MGA Forum

Might anyone know the size of the o-ring for the tachometer drive on the engine?

I've tried the Moss "new style" seal and it's pretty tight so not happy with it. Believe I tried the speedometer drive seal too with the same result.

The original tach drive seal was an O-ring. The original speedo drive seal was a garter spring seal. Only different because one was from the engine designer/builder and the other was from the gearbox designer/builder. Otherwise the cavity dimensions are the same, and you can use the speedo drive seal in the tachometer drive (which is what I always do).

1G2398 - Seal, engine tach drive pinion (O-ring), MGA 1500, 1600, Twin Cam,
Also MGB thru 1964. Can use 2A3254 or NKC105A
Shaft=5/16" - Housing=9/16" - width=11/32"

2A3254 - Seal, GB pinion, speedo drive - MGA 1500, 1600, Twin Cam,
Also MGB thru 1967 -- Superseded by NKC105A
shaft=5/16" - housing=9/16" - width=11/32" - see list at:
http://mgaguru.com/mgtech/gearbox/gt106a.htm

If using an O-ring, it should be slightly more than 1/8-inch in thickness to have a little bearing force on the shaft. Width of the seal cavity on the tach drive is adjustable, due to an insert sleeve used to retain the seal (but should not hurt to leave it a little loose, side wise).

---

Barney Gaylord - 1958 MGA with an attitude - http://MGAguru.com - barneymg@mgaguru.com - Ph: 630-946-3841
(Please email me direct, do not leave a PM on the public server).

Barney,
When reassembling my engine, I don’t remember putting a seal in the tach drive housing. Maybe the old one was still there and I just didn’t notice it. It’s not leaking (at least not there). Should I just leave it alone?

Jim

If the cable attachment nut is properly tight, then they commonly do not leak externally at the cable end. However, a leaking seal can put oil into the cable housing where it has a tendency to move up the cable and find its way into the back of the tachometer. Oil can even work its way completely up the longer speedometer cable. The speedo drive is below oil level in the gearbox, so the seal is more important there.

---

Barney Gaylord - 1958 MGA with an attitude - http://MGAguru.com - barneymg@mgaguru.com - Ph: 630-946-3841
(Please email me direct, do not leave a PM on the public server).

Mine had the original o-ring in it which was hard as a rock.

The new style Moss seal is very tight on the shaft and is too tall to allow the use of the original spring clip.

I'll go o-ring shopping...

I found the issue with the "uprated" seal (460-670 or 461-445) not fitting my tach drive.

The tach drive shaft measures 3/8" so 1/16" bigger than expected. Housing size is 9/16". Engine is 18G.

Looks like a #8 o-ring would be the right size for this (9/16 in. O.D. x 3/8 in. I.D. x 3/32 in. wall).

Cute. Apparently you have a tach drive from a different engine, or it has been modified. MGB 3-main bearing engines should have the same tach drive parts as all MGA, with 5/16" pinion shaft and 9/16" housing bore. Sourcing original type parts might be easier than hunting for the non-standard seal.

---

Barney Gaylord - 1958 MGA with an attitude - http://MGAguru.com - barneymg@mgaguru.com - Ph: 630-946-3841
(Please email me direct, do not leave a PM on the public server).

Pretty sure it's original but who knows... anyway, o-rings are cheap and plentiful.

The #8 o-ring specs matches up with the measurements I took from the hardened o-ring I removed from the housing.

Look for O-ring Dash No. 110 for 3/8" shaft and 9/16" housing.
McMaster-Carr part number 9452K22 ($2.43 per 100)

---

Barney Gaylord - 1958 MGA with an attitude - http://MGAguru.com - barneymg@mgaguru.com - Ph: 630-946-3841
(Please email me direct, do not leave a PM on the public server).

Lowe's has them in the plumbing isle. Buna-N type rubber.

Model # 96725
#8 O-Ring (AS568B# 110)
9/16 in. O.D. x 3/8 in. I.D. x 3/32 in. wall thickness

Old o-ring on the left: