G11 Upgrade of Oil Pump

Information relating to the Matchless G11 or AJS Model 30 600cc twin
Don Madden
Member
Posts: 2882
Joined: Mon Jan 01, 1990 12:00 am
Location: USA USA

G11 Upgrade of Oil Pump

Post by Don Madden »

The G11, which has the shorter stroke, usually do not have the failure rate of the 650cc but the sound he describes is typical of big end failure. If so, I would opt for the allen head through bolt mod. Those who ride their twins gently & change oil frequently have few problems, but I had frequent big end failures in my CSR until the bolt mod. Cheers, Don.
JEAN-NOEL
Member
Posts: 601
Joined: Wed Oct 12, 2005 10:45 pm
Location: FRANCE

G11 Upgrade of Oil Pump

Post by JEAN-NOEL »

Don, what are you replacements bolt ? Length, tensile, et c. Is it a sufficient cure ? What do you think about the new conrods CNC machined ?.
User avatar
MartinAmes
Member
Posts: 62
Joined: Thu Apr 21, 2011 12:30 pm
Location: WEST MIDLANDS UK

G11 Upgrade of Oil Pump

Post by MartinAmes »

Don,
Can you explain what the allen head through bolt mod is, and what it entails doing.
Don Madden
Member
Posts: 2882
Joined: Mon Jan 01, 1990 12:00 am
Location: USA USA

G11 Upgrade of Oil Pump

Post by Don Madden »

The newly made CNC rods or Carrillo are the best, albiet expensive. The through bolts are stronger than the original fitment but not as good as the new ones.

It entails removing the studs & retaining pins & drilling through the rod & trunnion all the way to above the shoulder. The new bore must be countersunk at the top to allow an allen head capscrew to be inserted from the top that has the same profile at the bottom as the original studs. The protruding edge of the capscrew head is then ground to allow clearance in the cases. This holds the cap in compression, as most other designs, instead to tension as the original AMC. I have seen the mod but not done it myself & do not have one to hand to measure. It is a job for a good engineering shop. Cheers, Don.
zwarts
Posts: 514
Joined: Mon Jan 01, 1990 12:00 am
Location: LANCASHIRE UK

G11 Upgrade of Oil Pump

Post by zwarts »

Don, I find myself confused on this. If you drill up through the trunnion and rod and then counterbore back down to the trunnion, how is this any different to the original setup? Other than you now have a new high tensile bolt retaining the cap, of course. In the original design, the cap is retained under compression, but the portion of the rod above the trunnion could be considered as under tension as the rod accelerates away from its stationary point at TDC. My own view is that rod ovality (at the big end eye) is more likely to be a consequence of age and use rather than an inherent design flaw. The trunnion method of stud retention has one big advantage over the more common design as used by every other manufacturer - it avoids the corner/change in section, where the head of the cap bolt is located and thus a potential stress failure point. The downside is that stud replacement which should be done whenever the caps are removed, gets overlooked as it is not as easy as just pressing out and replacing the bolts with new items. Stretched/overused studs could well be another reason for ovality in the big end eye.
Bob.
Don Madden
Member
Posts: 2882
Joined: Mon Jan 01, 1990 12:00 am
Location: USA USA

G11 Upgrade of Oil Pump

Post by Don Madden »

The section of the rod at the trunnions is very thin & I've seen some with the trunnion bores oval as well. I think the design sucks, BSA, Triumph, Norton & problably all autos use the through bolt method without problems. Aluminum works well under compression but is much weaker under tension. They also use steel caps, except for BSA who finnally went to steel in the last year or so of production. The Triumph plunger pump just pulses a shot of oil now & then, about like our singles, but has no problem with the big end. Do what you wish with your engines. Cheers, Don.
zwarts
Posts: 514
Joined: Mon Jan 01, 1990 12:00 am
Location: LANCASHIRE UK

G11 Upgrade of Oil Pump

Post by zwarts »

As far as I can see, the metal between the trunnion and the cap retaining nut IS under compression. Tightening the con rod stud nuts ensures that it is so. As I bet you are well aware, Triumph/Nortons/BSA rods are far from immune to failure!!! The Triumph oil pump, whilst being a plunger like our beloved single pumps, is actually quite different in operation. Being driven from the inlet camshaft, its pumping rate is considerable higher than the AMC plunger.
Don Madden
Member
Posts: 2882
Joined: Mon Jan 01, 1990 12:00 am
Location: USA USA

G11 Upgrade of Oil Pump

Post by Don Madden »

I was at Ed Crowell's to pick up my Typhoon crankshaft that he rebuilt. While there I took pics of his mod to replace the studs with allen head capscrews. The pics are of different perspectives & along with a standard rod.

It is clear to me that that when the rod travels upward the mass of the main part of the rod, above the cap joint with piston, pin, etc are pulling at the trunnions & held by the stud nuts. This applies TENSION to the section of the rod adjacent to the trunnions where it is thinnest. I have seen several rods where the trunnion bores were pulled by TENSION oval by the load. Oh, yeah! My aged brain just reminded me that I have an example hanging in my shop, It is a rod PULLED apart by the forces that I mangled with my CSR. Tomorrow I'll take a photo & post in the photos section. I have already posted the three of the mod rod under "Parts".

Cheers, Don, the California Curmudgeon.
zwarts
Posts: 514
Joined: Mon Jan 01, 1990 12:00 am
Location: LANCASHIRE UK

G11 Upgrade of Oil Pump

Post by zwarts »

Thanks for posting photos Don. Am I right in thinking that the counterbore for the head of the cap screw goes down to the trunnion, and that the head of the cap screw actually 'sits' on the trunnion with this mod?
Sorry to disappoint you but I find none of this 'curmudgeonly' - it is too interesting for that. I will add my analysis on where in the cycle compressive and tensile forces occur in the con rods later. If you have seen rods with loose trunnions, there is obviously a potential problem there. Of course, all this is largely irrelevant when good quality new rods can be sourced from at least 3 suppliers. We are all running around with 50 year old alloy rods in our twins and sooner or later they are going to give trouble. This is not just an AMC problem. BSA/Norton/Triumph/Ariel owners, to my knowledge have all had failures due to age/fatigue failure and sourced new rods as a result.
Bob.
Don Madden
Member
Posts: 2882
Joined: Mon Jan 01, 1990 12:00 am
Location: USA USA

G11 Upgrade of Oil Pump

Post by Don Madden »

The rods are not counterbored deep enough to reach the trunnions, leaving more metal the support the cap. I have posted a new photo this morning showing disasteroius failure of the rods at the trunnion. Cheers, Don.
Locked