omc jet sizes

**phillnjack** · 07-07-2017, 03:07 PM

would I be right in thinking that omc jets are sized by the imperial inch system ?
i.e 52D would be 0.052 inches and a 60d would be 0.060 inches and a 73D 0.073 inches ?

or is this not the case at all.
does anyone have a jet size chart at all ?

**racer** · 07-07-2017, 05:54 PM

No a 73D actually measures over .075. The C series are different 73C is .073

**71V153** · 07-07-2017, 06:26 PM

Yep. Part # doesn't necessarily jive.

I had a chart with all the dia's and numbers. Before the last computer croaked. Sorry ...

**phillnjack** · 07-07-2017, 06:28 PM

Thanks racer

my idea was wrong then ha ha .
any idea what size a 60D would be ?

I got told a 59D was 1.5mm ??? does this sound about right, if so I will give it a try.

**Hupiveneilijä** · 07-10-2017, 09:32 AM

I go by this and then change to mm. Next opening there is chart for drill sizes, in&mm.

**racer** · 07-10-2017, 02:47 PM

Except chart is not accurate on D series jets.

**powerabout** · 07-10-2017, 08:13 PM

IMHO when you drill a jet you have no idea what it now compares to

**Hupiveneilijä** · 07-10-2017, 11:55 PM

Originally Posted by racer

Except chart is not accurate on D series jets.

Okay, thanks!

**Hupiveneilijä** · 07-11-2017, 12:02 AM

Originally Posted by powerabout

IMHO when you drill a jet you have no idea what it now compares to

Thats true, but if i drill all up or down i know its larger or smaller than before and do checking by looking piston dome on plughole. Allthough drilling is done much greater steps than one jet size.

**Performance Unlimited** · 08-27-2017, 09:58 AM

We use drills to measure the jet size and mic the drill shank for the od. of the drill... we had a 67D which measures .067" and we also made our own jets out of brass set screws on a lathe , then drill and chamfer the hole.

**racer** · 08-29-2017, 06:38 PM

Problem with drilled jets is the swirl left by the drill which can effect flow. I just went out and measured 3 jets one was a 75D that would easily pass the shank of a .078 drill (shank measured .0775) then did a 78d which would pass the shank of an .081 drill then a 62D of which a .062 drill was loose in. Before I retired I had a full set of reamers to check sizes, factory jets are done with reamers. Before jets larger than 81D were available I had a fixture that indexed in my lathe that jets would screw into to ream them.

**powerabout** · 08-29-2017, 06:55 PM

Omc always claimed they flow tested them too

**phillnjack** · 08-30-2017, 06:36 AM

All very intriguing stuff,
how would you go about going up in jet sizes ?
I want to go from a 50D mains and 55D top jet.. to 55D mains and ??D top jet ...... it is the top jet size I would not know what size to start with.
do you go up the same numbers on each jet, like 55 to 60 for the top as well. ?

carbs are the large black top carbs on the omc 70hp oval port 56.

..

**Performance Unlimited** · 08-30-2017, 10:54 AM

Originally Posted by racer

Problem with drilled jets is the swirl left by the drill which can effect flow. I just went out and measured 3 jets one was a 75D that would easily pass the shank of a .078 drill (shank measured .0775) then did a 78d which would pass the shank of an .081 drill then a 62D of which a .062 drill was loose in. Before I retired I had a full set of reamers to check sizes, factory jets are done with reamers. Before jets larger than 81D were available I had a fixture that indexed in my lathe that jets would screw into to ream them.

If you measure several of the same size jet re: 75D you will notice that there will be tolerance of .0005"-.001" between the same jet number, since one jet can be on the upper limit of the id and the other can be on the lower limit of the id.. .re: a 75D might flow the same as a 74D, since 75D is low on the tolerance and the 74D is on the high side of tolerance and as well as there will be a difference in the flow between the jets of the same number which can be up to 3%.

So it is better to mic the jets you have so you know where the starting point or base line is and then increase to the numbers that you need and by either using a drill or reamer for the jet size and do a chamfer on both sides of the hole.

Reaming jets to a size has its cons as well by the hole having a bunch of flat areas (like a stop sign shape with many more flat areas) as well as reaming with lubrication or dry reaming will make the hole either oversize or undersize of the reamer size being used.

Drilling can leave a swirl pattern which can flow more......however either way that you might choose is so close you will be trying to split hairs and have the same result.

**racer** · 08-30-2017, 11:12 AM

I agree the jets vary a bit but since the D series came out in the 80's I have yet to come across one that was smaller than indicated size. I liked reamers due to size options available. I was lucky to have an aerospace machinest working for me. RPM jet is turned and feed rate also has an effect on hole.

Everyone has their own way of doing things. After building hot rod OMC engines since the 70's with great results I just like my way which may or may not work for others. What I do know is very few people have spent the time I did on a true dyno that I have.