The best bet may be to carefully take off one of the other bolts and match. Unless you do, or go to a dealer to order the exact replacement, you'll have no idea how long it should be. Of course a visit to the dealer is probably best, but a quick look at another bolt on the flange has a very high probability of telling you what you need to know. It sounds like you should be checking all of the other bolts anyway.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold