So, in an attempt to cure my L18 of its bad habit of breaking alternators by having the mounting lugs break, I'm looking to fit an underdriven crank pulley. Any recommendations as to size ? Is it possible to go too small and not drive the water pump hard enough for good cooling ?

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1970 ex Group A Rally P510
1971 P510
1972 180B SSS
1965 SP310 Fairlady
1966 SP311 Fairlady

Depends on what sort of rev range you are going to be using and how much of a percentage you are changing your pulley size.
Supposedly most alternators don't like anything over 10% change in RPM (you won't charge if you are underdriving at idle for example)

You could always change your water pump pulley as well so it maintains the ratio?

How much of a change are you proposing?

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1972 Datsun 1600, S14 SR20DET Engineered (204rwkW @ 17psi.)
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#SR20Datsun @SR20Datsun

Not sure - I have an underdrive pulley in the shed, I'll dig it out tonight and see ! I'm hoping to just whack it on and see.

Being pretty much track only nowadays, not charging at idle won't be a problem. Not as big a problem as breaking the mounting lugs anyway - I've gone through my stash of L-series spare alternators and have had to move onto R31 alternators from the wreckers. They just keep breaking the two lower lugs and end up just dangling from the top adjuster bracket.

Rev range ? Not sure, I should fit a tacho one day - I just rev it till it runs out of go-forward, then change gears.....

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1970 ex Group A Rally P510
1971 P510
1972 180B SSS
1965 SP310 Fairlady
1966 SP311 Fairlady

Think about a harmonic balancer too, vibration could be the actual cause.

The Ol skool recipe was to fit the balancer, and alternator pulleys from a 240K. But, the 240K water pump has a different nose length to line up with the different offset on the 240K Balancer. I can't recall whether you also need the 240K alternator bracket ?
If you needed sizes/diameters PM me and I'll brave the rats in the shed and see if I can dig them out.
Vibration could be an issue, but I've seen motors hard mounted with solid pulleys that don't break alternator lugs.
Btw; if anyone knows of a good alternator upgrade for an L18, I'd be keen to hear it. I've heard RB30 internals are the go, but couldn't find info to confirm that. Something a bit more modern casing wise might help out some of your issues as well.

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Happiness is watching the race.... in your rearview mirror

You could get a different size pulley for the alternator, easier than changing all the others.

A real 240k water pump has a thicker shaft and deeper impeller than stock L20 style. It will move more water.
You can pull off a stock flange, machine out the center and press it onto the 240k water pump shaft - more water flow and standard pullies!

I said "real" because these days a lot of the cheap L series water pumps are all the same just with a different flange.

Ive got the smaller 240K/Z type L24 crank pulley on my L20B. I had an original K pulley until the rubber perished that much it seperated and have since replaced it with a new (Powerbond?) pulley from America which is identical to the stock L24 pulley.
The L24 water pump pulley bolts straight onto the water pump and I think I used an L24 lower alt bracket to mount my alt but one of the bolt holes doesn't line up to the block.
Ive got another from a Z20? Urvan which I have been meaning to try but for now it works fine...

My KA headed L18 has a sr20 alternator. Is pulls 9000 rpm every change and has never had an issue. Fairly simple modification to make it fit. Change the pulley over to l series style

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Circuit Race 1600 in the build

So apart from the pulley, the SR20 alternator bolts in the same spot ? That might be just the solution I'm looking for to get away from the externally regulated stock unit.

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Happiness is watching the race.... in your rearview mirror

Yep identical spot, onto the cast standard bracket. I will try to get a photo when i get back from work

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Circuit Race 1600 in the build

Thanks guys, plenty of options to try.

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1970 ex Group A Rally P510
1971 P510
1972 180B SSS
1965 SP310 Fairlady
1966 SP311 Fairlady