View Full Version : TR5/TR250 TR250/6 machinists

07-19-2009, 03:30 PM
You have to love the Brits. There on the top of the 250 block, by each cylinder, is stamped a G or an H, designating which size piston was installed. I guess it was cheaper to make different size pistons then it was to improve cylinder boring quality control. What a hoot.

07-19-2009, 07:20 PM
This is the same as Chrysler. They also fitted bearing shells to match the crankshaft.

07-19-2009, 09:13 PM
No wonder our freeze plugs were different.

07-19-2009, 09:29 PM
If you check those letter grades, I think you'll find they differ by only a half a thousandth (ie .0005") for a total range of about .001". As an amateur machinist, I'd say that's pretty dang good. By contrast, my 1980 Chevy had an allowable variance of .003"

07-20-2009, 01:00 PM
Yes, I was a little harsh on the machinists. One has to consider the tooling they had then on the production line was not nearly as accurate as todays.

The process is somewhat of a production engineers nightmare. First the boring, then hone to size, then measure the bore, then stamp the block with the correct piston code, then select the right size piston and install, then inspect. The same thing happened in making the pistons.

But one has to consider the cost of having 2 or 3 different sized pistons to select from, then manually match each to the cylinder. Inventory management and distribution was not very efficient, but probably unavoidable.

07-20-2009, 01:34 PM
Which of course is why the American makers just put them together sloppy, with enough clearance to cover all the manufacturing variation. "Inspection" consists of making sure there is a piston in every hole :laugh:

07-20-2009, 01:34 PM
It wasn't just Triumph.

Cornbinders (International), Dodge, Plymouth, Willys, early Fords.

It's like the end of the month, see what we can do to fix failed blocks.

You haven't lived until you've taken apart an old Ford flathead V-8 and found sleeves on ONE bank, where "core shift" in the casting process made one bank too thin (or bored into the water jackets!).

Seen more than one of those.

The old books I have show various designators for changes in rods, mains, bore, wrist pins, etc.

It may have been a discoloration in the bores, caused a slight overbore to clean it up.
You never know.

They probably figured a certain percentage needed, oh, .005 or so over, ordered that many sets of pistons and rings, if it was more, the block probably got scrapped.

07-20-2009, 01:46 PM
GM cast iron V-6's and the 152 CI 4 bangers were no bargains for "tolerances" back in teh 80's either. Talk about worn out tooling........

07-20-2009, 03:04 PM
It was actually a good way to do it. As pointed out, the variances were quite small. They simply ordered parts to a particular size and then found that there were small variances.

They could have ignored them, or added machining steps (which cost money) or just added an apprentice with a micrometer to measure and stamp cylinders and pistons, which was the economical step thay chose.

The other manufacturers of the day had similar tolerances, but those that failed to do this sort of parts triage didn't offer as fine specs in the final product. I'd say that was a good use of not much money added to the whole process.