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Head gaskets: cars vs. motorcycles


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2 replies to this topic

#1 psychocandy

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Posted 02 June 2012 - 11:39 PM

So, being on a Subaru forum, we talk a lot about head gaskets. They're a big deal. But, I come from the motorcycle world. I apprenticed in a shop for a couple years.

Anyway, we didn't really do head gaskets that often. But when we did, it was no big deal. Sure, most MC heads are easier to access than in a car. Hell, on a boxer (say, a BMW) engine like our Subies have, it's criminally easy to get to the heads. I've done countless BMW head gaskets. It's a simple common job. Never once did we have send the heads out to get milled flat. We never really worried about warpage.

Considering how similar our Subaru engines to something like a modern BMW motorcycle engine. Our cars even have mechanical valve trains with shim adjusters like most motorcycles! So my question is, why is it so important on cars, when doing head work, to get the heads machined flat? And I know it's not just Subarus. My old Toyota's head gasket went bad years ago. It was a 4 banger, so taking the heads off wouldn't've been that huge a problem. But, when I talked to a shop about doing it, the cost of it (with the machining and everything) was so much it just wasn't worth it.

Why is it that this is so important for car engines? Considering most MC engines are tuned for higher performance (relative to cars, anyway), how come its cars that are so much more sensitive?

Serious question here. I'm really not much more than a shade tree mechanic, so I don't purport to know a whole lot. I'm just curious!

#2 Fairtax4me

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Posted 03 June 2012 - 02:57 AM

Due to the size of certain cylinder heads and metals used (steel/iron heads vs aluminum). Some heads warp much more easily, (gm vortec 350 heads for example) than others. Subaru heads are often within spec for warpage limit, but the MLS gasket used (in the 2.5 specifically) you need a very flat and extremely clean surface in order to seal properly. Otherwise the gasket doesn't form to the head correctly and you end up with leaks.
The older 2.2s don't care that much. You can scrape the crud off the heads with a butter knife, and put a new gasket on wet and they will still seal. The 2.2 uses a different style composite graphite gasket though. Subaru used the same style gaskets on the early DOHC 2.5 and the failure rate was nearly 100% by the time the engine reached 150k miles. Now granted, 150k is a good stretch for a head gasket on an American engine, but a Subaru 2.2 can go to 350k on original gaskets, so long as the cooling system is properly maintained. (Honestly improper cooling system maintenance is about the only reason for a 2.2 HG to fail IMO. )

I've done, in recent months, head gaskets on 3 SBC engines all with under 150k miles. One was only 98k, but it was also 28 years old, and by far the cleanest of the three. The heads that were put on the engine were from a junkyard car, and according to the machinist were warped .006". They came from a 97 Chevy truck with only about 120k miles. No indication the gaskets were bad but they weren't long for this world with the heads being so far out of spec.

#3 grossgary

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Posted 03 June 2012 - 12:12 PM

it's cheap and easy. if you think a headgasket job is easy you can't, with the same logic, talk as if having a head resurfaced is hard or problematic. there's even a DIY thread here on how to do it yourself in 5-10 minutes.

you mention high prices - resurfacing is cheap, so whatever the "machine" costs were must have been a valve job or something else, because resurfacing is cheap.

headgaskets are much harder to do on a car so a little preventative has higher value.

you also see more headgasket talk because there are ongoing headgasket issues for 10+ years on one particular Subaru motor - the EJ25.

it's *more* recommended on engines prone to headgasket issues. i wouldn't worry about not resurfacing an EA82 or ER27 depending on the circumstances, because they aren't prone to headgasket failures.

cars are typically acquiring more miles on them then MC's. i doubt wanting another 100,000 or 200,000 miles out of bike is anywhere near common practice.

vehicles are doing more work - heavier weights, climbs, off road, winter driving and more severe temperature fluctuations, towing....bikes get revved and that's it, they're limited, can't do much, and don't see what cars, trucks, off road vehicles are subjected to.




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