow the stroke on the HPFP to come out of the pump a bit further. What is “base of circle”? It is the lowest point for the fuel pump lobe, where the HPFP cannot go any lower. We’ll go over how to find base of circle next. One other thing to point out is that if you run a cam with a 32% fuel lobe, you MUST do a lash cap. Why? Because the lash cap provides the proper “preload” on the HPFP. Since your cam brings the base of circle farther in, the pump cannot reach the base of circle anymore without a lash cap to provide the necessary preload on the pump, thus, a lash cap is a necessary evil.

Now that you know how a 32% fuel lobe works on the cam, there is one more thing you will need to do a lash cap job, and that’s a special tool to ensure the cam is at its base of circle. The tool you need is made by Kent-Moore and the part number is EN-51092 (shown below. This tool looks like a big plug that takes the place of the high pressure fuel pump.



When the tool sits flat and there is no gap, it is properly on base of circle. If you’re not on the base of circle, simply turn the crank over until it sits flush with the valley cover. Example below:



Ok now that we are past that, let’s move on and go over how to check clearances, which will then tell us which lash cap we actually need to use. In our example, we are installing an LT4 high pressure fuel pump, a lash cap, and a Vengeance Racing cam with a 32% fuel lobe. The reason we need to check the clearance for this setup is what happens when you have a 32% fuel lobe, a LT4 HPFP, and a lash cap that’s too think? You may have TOO long of a stroke on the HPFP drive rod, causing it to bottom out in the pump and either shear off the heads of the HPFP bolts, or cause the HPFP to fail entirely. Both of these situations have happened to CorvetteForum members in the past.

Checking for HPFP clearances:


First, make sure you find your base of circle for the cam fuel lobe as I described previously. Once you are on base of circle, remove the tool. Now we need to use our HPFP to do measurements. You will need to remove the outer spring from the HPFP for this. It simply pops off as shown below:



The reason we take the spring off is it aids in reducing the pressure needed to finger screw the bolts in for the next steps.
Now you will need to install the HPFP, WITH the gasket, and see if there is any preload. The simplest way to see this is by asking, does the fuel pump go all the way down, or does it sit up a little as shown below:



If it sits flush, you need a lash cap. If it sits up a little like mine does, you have proper preload already, and now we need to check for installed clearance next. For reference, we are starting with a lash cap of size 8MM wide by 0.080” thick. You can see in the above picture that there is still some clearance left between the pump and the valley cover, with the gasket in there and no bolts to hold it down. This is “preload” and this is what you want at base of circle on the cam fuel lobe.

Great! We are half way there! Now we need to remove the HPFP, and we need to find the highest peak on the cam fuel lobe. Use your trusty Kent-Moore tool for this again. It may be hard to locate the exact spot that is at the highest point, so to help, you may want to use a marker on the tool where it goes into the valley for visual reference to know when it stops going higher. Once you have located the highest point on the lobe, now we need to install the HPFP WITHOUT the gasket, and HAND screw in the bolts evenly (a few turns each side to keep it perfectly level) as shown below:



Once it is down as far as it will go, does it sit flush on top of the valley cover? In my case it DID NOT! (shown below). This means that the 8mm x 0.080” lash cap is too thick as is causing my pump to bottom out! Oh no’s!!



Now we need to measure exactly how much clearance is left between the pump and the valet cover. To do this, use some feeler gauges. In my case, I had exactly 0.015” of clearance that needs to be eradicated (shown below).



Wait a second, “didn’t you tell me to install the pump without a gasket?” Yup. Sure did. That’s because the gasket is exactly 0.010” thick as shown below:



So if we take off the 0.010” from the 0.015” clearance we have, that leaves us with 0.005” of extra space we need to get rid of. Yes, that extra 0.005” WILL cause your pump to bottom out and fail!! We MUST change to a different thickness lash cap!

Alright, what thickness then? The next size down is an 8MM x 0.060”, (for clarification, the 8MM part of the measurement is the internal diameter of the cap that fits onto the lifter) which is what we will use. After installing the thinner lash cap, and by following the same procedure, we first need to reconfirm if we have proper preload on base of circle of the fuel pump lobe on the cam as we did previously. Great news, I still had preload! Now we need to follow the same procedure to check for clearance at the highest point on the cams fuel lobe again. Remember, install the pump WITHOUT a gasket, and finger screw in the bolts. Does the pump go all the way down to the valley without bottoming out? Yay! It goes all the way down!!



So what exactly happened there? We is gonna have to math it up for that:
To understand how we figure this out, we need to assume that ground zero means to be level on the valley cover. So any gap above the valley cover will be represented as a negative number. We know that the lash cap that was 8MM x 0.080” thick plus the 0.010” gasket left us with 0.005” of the pump that still needed to go into the motor. So representing this as a negative number since it will take the pump 0.005” to reach “0” where it is sitting flush with the valley cover, we will use -0.005”.
By switching to the 0.060” lash cap, we recouped 0.020” of space on the internal clearance under the valley cover. Doing this means that we take 0.020” and add -0.005” which comes out to 0.015” clearance. So, when the pump is installed, at the highest part of the cam’s fuel lobe, the pump will still have 0.015” of clearance before it bottoms out. Perfecto!

