Hi Alan,
I've considered upgrading my TR4's brakes, so have done a little research into this. Since my car uses 16P calipers, I'm uncertain if any of this applies to the 16PB caliper with metric mounting, like those on your car. I would guess that spacers and the four assembly bolts are same on 16P and 16PB, but don't know that for sure. Here's what little I know...
Hi-Spec Motorsport in England might be your best option. They offer brake upgrade kits for TR and will do the work to space original TR calipers. I'm not certain if you can just buy the spacers and necessary bolts from them, separate from their kits.
I know they offer a kit for home installation, that includes spacers for calipers, new bolts, rotors and EBC pads. They also offer kits with choices of at least three different versions of their own 4-piston calipers. Finally, they offer a kit that includes the service of widening your stock calipers, where you'd ship them in to have the work done. I believe the rotor supplied, in the kits that include one, are all EBC 347 ventilated (22mm thick). I think they use various EBC pads for most of their kits, or at least offer them as an option.
https://www.hispecmotorsport.co.uk/ is their website. I found some older discussion on the Internet of delays getting parts from them, hopefully that's all sorted out now.
Racetorations offers a high performance big brake setup by AP, I believe, which are very good but very expensive kits that include everything. Still, it might be worth asking about spacers and bolts, since Darryl seems to be able to get most anything for TRs.
https://www.racetorations.co.uk/ is the website.
Richard at Goodparts seems very knowledgeable about TR6 in particular, and might be a good source of info, even if he can't supply the parts.
www.goodparts.com
British Parts Northwest might be another source.
www.bpnorthwest.com
Revington TR seems only to supply standard thickness disc parts, but also makes a lot of parts and might offer what you need.
www.revingtontr.com
Cambridge Motorsport is another possibility, but also doesn't seem to offer any big brake parts.
https://shop.cambridgemotorsport.com/
Moss Europe might offer something,
www.moss-europe.co.uk Generally speaking, they tend to have more high performance items than Moss USA.
Moss USA offers a "high performance" brake kit of their own for TR3-4A, but this does not include calipers or ventilated discs and doesn't appear to offer individual pieces from that kit. They don't appear to offer a similar kit for TR250/5/6. Still, it might be worth asking,
www.mossmotors.com
Simon's point is one I've often heard. It seems to be generally discouraged to split original TR calipers, probably just because it's major safety issue. The manufacturer probably wanted to encourage sending calipers for factory direct or authorized repair. So, specifications for the process were never freely given and some of the necessary parts are hard to find. The bolts are special and two different diameter are used. They need to be torqued to two different settings, depending upon size. When we start talking about widening the calipers, longer bolts are necessary. The o-ring seal used in the calipers is a bit hard to find, but I understand Moss has them. Instead of the usual one-per-caliper, you'll need two for each caliper when doing the spacing.
All the technical info for splitting and rebuilding calipers used to be posted on the Buckeye Triumphs website, but was taken down on the advice of attorneys, because of liability, or so the story goes.
Still, a lot of folks do split TR calipers without any problem, for various reasons. I think some do so mistakenly, thinking it will somehow make rebuilding them easier.
Rob is right about the bolts being special, and possible problems finding correct, longer ones to use with spacers.
I think by the time I convert to 22mm ventilated discs on my TR4, I will just go to Wilwood Superlite II or one of the Hi-Spec calipers at the same time, to help offset the significant weight penalty of the heavier rotors.
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