• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Rotor arm woes !

Share this page

A

AndrewMawson

Jedi Trainee
Offline
Out for a spin with my daughter (honest!) nasty noise like a crackling EHT lead - engine dies. Coast to side of road - leads ok but lifting the distributor cap find the brass bit of the rotor arm lying loose on the points and fine brass dust on the base plate. Clean it all up, fit the spare (previously put there having read about them failing by tracking here on the list).

This is a 'rebuilt' distributor that's done less than 100 miles from a reputable UK dealer - the rotor is an aftermarket one and may well be the sole cause of the problem but when I check the play on the driving spindle at the cam there is descernable movement - could it have wobbled enough for physical contact between the rotor and the terminals to knock the brass bit off - maybe though I understand that there is 33 thou clearance.

One puzzle - the brass rotor rivet has completely vanished - gone - vamoosed.

How much shaft play is acceptable ?
 
HI Andrew, if you have a timing light start the engine let it idle and see if the timing is steady and let us know of your results.---Keoke
 
[ QUOTE ]
find the brass bit of the rotor arm lying loose on the points and fine brass dust on the base plate. Clean it all up, fit the spare (previously put there having read about them failing by tracking here on the list).

This is a 'rebuilt' distributor that's done less than 100 miles from a reputable UK dealer - the rotor is an aftermarket one and may well be the sole cause of the problem but when I check the play on the driving spindle at the cam there is descernable movement - could it have wobbled enough for physical contact between the rotor and the terminals to knock the brass bit off - maybe though I understand that there is 33 thou clearance.

One puzzle - the brass rotor rivet has completely vanished - gone - vamoosed.

How much shaft play is acceptable ?

[/ QUOTE ]
Hi Andrew,
It's pretty obvious that there was contact between the rotor end & the cap terminals. Yes, the rotor could have wobbled "enough". The .033" clearance actually varies a lot. Not all caps have their terminals in the same locations, not all rotors have the same center to rotor tip length. The rotor runout/play/clearance is affected by several things. The shaft top bearing, clearance between the cam & it's spindle, & obviously a bent cam spindle.There should be no discernable lateral movement of the cam in relation to the distributor body. The rebuilder obviously missed something. Clearance between the cam & it's spindle is common if the joint has not been regularly lubricated by putting a drop of oil on the top of the spindle under the rotor.
D
 
[ QUOTE ]


HI Andrew, if you have a timing light start the engine let it idle and see if the timing is steady and let us know of your results.---Keoke

[/ QUOTE ]

It certainly was steady at idle before the problem - I set it yesterday at 10 degrees static by measurement off the pulley, and then tweaked a bit to get 15 degrees on the strobe at about 600 rpm. Not tested it since and would be unpopular doing now as the suns gone down this side of the pond - I'll give it a try tomorrow.
 
[ QUOTE ]


HI Andrew, if you have a timing light start the engine let it idle and see if the timing is steady and let us know of your results.---Keoke

[/ QUOTE ]

Just strobed it again - no perceptable flutter.

I blued the edge of the rotor arm with an indelible marker and ran the engine again to see if there was any brushing contact with the terminals - none at all on this rotor, which is a genuine Lucas one.
 
Andrew,
Sounds like you are OK probably just got hold of a bad rotor. The brass rivet is probably inside under the base plate take out the Dizzy turn it upside down and see if you can shake it out.-Cheers--- Keoke
 
[ QUOTE ]
none at all on this rotor, which is a genuine Lucas one.

[/ QUOTE ]

Andrew,
There is no such thing as a good genuine Lucas rotor. Maybe a "lucky" rotor. The Green Box Lucas rotors are notorious for being bad. See this topic;
https://www.britishcarforum.com/ubbthreads/showflat.php?Cat=&Board=UBB1&Number=64536&Forum=,,All_Forums,,&Words=&Searchpage=1&Limit=25&Main=64498&Search=true&where=&Name=888&daterange=&newerval=&newertype=&olderval=&oldertype=&bodyprev=#Post64536

You first said that the cam has discernable side play. If it does, you are waiting for more trouble. The fact that there is no contact now doesn't tell you how close to contact it is.

I think you are relying on "luck" more than I would care to.
D
 
[ QUOTE ]
[ QUOTE ]
none at all on this rotor, which is a genuine Lucas one.

[/ QUOTE ]

Andrew,
There is no such thing as a good genuine Lucas rotor. Maybe a "lucky" rotor. The Green Box Lucas rotors are notorious for being bad. See this topic;
https://www.britishcarforum.com/ubbthreads/showflat.php?Cat=&Board=UBB1&Number=64536&Forum=,,All_Forums,,&Words=&Searchpage=1&Limit=25&Main=64498&Search=true&where=&Name=888&daterange=&newerval=&newertype=&olderval=&oldertype=&bodyprev=#Post64536

You first said that the cam has discernable side play. If it does, you are waiting for more trouble. The fact that there is no contact now doesn't tell you how close to contact it is.

I think you are relying on "luck" more than I would care to.
D

[/ QUOTE ]

Dave,

Yes it was that thread that prompted me to carry a spare!

