Jon J 1250 Posted October 19, 2021 Report Share Posted October 19, 2021 Hi, Does anybody know of an 80mm/3in Mechanical Rev Counter Tachometer suitable for the 4cyl 1147cc engine on a Herald 12/50. I have a spare Delco Distributor with the drive for the rev counter and an under dash panel for an 80mm gauge, but cannot find any suitable tachometers. Do I really have to convert it to Negative earth and go with an electronic one? Best Regards Jon Link to comment Share on other sites More sharing options...
Colin Lindsay Posted October 19, 2021 Report Share Posted October 19, 2021 Mechanical versions do come up all the time; I've no idea about compatability between models ie how accurate they would be if on different models but this one is cable drive and measures 80mm: https://www.ebay.co.uk/itm/255081885627?hash=item3b6410afbb:g:pw8AAOSwQJxg23Be I'm assuming your Herald is positive earth? You do find switchable versions coming up for sale, I sold a rectangular version a while back, but most are negative versions. https://www.ebay.co.uk/itm/255161457626?hash=item3b68cedbda:g:t2gAAOSw5~thWXX6 Link to comment Share on other sites More sharing options...
Pete Lewis Posted October 19, 2021 Report Share Posted October 19, 2021 bear in mind the orig dizzy drive is 3.55:1 ratio Pete Link to comment Share on other sites More sharing options...
Jon J 1250 Posted October 20, 2021 Author Report Share Posted October 20, 2021 15 hours ago, Colin Lindsay said: Mechanical versions do come up all the time; I've no idea about compatability between models ie how accurate they would be if on different models but this one is cable drive and measures 80mm: https://www.ebay.co.uk/itm/255081885627?hash=item3b6410afbb:g:pw8AAOSwQJxg23Be I'm assuming your Herald is positive earth? You do find switchable versions coming up for sale, I sold a rectangular version a while back, but most are negative versions. https://www.ebay.co.uk/itm/255161457626?hash=item3b68cedbda:g:t2gAAOSw5~thWXX6 Many thanks, have searched Ebay and was getting confused. I keep getting results with motorbike tacho's or others which don't look right, or drive the wrong way, have the numbers round the bottom etc.. Plus I don't think I understand the technical specs well enough. I sometimes see ratio's mentioned, doesn't the cam drive at 1/2 the crank speed, and the distributor the same. There seems to be so much to potentially get wrong, hence my question. Link to comment Share on other sites More sharing options...
Jon J 1250 Posted October 20, 2021 Author Report Share Posted October 20, 2021 14 hours ago, Pete Lewis said: bear in mind the orig dizzy drive is 3.55:1 ratio Pete Hi Thanks, do you mean the tacho output from the Delco distributor rotates 3.55 times to 1 rotation of the distributor? Link to comment Share on other sites More sharing options...
NonMember Posted October 20, 2021 Report Share Posted October 20, 2021 I think it's 1 revolution of the tacho drive to 3.55 of the crank (so 1.77 distributor revs per cable rev) but I'm sure Pete will confirm Link to comment Share on other sites More sharing options...
Pete Lewis Posted October 20, 2021 Report Share Posted October 20, 2021 apart from electronic all the Triumph mechanocal drive rev counters are 3.55;1 the cable drive is quite slow so its 355 revs in gives 1000 on the dial i dont have one to play with the 2000 is devoid of such luxury Pete Link to comment Share on other sites More sharing options...
Jon J 1250 Posted July 31, 2023 Author Report Share Posted July 31, 2023 Just to conclude this old topic. I have now installed a period gauge pod in the car with some period gauges to replace the Ammeter and temp gauges installed by the original owner. I( do keep everything I take off the car, and didn't have to drill any new cars so everything is reversible.) I mounted it where most would put the radio, but I have no need for a radio in a Herald, you can't hear it very well on the move, and I'd rather listen to the engine and gearbox whine! The tacho is a Smiths RVI 1003/00 pulse tacho, which is supposed ly the model for a 4 cyl engine, however it does read a bit fast even with the calibration pot wound fully off. I added a dual temp oil pressure gauge too (soory if that's a bit too MG for some! lol!) as I always wanted to monitor oil pressure. I'm pretty happy with he results. Does anyone know why the tacho reads a bit fast? JJ Link to comment Share on other sites More sharing options...
johny Posted July 31, 2023 Report Share Posted July 31, 2023 Looks nice but how do you know the tacho reads fast? Link to comment Share on other sites More sharing options...
Jon J 1250 Posted July 31, 2023 Author Report Share Posted July 31, 2023 34 minutes ago, johny said: Looks nice but how do you know the tacho reads fast? Thanks, just a suspicion, I'll meter that can read frequency to know for sure, but my tickover is quite low, but the tacho reads about 1100 rpm, so doesn't quite feel right. Link to comment Share on other sites More sharing options...
Colin Lindsay Posted July 31, 2023 Report Share Posted July 31, 2023 I think those were originally Mini Cooper tachos? Not sure if it will operate any differently in the Herald, but it looks good! Link to comment Share on other sites More sharing options...
johny Posted July 31, 2023 Report Share Posted July 31, 2023 According to the interweb you must have a good 12v supply to the tacho or it might read high so both dynamo? output at tickover and earthing of the tacho must be satisfactory... I like the rev limit youve set😁 Link to comment Share on other sites More sharing options...
Jon J 1250 Posted July 31, 2023 Author Report Share Posted July 31, 2023 4 minutes ago, johny said: According to the interweb you must have a good 12v supply to the tacho or it might read high so both dynamo? output at tickover and earthing of the tacho must be satisfactory... I like the rev limit youve set😁 lol! its not a target, I plan to keep my pistons safely inside my engine!. I think voltage is fine, but would be a touch higher than 12v, even with a dynamo. I connected it directly to the battery terminals to test, made no difference. Maybe is needs a bit under 12v? Link to comment Share on other sites More sharing options...
Jon J 1250 Posted July 31, 2023 Author Report Share Posted July 31, 2023 (edited) 25 minutes ago, Colin Lindsay said: I think those were originally Mini Cooper tachos? Not sure if it will operate any differently in the Herald, but it looks good! Thanks Colin, Often used in Mini's yes but the 60's A-Series is still 4 cyl/4 stroke engines with near identical Lucas ignition system so, i wouldn't have though it would make a difference. JJ Edited July 31, 2023 by Jon J 1250 Link to comment Share on other sites More sharing options...
johny Posted July 31, 2023 Report Share Posted July 31, 2023 3 minutes ago, Jon J 1250 said: I think voltage is fine, but would be a touch higher than 12v, even with a dynamo. I connected it directly to the battery terminals to test, made no difference. Maybe is needs a bit under 12v? Could the voltage be low for a while after a start as the dynamo tries to recharge the battery? Other than that theres just the calibration of the tacho itself so speak to the supplier? Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now