Gilmer Belt – Water Pump Drive

A Gilmer Belt Drive for the water pump was developed during the 2011 Engine Rebuild with the objective of reducing the pump RPM by about 30%. The motivation was to reduce pump cavitation above 5000 RPM and to increase horsepower. ( As discussed in Kastner’s  Triumph Prep. Handbook, p. 136-138, and an email to FOT from Greg Solow on 7/29/10.)


Two methods were used to determine the  new diameter ratio of the crank pulley to the pump pulley. One was based upon the diameters of the v-belt pulleys and the other based upon maximum RPM ratio.


The v-belt pulleys were the narrow belt  style and came with a harmonic balancer  kit from BFE in 1991. The crank pulley was attached to the harmonic balancer and the pump pulley was made of aluminum. Their diameters and ratio were:

Crank Pulley Diam. = 4.885 in.       Pump Pulley Diam.  = 4.035 in.

Crank/Pump Pulley Diameter Ratio = 1.21

Solow suggests reducing the pump speed by 1/3 which implies reducing the diameter ratio by 2/3 or:

New Crank/Pump Diam. Ratio = (1.21) x (0.66) = 0.81


Presume the stock pump was sufficient at the 1958 engine red line of 5000 RPM, and the estimated maximum modified engine RPM is 7200. By adjusting the pulley sizes so the new pump speed is  5000/7200 = 0.69  or (69%) of its original speed, the water pump RPM at 7200 engine RPM will be the same as with the stock engine at 5000 RPM. This approach gives:

New Crank/Pump Diam. Ratio = (1.21) x (0.69) = 0.83

This is a 31% reduction in pump speed.

Both methods give approximately the same pump speed reduction (31% – 33%) and crank/pump pulley diam. ratio (0.81 – 0.83).


Data for “L Series ( ½ inch wide) Timing Belt Pulleys” were examined on the McMaster-Carr website. The OD, pitch diam.,bore size, and the number, diameter, and bolt circle diam. of attachment holes were considered. The crank/pump pulley pitch diam. ratio of about 0.80 was sought, but other features were important. It was assumed that the stock water pump pulley and the stock crank pulley hub would be modified to accommodate the new pulleys, which also would be modified. The new pump pulley had to have a large OD to allow boring the ID to fit over a modified pump hub, but not so large as to contact the bottom of the water outlet on the cylinder head. The new crank pulley had to have sufficient material around its bore and attachment holes to be re-drilled to match a modified crank hub. The chosen pulleys and timing belt are listed below with their McMaster-Carr part numbers.

Crank Pulley:  Pitch Diam. =3.581 in.,  PN 6495K217.

Pump Pulley:  Pitch Diam. =4.297 in.,  PN 6495K219.

Timing Belt: L Series (1/2 in wide), 32.2 in. outer circle, 0.375 in. pitch,
86 teeth, Trade Size 322L, PN 6484K154.

These pulleys give a crank/pump pitch diam. ratio of 0.83, which is in the desired range.


I wanted to avoid running an idler pulley, but the closest available belt length, listed above, resulted in a small amount of slack, as determined from its installed lateral deflection. The remedy was to slightly raise the water pump. This was accomplished by enlarging two mounting holes on the pump housing and fitting short pieces of aluminum rod that were drilled with an off center hole. The pump exit port and block port had to be re-matched and more material removed from the top of the pump housing to clear the cylinder head.


The new crank pulley diameter was about 2 inches less than the stock pulley, so the timing indicator on the front cover would not be useable. I could have modified the timing indicator (Easy), or made a timing disc having the stock pulley diameter and fastened to the new pulley (Harder) to allow use of the stock timing indicator. I chose the latter approach.

SKETCHES and PHOTOS of the modified hubs, pulleys, timing disc and housing are attached. The sketches are not to scale, and all dimensions are in inches.

Crank Pulley New

Pump Pulley with text

Timing Disc New


Modified Gilmer Belt Pulley Glued on the Modified Pump Pulley

Modified Gilmer Belt Pulley Bolted to Modified Crank Hub with Timing Disc


Offset Holes in the Water Pump Housing


Timing Disc Bolted to the Back of the Modified Crank Pulley



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