I've just done some tests on the three rotor arms that I have intact using the DRO on my Bridgeport to measure the swept arc of the rotor tip - amazing variation:

used my 'original' Lucas one in the car when I got is as the reference against two aftermarket ones:

A/ plus 10 thou on the arc ie extra 20 thou on diameter
B/ plus 18 thou on the arc ie extra 36 thou on diameter

Now it's most likely that the Lucas on has significant wear but as I've personally driven with it for several hundred miles on my old distributor it's probably 'ok'.

I carefully took the aftermarket ones down to the same dimensions on a pattern makers disk sander, blued them and checked for interferance with the terminals on the cap - none thankfully.

Measuring the play in my 'rebuilt' distributor and the original (which may have been rebuilt at some time) they both come out almost identical at a max TIR (total indicated runout) of 8 thou, and that using heavy thumb pressure to bias them both ways measured on the true circle just above the cam.


XXX (better not name them!) have agreed to replace the rebuilt one when they have them back in stock but there seems no point in putting back the old one as they measure the same
 
Since I was the guy with the other post, I thought that I would add my two cents.

What is happening to my rotors is that the spark is working its way through the casing and grounding on the distributor drive. I cannot find a crack (even under a magnifying glass), but I can find what appears to be a tiny hole where the spark has burned through.

I do not know if it is related to the rivet in any way.

What I have been told is happening, and it would explain the problem I am having as well as the rivet problem, is that there is a problem with the strength of the compound that the arm is built with. This is exaggerated by heat, and as it weakens it can fail by either of the two ways that we are documenting.

The patch using dielectric grease along with a thin rubber barrier underneath my rotor is holding so far, but I do not consider it a permanent fix.

UK Healey (https://www.ukhealey.co.uk/) claims that they have commissioned a reproduction of the larger arm original style rotor. Here is what they replied to me when I asked them about their experience with their product.



[ QUOTE ]
We sourced a new manufacturer about two years ago who make a copy of the original Lucas design. Since then we have sold 800 and only had one fail. That was damaged and broke up! We don't have the insulation break down problems with them.

[/ QUOTE ]

I ordered two of them and the cost plus shipping came to around $15.00 for both. A little high for a rotor arm, but I have $12 in dead crappy ones.

I expect them here any day, and I will update with what I get, plus I will put something up on my website with pictures. If anyone has any good pictures of one of the rotors with a disintegrating rivet or separating arm I would appreacite it so I can include it on the page.

If you haven't guessed, this is rapidlly becoming a crusade for me.

Thanks
Patton
 
[ QUOTE ]

I've just done some tests on the three rotor arms that I have intact using the DRO on my Bridgeport to measure the swept arc of the rotor tip - amazing variation:

used my 'original' Lucas one in the car when I got is as the reference against two aftermarket ones:

A/ plus 10 thou on the arc ie extra 20 thou on diameter
B/ plus 18 thou on the arc ie extra 36 thou on diameter

I carefully took the aftermarket ones down to the same dimensions on a pattern makers disk sander, blued them and checked for interferance with the terminals on the cap - none thankfully.


[/ QUOTE ]
Andrew,
Excellent - You didn't give many details in a previous reply, so I wasn't sure of your measuring skill/instrument capability. Very high, I would say. I did much the same on my lathe, machined a mandrel to fit the rotors, found much the same as you & used the same remedy.

Imagine what this amount of variation, not even considering distributor shaft runout, combined with the fact that the cap contact spacing can vary from cap to cap, would do to the .033" design clearance. I haven't bothered to find the exact cap terminal ID to see how much variation there actually is or to calculate the specific rotor to cap clearance.

I think a fair number of rotor dilectric failures may be percipitated by rotor to cap contact stressing the rotor. Another factor is that if the rotor fits too tightly on the shaft, the expansion stresses on the rotor may cause dilectric failure. Some rotors are a bit undersized on their ID.

Maybe Patton will find a source of "precision" rotors.
D
 
Further "Google Research" has found referance to a Landrover dealer:

https://www.land-roverco.com/tech/genuineparts.htm

who has done some research on genuine new original Lucas distributors and found the shaft play to be similar to that I've measured on mine at just under 10 thou.

I'm tempted if I can find an old dizzy body to have a go at installing needle roller bearings instead of the sintered iron bush originally fitted to the 25D6, and see if I can get the side play below the figure of 1.75 - 2.25 thou quoted on that site for their version of a rebuild.

Sad when you have an obsessive personality - these things grab you /ubbthreads/images/graemlins/savewave.gif
 
[ QUOTE ]

I'm tempted if I can find an old dizzy body to have a go at installing needle roller bearings instead of the sintered iron bush originally fitted to the 25D6, and see if I can get the side play below the figure of 1.75 - 2.25 thou quoted on that site for their version of a rebuild.

[/ QUOTE ]
Andrew,
The original bronze bushings work well for a long time if properly fitted. Moss has a replacement bushing for the four cylinder distributors, part # 153-100. It is .490 ID X .752 OD x 2.472 LG. Of course it has to be fitted. I don't know how well it would adapt to your distributor, but maybe worth investigating. I do know that my distributor has less than .002" side play as measured by a dial indicator. You do need a little clearance.
D
 
Back
Top