About a year ago I received a batch of 7 mm MBP retainers from Guy Martin and I noticed that some fit a bit tight. We both didnt think it would be a problem but started to get calls from customers who were having problems installing some of them. We looked into the problem further and decided that some of the retainers were just over the high side of the specification. We found what caused the problem. We pulled the entire lot.
The current lot of retainers fit much better. They do not require excessive force to install and they can be removed easier also.
The statement about the closing shims changing in 1998 by adding a radius is simply not true. I owned a 97 916 and the closer shims had a radius. The 748 R bikes also have the radius on the closer shims used in them. These bikes were notorius for breaking the half rings during a race and even dropping valves and blowing up their motors. It was this problem that caused Guy Martin to come up with a better mouse trap to avoid engine failures. Hence the MBP retainer was born.
I have spent many and hour designing my shims to work with the OEM ducati half rings and the MBP retainers. Here is my opinion on this subject.
First of all the stock OEM half ring system is in use in thousands of Ducatis all over the world including the new 1198 superbikes. It works adequate. The round cross section ring fits into the groove in the valve stem and also inside the counterbore in the closer shim. The half ring is contained down inside the closer shim as the opener shim sits right on top of it. If the half ring should break the pieces are usually contained in the closer shim and no catasrophy results. The tighter the fit the better half ring performs. Unfortunately the OEM half ring is not precision made. It is simply wire stock round a round a mandrel and snipped in half. You can see it is literally snipped in half as the ends are not square. I also have measured the half rings and they are not uniform in cross section, meaning the thickness from one ring to another can be different. Also the half rings are not very hard and definitely deform over time.
In comparison the MBP retainer is precision made of A2 tool steel and heat treated (hardened) to a much higher hardness than the stock half ring. Also it fills the up much more volume and has more surface area of contact to hold the closer shim onto the valve stem. More surface area means less micromotion and less wear. See the diagrams of the two designs I have included.
From personal experience from bikes I have owned and from customers I can say that the MBP retainers will increase valve adjustment intervals over the standard half rings. You will see the advantage of the MBP retainers the harder you run the bike. I have owned 4 Ducatis, two track bikes and two street bikes. The street bikes (ST4, 46,000 miles, Monster 2V, 43,000 miles) have the stock OEM retainers. I started doing the valve adjustment every 6,000 miles but increased to 10,000 miles as in the last 20,000 miles I noticed the clearances not changing as much as when the bikes had lower miles. I never had a failure and the experience was a good one with the stock half rings. On one of my track bikes (916) I installed the MBP retainers at 6,000 miles and put on 3,500 track only miles in a 4 year time period. I checked the clearances at 10,000 miles and only one shim had changed by .001 in. I know the stock half rings would not have performed as well if used in this bike.
In conclusion, the stock half rings work adequately in street bikes. In track and race bikes they get hammered pretty good and can break. Even Ducati saw their limitations and came up with a completely different collet system in their "R" bikes (999R, 1098R, 749R).
If you want a better system to either increase your valve adjustment intervals or keep your piece of mind when racing your Ducati then the MBP retainers are a good alternative. If you simply ride your bike around the street and do not mind the recommended valve check intervals, then keeping the stock system is a good choice.
Mike