Tachometer accuracy

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A single point calibration of your tacho will not tell you how repeatable it is. If you simply rely on a magnetic tacho's reading at 7000 RPM, it is unlikely that it reads with a different error every time you use it. I used several chronometric tachos years ago - you never know where you are with them.
With the Briggs and Stratton vibration tacho, how would you know if the secondary vibration mode (rocking couple) of your crank was having an effect, when you use it on a Commando engine ?
 
I only really care if 7000 is 7000! Reading the tacho is limited to looking to see if the needle points straight up...change up! If it does it in top, gear up!

I can't say about secondary vibration. One was used on my Rickman Commando in the '70s, it indicated the required 3700 within a few hundred of what my mechanical tacho said!

3750 is 11m/sec piston speed with 89mm stroke. I doubt the device can be set more accurately than each 100rpm!

In truth I want to know if the current reading of my mechanical tacho at 7000 indicated is 6500 or 7500!

If I wanted something more accurate than that I would fit an electronic one!
 
I have an assembled engine on a stand. I put a shaft into the tach drive with a tape flag and found that 1 engine turn equals 1/4 turn of the tach drive, so the 4:1 mentioned earlier is what the instrument sees. If you have an 1800 RPM drill that you apply directly to the instrument you should see 7200 RPM indicated.
 
Mr. Rick said:
Looks like I might be able to get my iphone light to flash at 60 HZ. This wd be 3600 RPM?
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It would, but being British living in France, I am used mains at 50Hz, which is 3000rpm! But the mains here in rural France isn't that reliable that I would be inclined to trust it to keep to +/-10% over 24hrs!

Just discovered the same capability on my Windows phone, though it is tricky to set to specific frequency settings, it seems to be OK on 'BPM'.


So with the confirmation of the 4:1, for my 7000 test I will need to turn it at 7000/4 or input a drill turning at 1750rpm! 29.16666667Hz........

........so maybe 1800rpm/30Hz and 7200 indicated will be just fine!
 
Mr. Rick said:
On the face of the tach, we can see 4:1
Does this mean that turning the cable at 250 produces a reading of 1000 on the dial? Or is it the other way around?
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I hope the cable doesn't turn 24000 RPM to achieve 6000 RPM reading on the tach!

I've been using one of these and have been happy with it. Can't promise it's accuracy, but it is a simple device so probably pretty good.

https://www.amazon.com/gp/product/B00VKAT8A2/ref=oh_aui_search_detailpage?ie=UTF8&psc=1
 
That 4:1 ratio is not true for all Nortons of the era. I have an engine with the tach drive coming off the front of the timing case, circa 1967 out of an Atlas, but I believe the early Commandos were the same. If I recall, the ratio is 2:1. The last time I had that engine running I could not use my 'standard' Commando tach. You're probably all right with the 4:1 assumption but I wanted to point out that it is not an absolute constant for all big twins.
 
Tachometer accuracy
Tachometer accuracy


The DTI is good to 20k RPM. Just hold it near a plug wire and it picks up a signal.
The timing light/tach does not adjust for wasted spark so it reads at double the actual RPM. It max’s out @ 10k. Makes timing easier. I believe the upgraded model will adjust for the wasted spark.
Both are easy ways to check RPM.
Pete
 
Atlas Commando said:
That 4:1 ratio is not true for all Nortons of the era. I have an engine with the tach drive coming off the front of the timing case, circa 1967 out of an Atlas, but I believe the early Commandos were the same. If I recall, the ratio is 2:1.
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Early (20M3) Commandos were 4:1 tach ratio.
 
SteveA said:
Would it be rude to ask to see your sensor installation? Please.
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It would not be rude, but it will be late today before I can post a photo. The installation is on a vintage Perkins diesel on my boat after the failure of the mechanical tach last year. But this could be set up on any 12v battery for a temporary installation by sticking the magnet to any rotating surface. Holding the sensor in place would be simple through any number of methods probably including by hand.

I did not intend for the installation on the boat to be permanent. But a year later, in a marine environment, it is still working, giving better results than the mechanical tach and now I am curious as to how long it will last.

I was not suggesting it as a permanent item on a Norton.
 
I found a couple of earlier posts, this one from Ron L back in 2012:

"All Commando tachometers are 4:1 ratio. All P11/N15/G15/Atlas tachometers are 2:1 ratio. BG1508-06 tach drive is 2:1 and used for 4:1 tachometer. BG1508-05 is 1:1 and used for all 2:1 tachometers.
Tachdrives are driven at cam speed (1/2 crank speed) so final ratio must be 2:1, thus a 4:1 tach needs to be reduce by a 2:1 drive, but a 2:1 tach is driven by a 1:1 drive."

