EFI Fuel Pump O-Rings

[Captain Obvious]

I've been through O-ring sizing details in the past for stuff that I had a hand in designing, but the easiest thing to do on existing equipment is.... Use the same size that came out.

Problem here is that the O-ring has taken a set from years of compression, and the groove that it sits in is not round, so it's not like you can easily measure the ID and OD. Depth is no problem, but anything having to do with circumference is tough.

I measured my old O-rings using a gauge of appropriate diameter. I came up with 33 or 33.5 mm depending how much stretch I estimate occurred over the years. I suspect either size would work OK, and if it comes to that, I'll just order both sizes and use the one the feels right. Of course, I'd still rather have a recommendation from someone who's already had success.

McMaster stocks the 33mm in both Nitrile and Viton. The 33.5 they only have in Nitrile. Good enough, but Viton is even better.

hr369, I can definitely tell you that they are NOT 30 mm. When I get the correct sizes, I'm sure I'll have to buy more than I need, so I'll have extras. In other words, don't buy any yet... I think I'll have you covered.

Thanks for the help everyone, and keep the info flowing!

[rcb280z]

Capt, I will buy a couple from you. My pump is seeping right now. So it's time for me to replace my o-rings as well.

[Captain Obvious]

No problem. I'm sure I'll end up with a couple extra sets and I'd be happy to share the surplus.

I took another very close look at the O-rings last night, and it just may be that there is a small size difference between the two. I'm not sure if it's stretch from application or swell from gasoline exposure, but my two O-rings are definitely different in circumference at this point. Easiest to tell when you swap the two from their original grooves. They definitely fit different in the other grooves, and I don't think it's just from compression set. Still close enough that I think the same size would still work in both locations, but if I'm going to be ordering two different sizes anyway, it just may work out for the best.

The two grooves are a slightly different shape as well due to the geometry of the pump ports. I'm thinking they tried to make the two O-rings the same, but missed a little bit? :bulb:

[EuroDat]

Hi Captain,

I posted this morning, but for some reason it didn't work. Now 8 hours later I see it.

I found my notes and the old o-rings for my pump. You are right I noticed a difference in the groove width. The depth is 1.5mm for both, but the groove by the motor is 0.5mm wider 3.0mm.

The o-rings I used are 33.5x2.00mm and 34.5x2.00mm. They are both according to the JIS B4201 standard. Numbers S-34 and S-35.

I ordered them at a local rubber supplier "TechniRub Vizo". See photo for the workings.

I did this when I was changing spindle pins and suspensions bushes. Probably why it faded into distance memoryLOL

Chas

[EuroDat]

Just to clarify. I used the same cross section 2.00mm because the depth of both grooves was the same and o-ring squeeze should be between 20 to 30% depending on material and hardness (often noted as Shore A or Duro).

Chas

[Captain Obvious]

Thanks Chas. Awesome info. My measurements are a little different, but not by much.

I measured the depth of the grooves to be 1.6mm (instead of the 1.5 you got). It was a little tricky to measure because the grooves have a small radius at the bottom corners, and if you get hung up on the fillet on the inside corners, you'll measure a little less depth. I was careful to measure at the center of the groove, and I got 1.6mm. Slightly less crush than what you calculated, but more than enough.

I did not measure the groove width, but I will next time I'm in the shop.

I see that you calculated ID from linear length, and I was curious... How did you measure that?

I measured ID directly with gauges (rods of appropriate diameter) and found the IDs to be 33.0 and 33.5mm with the smaller O-ring located near the motor.

Also, I bumped your fuel line insulator thread with a question. I'm guessing you didn't see that?

http://www.classiczcars.com/forums/garage-hobbyist/50966-280z-fuel-line-insulators.html

[EuroDat]

Hi Captain,

I was having trouble with posting yesterday. I did see your bump and posted 4 photo's, but something went wrong there too. I post again after I finish here.

This all took place a little over two years ago when I was doing the rear suspension and I seen the fuel leak at the pump head and went into that "while Im at it" syndrome. I had good pressure so my plan was to just do the o-rings. That ended up in o-rings, all the hoses, the clamps and a coat of paint the freshen it up.

I noticed the different sizes in the grooves and took some measurements on the back side of the paper. See photo. I only had a set of the old non-digital type verniers at home. Shallow depth measurements are hard to measure, so my measurements could be a little off. I normally take fillet into consideration when measuring so the 1.5mm could be just a question of tolerances in my measuring:rolleyes: or from the factory.

The inlet cover o-ring is the larger diammeter. Looks like I got that mixed up on the front page.

I found the receipt for the o-rings and I ordered one size: The 33.5x2.00mm NBR 70A Part nr. 85000814.

Strange that they make two different size grooves.

The pump is German made so it could be DIN3771 standard used for the original o-rings. I know that in the seventies Germany used their own standards a lot so I don't think it will be an O-ring from the AS, BS or Le Joint Francias standards.

Linear method:

Where I work we use the linear length method to measure o-rings in odd shape housings like Lobe pumps etc. We use our standard elastomer suppliers for FDA/USP validation reasons. Some pump suppliers won't give the sizes, just their part numbers. To get around this problem we use a very thin plastic wire (a bit like plasti-gauge). Run that around the inside diameter and overlap the ends. Then cut though where it overlaps with a stanly knife. Remove it, lay it out and measure it with the calipers. Then use pye to calculate diameter.

Chas

[Captain Obvious]

Yeah, I'm in a similar situation with why I'm doing this now. I've got all four corners of the suspension off the car and I've promised everyone some cheap easily available check valve options for the pump, so there's no easier time to mess with the fuel pump than now. Then I figured while I had the whole pump assy off the car, I may as well make sure the insides of the pump looked good, and you know how it goes... Now I'm messing with an elegant mounting of a fuel filter back there using a mounting bracket that will hopefully look "stock" when I'm done. Scope creep.

I'm confident in my measurements to about +/- .01mm (a half thousandth over here at this part of the globe). Thankfully O-rings are pretty forgiving about sizing, and I don't think the differences between what you got and what I'm getting are really going to matter. Especially in a static face seal like we have here.

I'm going to order some O-rings tomorrow and go from there. Thanks again for your help with this!

And you made me chuckle when you mentioned the German standard. That was the first one I opened too. The second was the Japanese standard. And so on. Same thought process.