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MG TD TF 1500 - I need a knock sensor

1951 MD TD original XPAG engine

I'm getting the dreaded gear shift rattle at about 3500 rpms. I've got Len's roller cam, 1 1/2" H4s, bigger valves, .120 " overbore, and about a 8.5 compression ratio (calculated). I'm running premium, 1st tier gas. And I'm using a pertronix LS 146 EI. The car runs great, starts instantly, pulls like a racehorse starting at at about 3000 rpm up to 5,500 rpms (my self imposed upper limit) and no overheating. But the gear shift rattles past 3,500 rpms. I know some people discount the rattle as an indication of too advanced timing but to me that is the only indication I've had of pre ignition. Never heard pinging. Of course the rattle could be just the way the sympathetic vibrations act in an old car. But considering the possible damage that could be caused I'd like to know if it is simply vibration or advanced timing issues.

So, has anyone come up with a method of detecting engine knock on an XPAG other than the shifter vibration?

Timothy Burchfield

I've never seen any convincing evidence that gear shift rattle indicates engine detonation. There are a lot of things vibrating in an old car, even if you've done your due diligence balancing everything possible. Completely different gearbox of course, bur my TC gets a bit of a rattle around 4500 even on light throttle, so it can't be knocking. It didn't used to do it but the stick has loosened up a bit, as have the internals of the gearbox of course. I've never seen an engine of this age that you couldn't feel vibration in the shift lever as you drive down the road, so it has never worried me. MGAs and Bs do the same thing, and it's often cured by replacing the plastic cup underneath the lever. A lot of cars get body panel or other parts "buzzing" at idle for the same reasons. No harm in fitting a sensor of course.
Steve Simmons

I answered in the other forum, but check your ignition advance. Sometimes if it is too far advanced, a gearshift rattle can ensue.
Lew Palmer

As Lew said, check the timing. Had this on my TD, others in the club have had the same issues, Timing is one of the answers.
TRM Maine

Tim, I'm curious as to whether your shift lever housing has been re-bushed or is it still as original?
Richard Cameron

That is my thought, is my gearshift worn or timing too far advanced?

P G Gilvarry

Richard, I added bushings to the shift rod in the remote tower and the spring and ball in the end of the rod have been replaced.

Peter, that's the issue, one or the other. I'm trying to find a way to distinguish between the two. I've retarded the timing somewhat and the rattle moved up 50 rpms or so. But that may be because the car isn't accelerating with as much power. I thought if there was a simple way to install a knock sensor it would be a benefit now and in the future.

Timothy Burchfield

I installed a Knocklite some years ago on our supercharged TD (seldom more than 4psi boost). It runs on regular and never showed any symptoms of knock probably due to low compression ratio. Gear shifter rattles like crazy, though.

Also got a cheap Bosch knock sensor to hook up to the knocklite and bolted it behind the block down at the unused (middle) starter bolt hole.

Jim, that is encouraging. Some rattles are just rattles. But because the XPAG engine doesn't ping like other engines when over advanced it is less easy to ignore the rattle. Thanks for link.

BTW, where and how did you mount the sensor? Did you use the Bosch knock sensor?

Timothy Burchfield

Jim, it appears the Knocklite is no longer available. Sorry for asking the questions you had already answered. I thought I had edited the post to remove them. Probably forgot to hit Submit.

Timothy Burchfield

Tim, remember the checklist :
If your timing is correct il the plugs are OK, the head is polished and decocked, then there is no pre ignition nor detonation. Good idea to confirm that with a knocking sensor.
But most of the time something is vibrating at a certain frequency. And all your efforts should be focused on finding what is vibrating to fix it.
Of course, if you reduce the vibration generator (the XPAG itself) by reducing its power and its torque the passive vibrator will also decrease or disapear but this is only a cure, not the cancelling of the real culprit.

LC Laurent31

Or is the spring weak or broke?
W A Chasser

Laurent and Bill, the real problem is that there is, apparently, no way to distinguish between pre ignition and gear shift rattle on a TD. That's why I was thinking of a knock sensor. On every other car I've had it was quite easy to tell when your timing was too far advanced. But the TD (perhaps the TC and TF as well) really doesn't make the sound usually associated with pre ignition. A couple of people have stated that they have been driving with the "rattle" for some time with no disastrous results. I guess that is what I'll do.

BTW the two distributors I've used in the TD have
normal, stock advance curves as shown by my SUN Distributor Machine.

Timothy Burchfield

This thread was discussed between 26/08/2019 and 30/08/2019

MG TD TF 1500 index

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