High side fuel pump lash
Thread Starter
Intermediate
Joined: Nov 2018
Posts: 47
Likes: 17
High side fuel pump lash
I put this in the tec section the other day, zero responses. Hoping somebody had a thought here.
Put A&A kit on- time to put LT4 high side pump on z51 (stock engine, stock cam). Everything I've found says find top dead center #1, measure. It's a bear to get to the pulley with the kit on, can't turn it over by hand.
Got the pump, got the measuring plug. My question is- if I turn over the engine with the button, it turns over for a couple seconds, measuring plug goes up and down. As long as the plug bottoms out on the stroke we're good? I'd hate to blow up the pump if the lash isn't right... and I'd hate to pull the whole supercharger kit off to get to the pulley to turn it over...
Thanks in advance,
Tim
Put A&A kit on- time to put LT4 high side pump on z51 (stock engine, stock cam). Everything I've found says find top dead center #1, measure. It's a bear to get to the pulley with the kit on, can't turn it over by hand.
Got the pump, got the measuring plug. My question is- if I turn over the engine with the button, it turns over for a couple seconds, measuring plug goes up and down. As long as the plug bottoms out on the stroke we're good? I'd hate to blow up the pump if the lash isn't right... and I'd hate to pull the whole supercharger kit off to get to the pulley to turn it over...
Thanks in advance,
Tim
Thread Starter
Intermediate
Joined: Nov 2018
Posts: 47
Likes: 17
Since nobody seems to know, or care, somebody might find this useful at some point-
Since it wasn’t easy to turn the engine over by hand to find exact top dead center of cyl 1, this is what I did.
Measured the resting distance between top of pump plunger and pump base where it bolts to engine. The measurements were the same from lt1 and lt4 pump. Within a couple thou.
Then measured the stroke distance by compressing plunger on each pump. The distance happened to be .016” taller on the lt4 pump. Instead of shaving .016 off of the lash cap, I just put 2 gaskets under the pump. The extra .013” on the gasket got me within a hairs width of what I started with. Close enough to button it up and it runs great.
Since it wasn’t easy to turn the engine over by hand to find exact top dead center of cyl 1, this is what I did.
Measured the resting distance between top of pump plunger and pump base where it bolts to engine. The measurements were the same from lt1 and lt4 pump. Within a couple thou.
Then measured the stroke distance by compressing plunger on each pump. The distance happened to be .016” taller on the lt4 pump. Instead of shaving .016 off of the lash cap, I just put 2 gaskets under the pump. The extra .013” on the gasket got me within a hairs width of what I started with. Close enough to button it up and it runs great.
Instructor
Joined: Jul 2020
Posts: 243
Likes: 87
From: Raleigh, NC
I went with the cap when I did my LT4 fuel pump and it ran fine with a single gasket. Did all the measurements so I knew clearance was good all based on recommendations from CF members and YouTube videos on the topic. Then I took it to my tuner RPM motorsport in garner, nc. They said they never run with a lash cap on the stock cam and they seemed quite concerned I had it n there. So I pulled it off and have no issues even after thousands of miles on the pump. If I had to guess, there must be some loose tolerances in the valley cover height/thickness and/or block top surface height but I’m no expert. I think I got my engine to turn over by putting a wrench on the supercharger pulley using a torque wrench set lower than the final torque of the bolt to prevent over torque on it.
Burning Brakes
Joined: Nov 2016
Posts: 769
Likes: 88
From: Ewa Beach Hawaii
Yea sucks trying to turn the engine over. Ended up making a ribbed belt similar to oil strap to turn the crank by hand. There was a great write I think that is pinned for measuring for lash cap. When I switched to LT4 pump no lash cap was needed, but I just finished my build and threw in a big bore LPE pump and needed two gaskets.
