M20 500cc Twin Cylinder Heads

Arti51 · Post by **Arti51** » Thu May 01, 2008 10:09 am

Gents,
I am in the process of rebuilding my 1951 M20 AJS spring twin and have found a possible problem with the Cyl. Heads. In a engine rebuild book by F.W. Neill he states that the oil feed passages in the cyl. heads are metered by brass plugs. Three of these plugs are used on engines up to 1953, from then on four were used.
Now my cylinder heads do not have any plugs at all.
Q 1. With no plugs what would be the outcome here?
Q 2. Do I need to install plugs to the heads? If so what would be the dimension so I can fabricate them.
Kind regards
Arthur

paul knapp · Post by **paul knapp** » Thu May 01, 2008 4:42 pm

G'day Arthur, without the plugs to restrict the oil flow, I think the rocker box's would be flooded with excess oil.
I can't give you the dimensions, but the flats on the plugs are quite small and appeared to be inadequate.
Without any plugs in the oil passages, you could find that the rocker that was meant to have no plug in the oil gallery may be starved of oil as the oil takes the easiest path in the unplugged gallerys.
The plugs were put there for a purpose!
Do you have the correct heads for the year? they should be the small fin, sand cast ones, not the die cast, bigger fin ones which appeared a year or so later.

Paul

Rick.Edwards · Post by **Rick.Edwards** » Thu May 01, 2008 9:21 pm

And four bolt rocker box covers.the metering jets where discontinued for a time then reissued 1956 on i think.

CR · Post by CR » Thu May 01, 2008 9:43 pm

As stated, two brass plugs were used on this model to meter the oil supply for both rockers on the left hand cylinder head. A similar plug is used for the inlet rocker only on the right hand cylinder head. The oil passage for the right-hand exhaust rocker is NOT plugged as it's at the end of the feed line.

I've previously bought the plugs from Jampot Spares Limited. Suggest you look on the JSL website for the part number (search on 'metering plug' perhaps).

Hope this helps.

Biscuit · Post by **Biscuit** » Thu May 01, 2008 9:46 pm

I have just dealt with a pair of early heads, the casting was terrible. Fitted new metering plugs and they had holes and not flats.

Re the search of Jampot spares, it likes it simple just enter 'jet' Part number is 018890.Edited by - Biscuit on 02 May 2008 08:51:22 AMEdited by - Biscuit on 02 May 2008 09:25:42 AM

wilko · Post by **wilko** » Fri May 02, 2008 1:00 am

I've found the plugs lurking in the barrels sometimes, which would still give the same effect. Where's Don on this one?? Dugless

Arti51 · Post by **Arti51** » Fri May 02, 2008 2:35 am

I like to thank you (those who are blessed with the knowledge) for the replys. To Paul, I was thinking with no plugs of flooding LH Head and starving the RH. Seeing as I am a novice here I think the heads are big fins.
To CR, checking the JSL list I find P/N.018890 Metering Jet Cylinder Head Oilways. Is this the correct part?
To Buscuit, Did you install the above plugs and did you use 3 or 4 plugs in the heads? Also did you modify the oil distributor bush (enlarge the oil drilling from 3/64 to 1/16 or larger?
To Wilco, there is no plugs in the barrels.

Don Madden · Post by **Don Madden** » Fri May 02, 2008 3:40 am

Who summoned my from the grave? Was it Dougles or Douglas?

The oiling system configuration on twins is a thorny question as they seemed to have changed something every year, often reverting to prior setup.

As already mentioned, they started with the 3 plugs with flats with none for the R/H exhaust. For 1954 they used drilled holes instead of flats & 4 were fitted. The plugs were not fitted in 1959, but they went back to them in 1961.

If that is not enough, they revised the oil pressure relief, filtering, rocker arm oiling & crankcase venting system as well during production with some reverting.

I need a drink!

Cheers, Don.

Biscuit · Post by **Biscuit** » Fri May 02, 2008 9:46 am

Four were fitted when I received the heads. They were removed because every thing was full of what appeared to be grit. Four new ones were fitted.Edited by - Biscuit on 02 May 2008 09:26:44 AM

Groily · Post by **Groily** » Fri May 02, 2008 2:16 pm

I know nothing of pros and cons of holes, flats, 3, 4 or no plugs, but can confirm my '54 heads have plugs all round, with 'oles not flats, always have had and have never caused any grief. Never replaced a rocker shaft, pushrod cup nor a valve guide. I've looked at them enough times lately to be on tediously intimate terms. Luckily they're now out of sight again after a spot of gasket bother, now resolved.