I have a question about the oil pressure relief pistons on the 914-4. I fully understand the purpose of the primary one's purpose being that it bypasses the oil cooler when the oil is too thick and cold. I am not sure of the reason for the secondary one located under cylinder # 1 in the lifter and cam bearing galley. I can see that if the pressure is enough the piston moves and dumps the oil directly to the crankcase and looks to me like would rob the exhaust valve lifter for cylinder # 1 of it's oil pressure . What is the purpose of this relief valve?

I have never seen a 'definitive' explanation for this. It is called the 'control valve'. My thinking on it is that since it is smaller than the relief valve, it can provide more finite adjustment of the oil pressure, 'controlling it'.

Why it was not on later engines isn't clear also. By then they weren't putting the 2nd valve in bus engines prsumeably because of hydraulic lifters needing all the pressure they could get.

It may have been with the introduction of hydraulic lifters in bus engines that they became aware of the 2nd valve doing more harm than good on account of it's leaking internally? -just speculating.

I have tried holding it closed by putting like a welding rod in the middle of the spring - I didn't notice any difference. - probably because of the seating issues on the main valve were so bad that it made no difference is my guess. The bore of the big one, the relief valve, scoring on that is a big problem with these engines.

Not that it matters, but .........

Single relief cases have 8 mm galleries

Dual relief cases have 10 mm galleries.

The first valve is the oil pressure relief valve and is located at the pump end of the case. It controls passages leading from the oil pump to the oil cooler, oil gallery and sump return. This is to avoid cold oil from rupturing the oil cooler as the cooler is bypassed by bleed to the sump.

When oil is warm, the first valve closes and pressure is regulated to the galleries via the second control valve at the flywheel end. When experiencing excessive pressure, the valve opens and bypasses the excess pressure to the sump. The first valve closing routes the oil through the oil cooler.

Spring pressures are set at 42 psig, 30 wt oil at 70 C (158F), 2500 RPM. Wear limit is 28 psig.

Loaded length of 1st control springs on Type 4s is 1 9/16" (39 mm). Load @ length is 15.0 - 19.4 lb

Loaded length of 2nd control spring on Type 4s is 1 1/16" (26.00) Load @ length is 3.8 - 4.4 lb.

Thank Tom Wilson for the information ....................

And, Why would you want to control oil pressure? Running excessive pressure to the bearings will pit them (remove what used to be called babbit material).

Hey guys, Thanks for the very useful information about the dual oil relief system.

I have noticed that you can buy pressure booster pistons for these 2 relief ports, and can't help but wonder why would you want to increase the pressure on the one that bypasses the cooler when thick and cold.

Would it be so the warmer, thinner oil would all be directed to the cooler instead of being partially bypassed?

Would the secondary piston, being longer raise the oil pressure for the entire oiling system from the pump to the last lifter bore?

I've always marvelled at is the differences in specs between the 1.7 and the 2.0 engines listed in the little book. I think it is the same as the bus 2.0 engines, the oil pressure is a lot higher. I don't have the specifics on hand.... Seems like this would explain that.

Interesting.....I have an old W case from a 1970 1.7 that is the dual relief version.

Dual relief cases began in 1970 and they should be aluminum, not magnesium.

I thought all 914 engine cases were aluminum, tell me more about this please.

It's 66psi for the 2.0, 42psi for the 1.7, 1.8, both at 70*F at 2500 rpm, sae 30 weight oil at 158*F. The wear limit is the same for both though, 29psi at the same. Impressive though that is 150% the pressure. I wonder if some of it is to do with tolerances in the parts..

Magnesium cases were like pre production or else just really early, not sure. They are pretty rare, but not that desirable since aluminum is better for everything but the weight of it.

From the book, magnesium Type 4 cases had a designation of V or VO (don't remember which one). They were European only, but they figured some made it to the Amerikan market.

Lighter, but weaker supposedly (they burn real nice too!).