GM Gen V V8 GDI Pump Preload Calculator

Calculation

Enter the values from your record sheet. Blank rows are ignored, so one lobe works as well as three. Readings from the in-engine and pump setups are entered exactly as the indicator displayed them — the page subtracts that setup’s reference reading F for you.

GDI pump preload, clearance and lobe lift

Every field below is a value you measured or supplied. Calculated results are named in words, so any letter in a formula is something you entered. The tool’s own fixed dimensions — the 25.40 mm (1.000 in) preload constant and the 44.45 mm (1.750 in) free length constant — are built in and never entered.

Preload = 25.40 − R F − L BC + C − G · Clearance = R C + L PL − 25.40 + G − C · Lobe lift = L BC − L PL · Available travel = R C − R F

Job optional, carried onto the printout

Engine or customer

Camshaft

Pump

Valley cover

Units and limits the limits govern the pass and fail flags — a published supplier specification overrides the 0.50 mm default

In-engine, camshaft installed — section 7 reference F, then depth to the lifter at base circle and at peak lift, per lobe

Pump, off the engine — section 8 its own reference F, then piston free and piston at end of travel

Gasket, lash cap and journal diameter — sections 13.1 and 13.3

V-block, camshaft off the engine — section 9 optional; characterizes the camshaft on the bench and does not feed preload

Reference data

The published and PSS-measured values behind the defaults above. Section numbers refer to the operating instructions.

Formulas, section 10.1

Every formula works in millimeters or in inches, provided every value including the tool constants is in the same units. For the in-engine and pump measurements each value below is the displayed reading minus the reference reading F for that setup, taken as an absolute value — subtract F first, then take the absolute value. For the V-block measurements every value is an absolute-valued difference from the journal reading J.

Lobe lift, in the engine = L BC − L PL Lobe lift, on V-blocks = B − K Base circle diameter = D J − 2 × ( B − J ) Peak lift diameter = D J − 2 × ( K − J ) Available piston travel = R C − R F Pump free length = 44.45 mm (1.750 in) − R F Preload = 25.40 mm (1.000 in) − R F − L BC + C − G Clearance to end of travel = R C + L PL − 25.40 mm (1.000 in) + G − C Check lobe lift + preload + clearance = available piston travel

Target values, section 11.1

Verify the preload and clearance specifications with the pump, camshaft or engine supplier. Any one of them may publish figures for the combination, and a published specification governs. Where none of them publishes anything, use the minimums below.

Item	Value	Notes
Preload at base circle	0.50 mm (0.020 in) min	A floor, not a target
Clearance to end of travel	0.50 mm (0.020 in) min	A floor, not a target
Measurement uncertainty	±0.05 mm (0.002 in)	The floor sits about ten times above it
Lobe-to-lobe spread limit	0.05 mm (0.002 in) total	Total spread, not plus or minus
Preload constant	25.40 mm (1.000 in)	Tool dimension, never entered
Free length constant	44.45 mm (1.750 in)	Tool dimension, never entered
Standoff length, S	63.50 mm (2.500 in)	Both constants derive from S and E
Flange to spring support, E	19.05 mm (0.750 in)	Both constants derive from S and E

Where all three lobes were measured, check these targets against the worst case rather than the average. Minimum preload occurs at the lobe with the largest L BC. Minimum clearance occurs at the lobe with the smallest L PL. These can be different lobes. The average describes the camshaft; it does not govern whether the combination is safe to run.

Published specifications run well above that floor. As one example, the Stanadyne RGX pump installation guide, specification 70640 Rev 1 dated 13 April 2026, calls for an installed plunger preload of 1.20 to 1.80 mm (0.047 to 0.070 in), publishes no clearance figure, and constrains the other end of the travel by limiting the camshaft instead — a maximum tested lobe lift of 8.2 mm with 7.85 mm recommended. Check the current revision of any supplier document before relying on the numbers in it.

Camshaft journal diameter, section 13.1

Published journal outside diameter for the Gen V LT series camshaft. The tool does not measure this dimension — a value measured with a micrometer is preferred.

Item	Millimeters	Inches
Published range, Gen V LT series	54.990 to 55.015	2.164 to 2.166
PSS default for D J	55.00	2.165

OEM camshaft GDI lobe lift, section 13.2

For comparison against a measured lift. An OEM stock combination of parts is acceptable as assembled and generally does not need these measurements.

Engine	Lobe lift, mm	Lobe lift, in
LT1, L83, L86, L84, L87, LT2	5.70	0.224
LT4, LT5	5.99	0.236
L8T	6.40	0.252

Pump gasket thickness, section 13.3

Both thicknesses confirmed by direct measurement at PSS. Because both measure the same, the choice between them does not change the calculation — but you still have to fit the correct gasket for your pump. Identify it by the inner feature. Where the gasket to be fitted is neither of these, measure it. The gasket is a one time use part.

GM part number	Model years	Inner feature	Thickness
12623308	2014–2026	Round	0.30 mm (0.012 in)
12679867	2019–2026	Square	0.30 mm (0.012 in)

12623308 is used on Gen 1 and Gen 2 Stanadyne pumps (LT1, LT4 and similar). The PSS default value for G is 0.30 mm (0.012 in).

Correcting the result, section 11.2

Available travel is fixed by the pump and lobe lift is fixed by the camshaft, so every adjustment you can make moves the division between preload and clearance. Clearance to end of travel moves the opposite way by the same amount in every case. Remeasure whatever combination you end up with.

Adjustment	Preload	Ratio
Machine the OEM valley cover pad	Increase	1:1 with material removed
Thinner pump flange	Increase	1:1
Thicker pump flange	Decrease	1:1
Aftermarket valley cover	Either way — measure it	1:1 with pad thickness difference
Lash cap on the lifter tip	Increase	1:1 with cap thickness

A lash cap adds a part to the drive, and with it another interface to wear and another thing to come loose. It is not the first choice, and it can be ruled out two different ways: the pump manufacturer may prohibit one, or the hardware may simply not accept one, since the end of the piston is recessed inside the spring retainer. Machining is not reversible — take the cut in steps and remeasure.