Joe Curto.
I’d have a word with @DavidBoger at http://www.everydayxj.com/ and see if he can’t help you with the right bits. A 4.2 with carbs from an XJ6 shouldn’t cost too much.
Nick
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(Shameless thread hijack in progress ;-))
I have often read that about 3.8 cranks, but I’ve never seen it documented objectively with a part number or such. I’m wondering if it is more a case of when it was produced than which engine it was installed in?
From an engineer’s (even less from accounting, I’m sure!) perspective, it would be illogical to produce two part numbers for engines so close in displacement and performance. I could see an incremental evolution that would have been introduced along the same time as the E-type, a more serious bit of kit. By then they would have had more than 10 years of experience with the 106mm stroke cranks, including 5 very successful runs at Lemans, so the timing would be reasonable for a update.
Back when I built that engine I had 3 crankshafts on shelf to pick from. Two were from different models produced in 1959, a 3.4L from a XK-150, the other from a 3.8L MkIX. The third was from a 1964 3.4L Mk2. The cranks from the two cars produced in 1959 were the same. The 1964 Mk2 crank had noticeably thicker webs between the journals. As a mechanical engineer, I would say the thicker webs provided greater section modulus and hence, it was stiffer in torsion. I would reject any “better balance” argument outright. Things are either dynamically balanced or not, particularly inline sixes, which are fundamentally sound in the first order harmonics. I would be agreeable to an argument for reduced probability for cracking from higher horsepower based on the superior torsional rigidity. Torsional “springyness” would contribute to a shortened fatigue life. I would hypothesize that there was an increment update in the 106mm stroke crankshaft and all non-4.2L engines probably got the upgrade.
You mentioned the S3 rods, I would note that the billet rods in that engine are quite different from original. They are 0.400“ longer than stock and made to accept Chevrolet V8 sized rod bearings, so S3 rods were not an option for me. There is a vast array of very good bearings available for the Chevy V8, Clevite 77 being my favorite, and very few choices for the Jag XK.
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If memory serves the early cranks are
approx 10lbs lighter and compared to the later cranks the counter weights are positioned differently. I am fairly sure the change of crank coincided with the introduction of the rear rope seal.
Not aware of a difference between the later 3.4 and 3.8 cranks.
Peter B.
Mike, there are two part No’s., C4808/1 for the 3.4 and C18350/1 for the 3.8/3.4.
I can’t give you a definitive date the later crank was introduced, but the early 3.8 MK II had the C4808/1 crank up to Eng. No. LB 8246, from Eng. No. LB 8247 to LB 9999 and then from LC 1001 on, C18350/1 was fitted, and also to the 3.4 MK II from KH 2794.
The C18350/1 had more counterweights than C4808/1 to more evenly distribute the bearing load by having a “better balanced” shaft.
I would say that the 59/64 cranks you had were the C18350/1.
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Thank you for that! The block and crank I used were from KJ-7024. The castng date on the block is 21-10-64.
Greetings All,
SU’s have 4 moving parts, Webers a whole lot more and you will find it necessary to buy components to get it closer th perfect. Needles only on SU and getting them staged or positioned correctly.
Lovell,
Just off the top of my head don’t Weber DCOE’s only have six moving parts - accelerator pump plunger, float valve, throttle butterfly plate, accelerator pump intake jet/check valve and accelerator pump weights. The moving parts aren’t the challenge, it is the one thousand combinations of the two venturis, main jet, emulsion tube, air jet, idle air jet, idle jet, plate angle, progression holes, accelerator pump plunger length, accelerator pump spring strength, accelerator intake jet, accelerator pump discharge jet size and direction, float level to the exact mm.
Or you can just go with the SU and get float level right and select main jet, needle and spring strength. If you want to get real tricky play with the plunger oil viscosity and the air bleed rate for the piston.
Matt
Exactly: for street purposes, the SUs are simpler, and provide every bit as much horsepressure.
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Greetings All,
The tackle box of parts needed to fine tune on race day isn’t exactly inexpensive either.
I remember someone big in Jaguar’s racing department stated the SU’s flowed better but the driver’s thought the Weber’s “felt” faster. Racing department felt it wasn’t worth a debate as they felt the driver would try harder.
With the higher manifold pressures, at mid-throttle, caused by wilder cams, the Weber’s, having an accelerator pump, were faster in throttle response.
Plus, on a highly tuned engine, Weber’s gave what was essentially one throat per cylinder. On a full-blown race engine, Weber’s did indeed work better, but on a stocker, they’re just a pretty PITA.