Friday, December 9, 2016

O-ring confusion for 250-305 Honda twins

Super Hawk and Scrambler twin carburetor models

While the port sizes on the twin-carb heads are basically the same for either a 250cc or 305cc engine, the Bakelite insulators have different markings and ID holes. The 250cc bikes received 22mm carburetors and the 305s are fueled by 26mm mixers. The 29mm o-rings with part number 16173-260-004 should be used on both the carburetor flanges and the matching insulators for the 305cc carb installations. 29.0 X 2.4 16075-GHB-B70 and/or 91304-KPH-700 are other part numbers with similarly-sized o-rings for 305 twin carb applications.

The 250cc Super Hawks and Scramblers can use the 91302-PF0-003 O-RING (26.9X2.4) parts on both the carburetor flanges and the insulators. Unfortunately, many Honda parts listings are showing the 91302-PF0-003 O-RING (26.9X2.4) as appropriate for the 305s, when they are actually sized only for the 250cc 22mm carburetors used on CB/CL72 models. Many Honda part numbers have been superseded to something newer that “almost fits” with some effort and in a few cases the “interchange” attempt has gone wrong and often unnoticed except for people like me and a few others! Check current online Honda microfiche listings for CB77/CL77 cylinder head and you will probably see the PFO O-rings listed there instead of the 260 code parts. They won’t work!

CB72 listings show O-RING (27X2.4) 16173-253-004 for the carburetor. Cylinder head listings show O-RING (26.9X2.4) 91302-PF0-003 (replaces 91301-253-000), but that 91301-253-000 O-ring is shown as 28.5mm.

Dreams (single carb models)
On the CA77 images for the cylinder head, we see: 
O-RING (26.9X2.4) 91302-PF0-003 (replaces 28.5mm 91301-253-000 ?) 
For the carburetor section the listings show O-RING (24X2.4) 16173-250-004 (replaces 16173-202-004. (FYI a -202- code part is for a CA95 Benly with a 20mm inlet size). The problem being that the Dream carburetor inlet is 22mm size, but the carburetor flanges have a 32mm groove in it, so these do not fit.

O-ring options from Honda for the Dream carburetor flanges are now:
31.7 x 2.4 91307-958-003
32.5 x 2.4 91356-ME4-003 
32.5 x 2.5 91307-MC8-005

When working with “almost the right size” o-rings, you can often get them to seat into the grooves well enough for assembly and effective sealing. Ones on the small size can usually be stretched into place. If they are a bit on the large size, a little squeezing down can be tolerated within the groove space. There are some SAE near equivalents at and if all else fails, but measure carefully, as some offerings are listed by OD (outside diameter) and others are ID (inside diameter).