Fast Fred's Secrets of the OMC MOD50's Thread #1

**Bill Gohr** · 01-30-2009

I used to get this call at OMC all the time because machine shops would tell dealers they need the pistons to fit them to the block which is wrong.

Piston fit is generally based on what the engineers of that engine set it at. As well as if you you change piston manufacturer and material then the fit changes accordingly.

Lets say you are building a 56 cu in 3 banger, what should you set the bore at for a .030?

Simple .030 over standard. The piston manfacturer set the "fit" at what they wanted it per the material they made their pistons out of. If I measure a Cast OMC piston and a Forged Wiesco, the Wiesco (if they did their homework correctly) should be smaller, because it expands more.

Further more, a given engine, like the 56 incher, have different fits for different years so you have to watch that too. So if you're boring your engine, you see what it says on the top of the piston and bore it that much over standard. And like Sam said on some applications we will tighten a motor up and make a little more power but how much you do that it based on what the piston is made out of and how you break it in.

If you want to know what the original clearance is on a Mod 50 TRW and don't have a new one to measure, I have a set to measure if necessary.

As far as heat goes, the motor needs temp so never run them stone cold, they just wear out faster.

**sheuninck** · 01-30-2009

great discussion. What is the word on minimum and maximum ring end gap?

**850cc racer** · 01-30-2009

hey there fred i have a couple of questions:

page 4 post 39 "the straight in" is 814cc by the look of it.. straight in ..what exactly that mean?

also post number 422 you refer to the 56er with long rods and modded exhaust.. what are the long rods. and whats a modded exhaust exactly?

also i have the 1 inch 2bbl carbs.. she came off the 56cuber that went BANG.. i pulled the jets out to have a look at size.. they have 54 c's and 54d's in there

why

i thought they should be around 34-38??

Help!!

850

**850cc racer** · 01-30-2009

also.. a mate has a 56cuber with FT front. complete motor 2-1 gearbox, 15 inch mid.. anyone interested or whats it worth?

**Fast Fred** · 02-01-2009

"the straight in", had bigger reed blocks, the front half went straight in ( the down sweep was removed), had press in cyls with a single vert exhaust bridge.
made about 10more hp than the 19s. she moved heat better, had behind the liner ports ( boost port was semi behind the liner)

no FT model listed

but we can make one

**850cc racer** · 02-01-2009

ahhhh okay well ill have a look at what i have i think i may have a straight in..

so what rods classify as long rods for a 56er?

so if i have 1inch 2bbl carbs and its on a 56cuber what are the jet sizes??
54's seem wayyy to big and ive been told they should be in the 34-38 range..

**Bill Gohr** · 02-01-2009

The long rods are Merc rods.

The FT motor which you speak of, is that the orange one that was pictured somewhere? That is not an FT engine, it looks like a stock block with an adaptor plate bolted to it. An FT came with a mod 50 front half.

Anyone have a real FT or OE center section, the one that came before the mod 50??

**Bill Gohr** · 02-01-2009

one

**Fast Fred** · 02-01-2009

big twin 40hp rods or Mod50 rods.

two ways to fit a piston, one is, custom hand fitted, the other is production style, nether is wrong, has to do with what your buildin.

**850cc racer** · 02-01-2009

hey Bill, well i dont think i have posted any pictures of an orange block?
there was one on ebay a while ago which was 814cc with the 2bbl front? that may be the one your thinking of?

so i havent seen that gearcase before..

Fred: i have a set of those rods. was going to use them in the 814cc with those trick pistons i got from you, so your saying i can use them in the 56cuber aswell?

this is the m31 box..