thread form for rockerbox bolts

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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thread form for rockerbox bolts

Postby clive » Thu Sep 07, 2017 9:12 pm

The heady steady bolts on my model 18 have failed. I want to fit helicoils and wonder if someone can tell me the thread form and size, I think its 5/16 and quite a fine thread despite going into aluminium. I feel an internet search for the helicoil coming on not to mention an order for Jampot spares for some bolts with better threads .
clive
if it ain't broke don't fix

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Re: thread form for rockerbox bolts

Postby Rob Harknett » Thu Sep 07, 2017 10:03 pm

Clive see this
Ashampoo_Snap_2017.09.07_22h51m48s_003_.png

if the bolts are 015355 those like all others have 22 tpi. 26 for the nut that holds the head steady bracket. These bolts are nr. £5 each + p&p from club
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Re: thread form for rockerbox bolts

Postby Andy51 » Fri Sep 08, 2017 6:47 am

5/16 x 22 tpi is British Standard Fine (BSF) size; helicoils readily available. Andy

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Re: thread form for rockerbox bolts

Postby clive » Fri Sep 08, 2017 8:01 am

Many thanks all, I have now been offered use of the correct helicoil kit.
Interestingly when I took the rockerbox off I found 2 of the hairpin springs were not sitting with their legs flat on the valve spring tray. This means the coil would be sitting against the rockerbox cover and might explain the rather noisy top end.
As to why, I think it must be that when I sorted through my pile of springs I choose the least worn without reference to the length of the legs. On reflection I am aware that when they changed the fixing method from a tray with holes in it to secure the spring legs to the open tray they also increased the length of the spring legs. I did not check, resolution will probably be to fit the earlier type of tray which requires the half moon compressor to fit the springs.
Still my bitsa, sorry special, made it to Poland and back despite all the mismatched bits.
clive
if it ain't broke don't fix

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Re: thread form for rockerbox bolts

Postby Rob Harknett » Fri Sep 08, 2017 10:07 am

I am sure I read somewhere springs must be fitted the correct way round or they will hit the rocker box roof. I never knew about the changes you mention Clive. Back in 1956 onwards I had to use the half moon tool to get springs out and in. On the same 55 G3LS model I now have, I can pull out springs by hand and shovel them in with the other tool. If you ever pick up a set of hairpin springs that appear a bit different and have very long prongs. You may have picked up a very rare set of prewar AJS cammy or Matchless OHV hairpins, of which there were different types.

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Re: thread form for rockerbox bolts

Postby 56G80S » Fri Sep 08, 2017 1:04 pm

I'm a bit late onto this, Clive, but I did helicoil (didn't use brand Helicoils but a generic kit). All went well but read instructions with care and follow them exactly. I found that it was important to leave the inserts a bit short of the top of the thread, this allows the end to catch and not unwind when the bolts are removed. I also took the opportunity to use a small rose to open out the entry for the rocker box bolts into the head to give an expansion route for the gasket around the bolts. Use the right length inserts and they don't have to be all that long as they are steel versus the original thread in aluminium.

Take absolute care in drilling with the supplied correct drill for the supplied tap and getting all properly right angled to the surface of the head. If using a pillar drill don't forget to pack for the bulge at the exhaust pipe. Remember to knock out the tang at the bottom of the insert once it's in.

I didn't do it quite perfectly and a couple tighten up towards the bottom of travel but an additional washer under the bolt head was all that was required.

I didn't get all new bolts but did run a die down the old ones and they tighten up fine. I'll be putting in the new valve lifter cable a bit later and will confirm the die size for you.

Good luck.

Johnny B

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Re: thread form for rockerbox bolts

Postby Mick D » Fri Sep 08, 2017 4:39 pm

Hi

If you're helicoiling blind threads you need to use a plug tap to finish the helicoil thread, otherwise you will find the threads tighten up towards the bottom as observed above.

A touch of Loctite is beneficial when fitting the wire threads.

Regards Mick

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Re: thread form for rockerbox bolts

Postby 56G80S » Fri Sep 08, 2017 7:12 pm

Mick

I just used what came in the box - wished I'd had your comment beforehand. But it all works fine.

Just back from 25 mile run with the relined barrel and new p - rocker box not leaking yet!

Clive - forgot all about checking the die.

Johnny B

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