Compression numbers, never been rebuilt “F-series” (1 Viewer)


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Sep 9, 2004
Boise - Idaho
So we often do compression tests on Cruisers, 40/60/80’s either for PPI or for “150-point” inspections.

We have had a number or low 100,000 mile 60’s come in that last six months that tested not only interesting on compression but really good. We also just tested three FT/FTE diesel recently so it’s something we do often in the shop.

Anyways, I can’t imagine there are a lot of 40’s running around with engines that have never been cracked open? They do exist though! I personally enjoy doing a compression test followed by a leak down test and or a smoke test. It’s fun to look at something really old and perhaps crusty on the outside but not knowing what is inside without performing the tests or opening it up. In some ways as you do it more you get a real feel for what to look for or what weird things happening with your test equipment might mean.

So we have this 12/1970, one owner 60,000 mile 40 in for some work. As soon as we started working on it we could tell it has never been touched other than normal regular maintenance. There is so much original going on here it is a little mind blowing. 🤯

On with the point of the thread, we tested compression and did a leak down on it to determine health of the engine. At 60,000 original miles obviously it has sat or barely been driven many years.

Here is compression;

#1: 152
#2: 135
#3: 137
#4: 130
#5: 135
#6: 133

Obviously other than #1 being high, man that is really equal across the cylinders. Factory specs are, on the F155,

Standard: 150psi
Low: 128psi
Spread: 14.2psi

Pulling that out of my head so hope it’s spot on! 😉

Leak down test showed exhaust valves, but really not much “leak down” happening on it in general. I “theorize” #1 is high due to being #1 on an in-line six which just gets the best cooling, best lube and least amount of carbon build up, as well as perhaps least amount of ring wear from sitting. We did try the cylinders wet and dry in a warm test and they didn’t come up but 1-3psi. So overall rings are good and equally worn.

Pretty awesome to see a 50yr old Cruiser engine this solid. Sure it would be better to see 145-150psi across them all but come on man, that’s a really good spread besides #1 being high. Like I say we had some 2F 60’s in with some rad numbers too but this is the first 50yr old Cruiser we have seen with these “smooth” of numbers.

If you got one that has never been cracked open let’s see some numbers! Even if it is only on its first rebuild 20-some odd years ago let’s see some numbers! 😆


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