Tweaking a G80...

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
Member
Posts: 162
Joined: Sun Apr 30, 2017 9:42 pm
Location: ARGYLL UK

Re: Tweaking a G80...

Postby Stephen G80 » Tue Nov 21, 2017 9:28 pm

so, if one was thinking of these mods re the cams etc, and the engine was due for a re-build anyway, what mods can be done to the big end bearings, ie with a bit of machining can they all be upgraded to bullet proof rollers - is there a standard mod for this?

Member
Posts: 320
Joined: Tue Mar 15, 2016 8:51 pm
Location: Bedfordshire, UK

Re: Tweaking a G80...

Postby Pharisee » Tue Nov 21, 2017 11:27 pm

raffles wrote:All the comp engines use a roller bearing on the inside on the drive shaft.
Tony


Hmm... Just looked at the parts list for (example) the '55 heavyweights and the same big end bearing and drive side main bearing ball races are specified for all engines. There's no mention of a roller bearing for the 'C' variant.


Image
Image

Member
Posts: 41
Joined: Tue Mar 29, 2016 6:19 pm
Location: CALVADOS FRANCE

Re: Tweaking a G80...

Postby Dave T » Tue Nov 28, 2017 3:36 pm

Hello everyone. I have been following this thread for a while and have taken the advice to heart. My G80C had an old SS inlet cam in it,(exhaust is H) and the bike ran but not really zipilly (if that is a word). When I got the boxes of bits that came with the partially dismantled bike there was another pair of H cams but a tad rust marked so I kept the SS. I have now changed the inlet for the H and set the timing on 3. What a *@#§¤ing difference. It accellerates, it ticks over, it rides much more like the big thumper I had hoped for! Thanks enormously everyone for giving me this info. I'm not worried about the bottom of the engine, my piston is flat topped seven and a bit compression so I think my bearings should hold up.

The only snag is I'll have to wait 'til spring now to enjoy the bike. Hmm, might fit a 22 tooth engine sprocket over winter to use the extra ponies now available, hmm?

Again, thanks. Dave.

Member
Posts: 162
Joined: Sun Apr 30, 2017 9:42 pm
Location: ARGYLL UK

Re: Tweaking a G80...

Postby Stephen G80 » Wed Nov 29, 2017 10:55 am

Raffles/anyone - would these be the correct rocker arms for this mod (presuming they have been listed correctly as G2 250, seeing as they have been listed as "followers"!).

https://www.ebay.co.uk/itm/USED-CAM-FOL ... Swe9dZowlv

Member
Posts: 506
Joined: Mon Jan 01, 1990 12:00 am
Location: LANCASHIRE UK

Re: Tweaking a G80...

Postby zwarts » Wed Nov 29, 2017 12:44 pm

These are correctly listed as cam followers, they are not rocker arms so are not the items you are looking for.

Member
Posts: 162
Joined: Sun Apr 30, 2017 9:42 pm
Location: ARGYLL UK

Re: Tweaking a G80...

Postby Stephen G80 » Wed Nov 29, 2017 1:50 pm

ah yes getting confused with what an overhead cam rocker looks like. OK got it - ta.

So the 350 or250 light weights rocker assemmblies needed will consist of mainly 3 parts - the arm which the push rod end goes into, the spindle and the arm at the other side that pushes the valve down. Do I need both arms (presuming the spindles are the same) or just the PR end arm or the side arm valve?

Member
Posts: 320
Joined: Tue Mar 15, 2016 8:51 pm
Location: Bedfordshire, UK

Re: Tweaking a G80...

Postby Pharisee » Wed Nov 29, 2017 6:23 pm

This is the only part you need... the lightweight pushhrod rocker arm. It looks the same as the heavyweight but it's about 1/8" shorter. You only need to change the inlet. You don't need to change the exhaust as there is very little to be gained by doing so but you can do so if you wish. Don't change the rocker arms at the valve end of the shaft.
You do not have the required permissions to view the files attached to this post.

Member
Posts: 162
Joined: Sun Apr 30, 2017 9:42 pm
Location: ARGYLL UK

Re: Tweaking a G80...

Postby Stephen G80 » Wed Nov 29, 2017 9:00 pm

Thank you Pharisee - that's all clear now :-) . Just in case anyone else is thinking of doing the same thing this is the part number.

Capture.JPG
You do not have the required permissions to view the files attached to this post.

Member
Posts: 162
Joined: Sun Apr 30, 2017 9:42 pm
Location: ARGYLL UK

Re: Tweaking a G80...

Postby Stephen G80 » Wed Nov 29, 2017 10:16 pm

Pharisee - just because my brain always needs "closure", am I correct in thinking that one would not change the valve end rocker because it would negate any additional lift afforded by the shorter arm on the input side of the rock, so to speak, mess up the geometry of rocker/valve stem and, finally, their might not be enough scope in the push rod adjustment to take up 1/4" slack?

Member
Posts: 320
Joined: Tue Mar 15, 2016 8:51 pm
Location: Bedfordshire, UK

Re: Tweaking a G80...

Postby Pharisee » Wed Nov 29, 2017 11:51 pm

Stephen G80 wrote:Pharisee - just because my brain always needs "closure", am I correct in thinking that one would not change the valve end rocker because it would negate any additional lift afforded by the shorter arm on the input side of the rock, so to speak, mess up the geometry of rocker/valve stem and, finally, their might not be enough scope in the push rod adjustment to take up 1/4" slack?


Yes... the idea is to change the ratio between the push rod rocker arm and the valve rocker arm. That way you get a little more valve lift for the same movement of the push rod. As the top end of the push rod will be a little closer to the valve, check that the rod doesn't rub against the top edge of the push rod tube, particularly if you're using pattern replacement tubes. These can sometimes have a distinct 'narrowing' at the top edge. It may need to be filed a little to provide the necessary clearance.

PreviousNext

Return to G80 / Model 18

Who is online

Users browsing this forum: No registered users and 6 guests