Implication is that yes, tachometers were all 4:1, but two drives are/were available. Although my memory is always questionable, I had fitted a Commando tach to a '67 Atlas engine with the drive mounted on what would later become the timing points cover. The tach read double so I couldn't use it. I suspect at least some early '68 Commandos were fitted with the same drive, possibly evidenced by this post from bluesman3000 in 2017:

"Five years later...restoring my 1968 Commando, I found that the tach was reading very high. The drive gear on the bike was the BG1508/05, a 1:1 drive, while the tach is a 4:1. I got the correct 2:1 drive gear from Andover Norton and now it reads correctly."

Sorry for beating a nearly dead horse, certainly not important to the original poster........
 
I have three RG250 Suzuki tachos which are all 2 : 1. One is on my Seeley Commando 850, another on a Kawasaki two stroke. The third I have because I bought it for a friend who has a 750cc Triton and the ratio turned out to be completely wrong. With the Commando motor, what comes off the camshaft is a 2:1 reduction and the tacho head needs to be 2:1. On a pre- nit Triumph motor, the racing tacho drive runs off a small cog driven by the exhaust cam-wheel, the tacho head needs to be 4:1. However there are some Triumph tacho drives which are part of the timing chest and come out at the same 2:1 ratio as a Commando and the tacho head needs to be 2:1. With a Commando you could probably use a tacho head off a later Bonneville, but the direction it turns might be wrong. The RG250 tacho's direction of turn can be changed by simply undoing two screws on the back and turning the helical drive to the other side of the spline
 
Acotrel, I believe L.A.B. when he says all Commando tachs are 4:1. The most common tach drive is 2:1 (probably true for 99% of Commandos) but that is 2:1 driven off the cam which is 1/2 engine rpm. The 4:1 tach brings you to where you need to be.
When I ever get my Atlas engine running, it is equipped with a 1:1 drive, coming off the cam. I will have to find a 2:1 tachometer if I want one to show correctly. It's good to know there are certain Triumph units out there that I might be able to find in addition to possibly finding a 2:1 Atlas tach.
 
Atlas Commando said:
Acotrel, I believe L.A.B. when he says all Commando tachs are 4:1. The most common tach drive is 2:1 (probably true for 99% of Commandos)
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99.999% :) of all Commandos as even the early 20M3 model had (and are shown to have) a Smiths 4:1 RSM 3003/10 9000 RPM Green blob tach and therefore had a 2:1 (BG1508/06 reversing) gearbox (Norton part 060710), unlike the Atlas and P11 from the same period which had a 2:1 RSM 3003/00 tach and 1:1 (BG 1508/05 reversing) gearbox.

https://andover-norton.co.uk/en/shop-drawing/451/engine
Item; 136
https://andover-norton.co.uk/en/shop-details/15382/tachometer-drive-gearbox-2-1-c-w-gasket-screws


https://www.ebay.ie/itm/NOS-Smiths-...on-Commando-/163131806071?hash=item25fb69d177
 
RoadScholar said:
I have an assembled engine on a stand. I put a shaft into the tach drive with a tape flag and found that 1 engine turn equals 1/4 turn of the tach drive, so the 4:1 mentioned earlier is what the instrument sees. If you have an 1800 RPM drill that you apply directly to the instrument you should see 7200 RPM indicated.
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Thank you, sir!
 
All I know is I have an RG250 Suzuki Tacho which I believe is 2:1 ratio and my motor has not blown-up YET. Do standard Commandos have something between the output on the motor and the input at the tacho head ? Perhaps I have got another 7000 usable RPM ? - When my tacho says 7000 RPM, the Commando motor sounds as though it is doing 7000 RPM. I might need weaker throttle springs.
 
MY
Atlas Commando said:
Acotrel, I believe L.A.B. when he says all Commando tachs are 4:1. The most common tach drive is 2:1 (probably true for 99% of Commandos) but that is 2:1 driven off the cam which is 1/2 engine rpm. The 4:1 tach brings you to where you need to be.
When I ever get my Atlas engine running, it is equipped with a 1:1 drive, coming off the cam. I will have to find a 2:1 tachometer if I want one to show correctly. It's good to know there are certain Triumph units out there that I might be able to find in addition to possibly finding a 2:1 Atlas tach.
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I don't know about the reversing gearbox on the Commando. But the 2:1 ratio RG250 Suzuki tacho works fine ($50). My friend simply bought a small magnetic tacho of the right ratio off Ebay and mounted it on his Triton. Cheap and easy. For a while there, we were tearing our hair out trying to find the right drive etc. - not necessary and too expensive. I have about three chronometric Triumph tachos, one is 2:1 and the other two are 4:1. I think the 2:1 unit is off a late 60s 650 Triumph, the two 4:1 units are off earlier Triumphs. On the later Triumphs, the tacho drive is taken straight off the end of the exhaust cam with a slotted nut. I think the right angle gear box in the Triumph timing cover is the same ratio as the one on the cam of the Commando motor.
 
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