15 replies to this topic

[jiminwatford]

Just watching Twincam's youtube video where AC Dodd mentions that if the cam hasn't been changed the engine number can be used to identify the cam. Apparently some Metros were fitted with 998 cams

 

The engine is from a 1988 1.3 Metro Jet Black. The engine number is 12HD09 147873 

 

i haven't been able to decipher the cam using various online engine number decoders

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[timmy850]

Minispares list this:

12HD09  9.4cr  1988 SPEC 65DM4 DIZZY WIDE RATIO BOX

 

Although as AC says in the video it's always worth measuring as it could have been changed in the past +30 years

[Spider]

From the 1989 Factory Manual;-

 

 

[jiminwatford]

Thanks

I don't fully understand cams and how they are measured

 

I think AC explained it in the video but i found it quite hard to follow

Do either of these posts show if it has the 998 cam or the Metro one?

Edited by jiminwatford, 09 June 2024 - 07:22 AM.

[Spider]

Do either of these posts show if it has the 998 cam or the Metro one?

 

It should have a 1275 Metro Cam in it.

The 998 one has a shorter duration on the Exhaust.
 

[jiminwatford]

 

Do either of these posts show if it has the 998 cam or the Metro one?

 

It should have a 1275 Metro Cam in it.

The 998 one has a shorter duration on the Exhaust.
 

 

 

Great. Thank you

[ACDodd]

Engine numbers are pretty meaningless nowadays. Do a lift check, compare inlet to exhausts lifts. Since the 998a+ cam has a nominal 0.030” less lift on the exhaust lobes it’s pretty easy to check. In short the a+1275 cam has the same lift on both inlet and exhaust lobes the 998a+ cam does not.

Ac

[Spider]

Engine numbers are pretty meaningless nowadays. Do a lift check, compare inlet to exhausts lifts. Since the 998a+ cam has a nominal 0.030” less lift on the exhaust lobes it’s pretty easy to check. In short the a+1275 cam has the same lift on both inlet and exhaust lobes the 998a+ cam does not.

Ac

 

It's actually only 0.018" less, not always easily spotted by the untrained eye.

[ACDodd]

It’s 0.263 to 0.265” inlet list and 0.235” inch exhaust lift when measured at the lobe, which in my book is 0.030”,

Here is a reference to the 998 cam fitted in 1275 engines for the economy models of the 1275, from AKM 4694 (5th edition) 1985.


According to AKM 6139 (1988) 12HD09 is the same short ignition timing as that found in ED’s car from the video and uses 9.75:1 compression. Therefore it is most likely to be fitted with the 998cam. As I said above do a lift check to confirm.

Secondly your engine would also respond just as well as Ed’s did in the video to a decent tune up, assuming it is in good condition.

It’s also worth mentioning, that none of the manuals I have make reference to the European version of the mg metro cam having shorter duration than the uk version, but they do as do they have different advance curves in the dizzies to match.

Ac

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Edited by ACDodd, 09 June 2024 - 09:28 AM.

[ACDodd]

To sum up original engine numbers with the 998 a+ cam fitted can be as follows;


12HA08AA, 12HA60AA, 12HA61AA, 12HA62AA, 12HA72AA, 12HA80AA, 12HA86AA, 12HA87AA, 12HB09AA,
12HA71AA, 12HB36AA, 12HC06AA, 12HC04AA, 12HC05AA, 12HC02AA, 12HC03AA, 12HC07AA, 12HC01AA, 12HC08AA, 12HC09AA, 12HC10AA, 12HD09AA, 12HD10AA, 12HD11AA, 12HD12AA, 12HD13AA, 12HD21AA, 12HD22AA

As ever a dial gauge check must be performed to confirm.

It’s also worth mentioning that those engines after ‘84 that were fitted with the 998 cam also
Cam with the Big inlet valve MG metro head until the introduction of unleaded fuel.

Ac

Edited by ACDodd, 09 June 2024 - 09:51 AM.

[Spider]

This is the factory Manual I'm working from here, published in 1989;-

 

 

For completeness, these are the 998 engine's cam timing published figures;-

 

 

 

And it's lift figures;-

 

 

 

For completeness, the 1275 Lift Figures

 

 

 

I also have the manual you have there and found errors in it. The way it's put together, it actually appears a draft but somehow ended up being published.

 

 

[ACDodd]

Fact is spider I just went out and measured a 998 A+ cam and the figures I got was 0.245” lift on the exhaust lobe. Loose 0.010” for follower clearance and you get 0.286” is what I measured on Ed’s car. The factory figures are not in line with reality they are a design figure I believe and not what you actually see when checked on a real engine, which is why factory manuals are and can be misleading. Hence the real world isBased on measurements I actually see on a real engine. The point is engines are old and meaurements must be taken from what you have not assumptions from manuals. I admit I quoted yhe exhaust lift as 0.235” well it is if you take the follower clearance off the actual lift.
If you do the same to the inlet you get 0.253” x 1.22 ratio and you get 0.308” lift and not 0.318” as the factory stated. In Ed’s case we got 0.311” which is again less than factory spec due to tolerances wear and using the design ratio rather than a typical real world figure. If you have a lift jig it’s easy to spot the descrepancy in the figures, it’s the load from the valve springs when causes bending in the system when loaded. Using light springs for checking purposes usually adds 0.006 to 0.008” to those measurements and therefore you can see where the factory get there figures from.


This is of course all largely irrelevant and the fact remains that a large proportion of the factory 1275 engine from 1983 to 1989 used 998 cams.

AC

Edited by ACDodd, 09 June 2024 - 11:29 AM.

[Spider]

Fact is spider I just went out and measured a 998 A+ cam and the figures I got was 0.245” lift on the exhaust lobe. Loose 0.010” for follower clearance and you get 0.286” is what I measured on Ed’s car. The factory figures are not in line with reality they are a design figure I believe and not what you actually see when checked on a real engine,

 

Ummmm,,,,, the factory figures quoted are ummm,,,,, valve lift, not lobe lift.
 

[ACDodd]

Yes I’m not stupid.

Ac

[coopertaz]

is this measured at the top of the valve or the adjuster just for clarity and wishing to avoid the spat going on?