Merco 61 bolt sizes?

[Eddie Clanzy-Hodge]

Does anyone happen to know the thread size of the cylinder head and backplate bolts on a Merco 61 Mk II?

According to Harry Brooks' REB plan it shows a Merco 49 and says to rear mount it with 4BA allen bolts. My Merco 61 does not have either 4 or 6BA threads. I have just tried it.

I was going to replace the Philips screws with allen bolts.

[roger graves]

I dpm't recall any Merco using phillip head screws.

Perhaps someone has recut the threads.

[PatMc]

I think they are 1/8 Whitworth. I once had to destroy the phillips head screws of a Merco 61 (twin plug) to get them out. Replaced them with 4BA allen bolts after drilling & tapping the crankcase.

Edited By PatMc on 05/11/2017 16:05:54

[Jonathan W]

The original screws in the Merco are 1/8" Whitworth, which you will struggle to obtain. 5-40 UNC is a very good direct substitute. You can obtain these from Model Fixings

5-40 UNC is same diameter and pitch as 1/8" Whiit, Only difference is a 60° thread form instead of 55°. This will make no practical difference.

I'd recommend avoiding BA threads these days, as supply of the fixings is very limited. Also, it's not a good thread form for use in aluminium, very easy to strip.

[Eddie Clanzy-Hodge]

For Jonathan W

Thanks for the advice - it makes sense.

I will try Model Fixings since I have used them before.

Do they have an equivalent for 6BA? I also have a Merco 35 which I have just cleaned up. It looks to be almost new with very little burnt caster. This uses 6BA screws.

[Jonathan W]

Hi Eddie, no problem.

If the 6BA threads in the Merco 35 are still in good condition but you want to replace with cap screws, you can try HERE.

If you wish to do away with the 6BA entirely, you could try tapping them out to metric M3. However, if the BA threads remain sound, I don't think it would be worth the trouble doing that.

[Eddie Clanzy-Hodge]

For Jonathan W

The Merco 6BA threads are fine. I haven't got the tools to tap them out accurately so will try your link..

Thanks again for good advice.

[Eddie Clanzy-Hodge]

For Jonathan W.

I have tried the 5-40 bolts and they do fit OK as you said.

I also managed to get the 6ba allen bolts from the web.

So, thanks for the good advice.

Now I am looking at the silencer bolt threads. They look as if they could be 3/32 Whit assuming Merco used all Whit sizes on the 61. A 6ba bolt will start to grip but it is just too big.

Would a 3-48 UNC fit? It is about 5 thou bigger than the 3/32 Whit but has same TPI. Is that too much to get it in the thread?

If I cant get the silencer bolts then I can resort to a silencer with a strap.