cam journal clearances

#1

Hi guys first post here, not into spece30 but rather building fun m20’s.

Can’t really find much info on the topic so i’m just curious what you guys are seeing / running for serviceable cam journal clearances?

#2

That’s a good question. I’d ask at e30tech. I’d be curious to know the answer too.

I’ve retired a couple take-off heads because of worn cam journals (on the head, not the cam), but I didn’t actually measure them…They just had bad wear grooves.

#3

Haha, I normally post over there too but figured i’d try here cause you guys seem to do a lot of m20 building and get quite paricular about the oil clearances.

So you nomally just do a visual inpection for scoring other obvious defects?

#4

[quote=“tinkerputzer” post=67044]
So you nomally just do a visual inpection for scoring other obvious defects?[/quote]Yes, but that’s because I don’t know what I’m doing. If I knew the gap that was supposed to exist, then I could check it with a bore gauge.

It seems reasonable that it would be about the same as a standard bearing clearance so 10 tenthou per inch of bearing, but guessing about the inside of an engine is risky business.

#5

I normally just do a visual inspection and look for journals with minor to no grooving. It would be difficult to measure some of the bearing diameters on the back half of the head; that said, clearances should be .001" for every inch of bearing journal diameter.

#6

The head in question looks great. No scoring or defects of any kind. So i’m wondering if the .001/" of journal applies to the cam bosses in the head since it is made of aluminum. Is the expansion rate of aluminum greater thus shrinking the clearances?

#7

I don’t know if the 10 tenthou rule applies, but certainly most aluminums expand more than most steels.

I’d be willing to bet that the common kind of wear in the head’s cam journals is grooving, not smooth wearing away that would be hard to eyeball. That is to say…if there’s no grooving, maybe you’re ok.

Support for theory. Anyone who’s seen a half dozen cams removed has seen grooved heads, but have you ever heard of someone saying that their cam bore journals are worn oblong? I’ve not. Also, the load on a cam bearing is pretty light compared to a rod bearing. The other day I measured how much torque it takes to turn a cam and got 21 ftlbs. That’s spread across, what, 7 cam journals?