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Bought a 1990 Classic that needed a rebuild. Found tin gaskets and 14 bolts per. Swapped the tin for composite and went with 10 stretch bolts. I used this method on our other Classics without issue. Once assembled I found I had low compression on the drivers side only.  Also did her rings and bearings. 1st cylinder came up to only 65psi dry (wet 85), 2nd was 90, and the last two were ok-ish at 120. Before I pull her apart again I thought posting this might initiate some talking points I'd find useful.  I don't think I installed any of her rings upside down. Any thoughts appreciated.

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I don't know to what extent the lowering of compression ratio, due to the use of composite gaskets, would have as I've only ever checked the values on an engine with tin items, but your readings are very low & you are right to be concerned. Even on a 3.5L I've had an average of 155.

I believe when reassembling with just 10 bolts it's advised to install the 'unused' bolts as well & just nip them up. but I doubt that would be a contributory factor to your problem (?)

I'm sure qualified members will be along with advice shortly, good luck !

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  • 2 weeks later...

Found the time to break her down and discovered 2 issues.

The Valve stem seals had all displaced. When I had originally disassembled the heads I found that the prior seals had all disintegrated to almost nothing.  Put the new guides on and now that I have once again torn her apart, I find that they do not stay put.  Seems a good question to ask is 'Do we need to use Valve stem seals on a 1990 at all?'  The stem seals for 1990 differ greatly from following years for which I've never had issue. The small blue seals work and stay put.  The 1990 stem architecture is much different in its design and makes me think that most probably it ran without its seals in place for some time, I suspect a great long time.  I'd even speculate they just may be completely unnecessary altogether. So, how many of you readers would argue that a 90RRC needs its VSS's?

As for the compression, I only thing I could find suspect were the headbolt installations. Each bolt input needed a retapping and perhaps the previous headbolt installation never torqued correctly because of this.  Rings placement were all correct and valve seats were good.


Edited by RoverDad
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