Here is where things may get a little controversial, as most all performance shops are only selling lash caps with the larger size, (8MM x 0.080” thick), which is too thick for this setup as math has just proven, and causing the pump to bottom out! However, I will not call out any shops on this because it is the job of the INSTALLER to check clearances and ensure the proper lash cap is used! So now we know how all about measurements. What does this mean if I am not doing a cam? This means you should be good to use the 8MM x 0.080” lash cap if you are doing the lash cap, and ONLY the lash cap for more fuel. It also means that if you are doing the LT4 HPFP and ONLY the LT4 HPFP, you do not need to do a lash cap. If you are not doing a cam, and are doing BOTH the LT4 HPFP AND a lash cap, you will still need to check for proper clearances!

One last thing, make sure to put the spring back on to the bottom of the fuel pump before you install it!!!

Should you need to get a lash cap, here are the two options:

8MM x 0.080" thickness - Comp Cams # 633-1
8MM x 0.060" thickness - GM part number: 12596509 (will show as either "PAD, Engine Valve Rocker Arm" or "PAD VLV RKR". This correct, and is a LS7 rocker arm lash cap pad).

Here are a couple of CF posts that I would like to make mention, as they have very good information in them also:

Theta’s HPFP How To:
https://www.corvetteforum.com/forums...e-for-lt1.html

CamMotion post regarding lash cap requirement and clearance checking:
https://www.corvetteforum.com/forums...post1593117869

More guides coming your way!

Ant

Last edited by FYREANT; 06-22-2017 at 10:47 PM.
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Old 06-21-2017, 10:42 AM
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Pure gold.
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Old 06-21-2017, 10:45 AM
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So bad ***. This is a big deal.

Last edited by C7&7; 06-21-2017 at 10:47 AM.
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Old 06-21-2017, 11:36 AM
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Great info
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Old 06-21-2017, 08:00 PM
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Great info and write up!! I thought many were getting away from lashcaps due to them being improperly installed/removed and falling into the motor?
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Old 06-21-2017, 09:49 PM
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Fyreant, great job man.

Boossted, you are right, but I am sure most of the lash caps were not installed with this level of measurement, when they should be. This should almost definitely reduce/remove the failure rate with lash caps.

Fyreant and I both had lash caps "get lost." I also had a pump bust off the engine as well ( didn't do the install, btw... ;-) ). I know some shops that have had that same issue. That's really a part of pioneering mods on a new platform, some experiments are costly.
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Old 06-21-2017, 10:48 PM
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Originally Posted by Higgs Boson View Post
Fyreant, great job man.

Boossted, you are right, but I am sure most of the lash caps were not installed with this level of measurement, when they should be. This should almost definitely reduce/remove the failure rate with lash caps.

Fyreant and I both had lash caps "get lost." I also had a pump bust off the engine as well ( didn't do the install, btw... ;-) ). I know some shops that have had that same issue. That's really a part of pioneering mods on a new platform, some experiments are costly.
Thanks man! What's even funnier is that the lash cap I ended up using is the same one that I lost in the motor and then found when I took the engine apart! It's just plain bittersweet! Lol

Speaking of the engine build, it's done! I fired it up tonight and it runs! Tomorrow I have to grab some more coolant to top it off and load the updated base tune you sent me, as well as tap my new charge pipe for dual meth and IAT breakout. Then she's ready for tuning. Howard at Redline is busy this week, so I'll have to try and get over there next week sometime.

Ant
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Old 06-21-2017, 11:29 PM
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Great post!!!
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Old 06-22-2017, 08:30 PM
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Guys some really good info on this & why people had problems with this in the past. Robert
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Old 06-22-2017, 10:48 PM
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Thanks guys! I edited the end of the original post to add the part numbers of the different lash caps to help out with locating them.
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Old 06-23-2017, 05:11 PM
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From where did you get the measuring tool, searched everywhere but didn't find it
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Old 06-23-2017, 06:05 PM
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Originally Posted by black_c7 View Post
From where did you get the measuring tool, searched everywhere but didn't find it
What measuring tool? Or do you mean the base of circle tool?
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Old 06-23-2017, 06:10 PM
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Yes i mean the base circle tool
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Old 06-23-2017, 07:42 PM
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Originally Posted by black_c7 View Post
Yes i mean the base circle tool
I got mine off of ebay actually
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Old 06-24-2017, 01:52 AM
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Originally Posted by FYREANT View Post
I got mine off of ebay actually
I already searched ebay its shows that its been sold do you somewhere else i can get it from?
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Old 06-24-2017, 07:56 AM
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Originally Posted by black_c7 View Post
I already searched ebay its shows that its been sold do you somewhere else i can get it from?
There are a few websites that sell it, so you'll have to google "Kent Moore EN-51092" and start looking for them. I know there is a place called Tool Source that says they have them
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Old 06-24-2017, 08:37 AM
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Originally Posted by FYREANT View Post
There are a few websites that sell it, so you'll have to google "Kent Moore EN-51092" and start looking for them. I know there is a place called Tool Source that says they have them
Already went to the website but its kind of complicated to find the prt in their website, would you be able to measure the length of it?
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Old 06-24-2017, 09:30 AM
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Originally Posted by black_c7 View Post
Already went to the website but its kind of complicated to find the prt in their website, would you be able to measure the length of it?
measure the length? No, you shouldn't try to do it that way. I'll PM you with a link to the part on their page..
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Old 06-24-2017, 04:00 PM
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Originally Posted by FYREANT View Post
measure the length? No, you shouldn't try to do it that way. I'll PM you with a link to the part on their page..
Sounds good, or just post it here so if anyone need it they can find it with the link, thanks
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Old 07-16-2017, 08:32 PM
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awesome buddy
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