Old TRW piston clearance.

[Dave Koehler]

Bill,
The L2300 is on page 642 in the 78 book; also has "Except High Performance & Special performance" in bold print below it. The L2268 is on page 644 0f the same book. Both pistons are listed as 66 to 69. Just noticed the L2308 for the ZL1 and L88 is also listed as 4.251; they all list the oversize's accordingly. +.030 is 4.281,+.060 is 4.311 etc.. The pages are yellowed but still readable after 40+ years. I must have "loaned" my interchange, haven't been able to locate it. The truck pistons are listed @4.250 for recommended bore.

Interesting. I have never seen that before. The oldest book I have is 84 and it shows 4.250 bore. Learn something every day

I can attest that indeed that the original oem specs had the bore size at 4.251.
Over time and the aftermarket it morphed into 4.250 in the catalogs.
It's an oddity of history.
I believe the reason was back then they did not want a 426 and in order to one up Chrysler came the strange specs.
Hence the 4.251 x 3.76 combo = 426.924 CI or 427 rounded off.
4.252 would have put them just slightly past 427 at 427.125.
Perhaps they did not do that for Nascar or NHRA rules.