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I have definitely seen thinner base gaskets available in the past, but can’t seem to find them now.

So, if anyone has a lead on a source I would be grateful.

This is for my 501, but failing that if anyone knows a source for 350/500 then contact details would be appreciated.

I’ve been meaning to fit some for years but never got round to it. Now the engine is apart seems like a good time. The aim is to recover a bit of compression ratio lost by the 2+2 cam.

I’m not sure how thin is sensible with a 2+2, again any experience of having done this in the past appreciated.

And I don’t really want to risk no base gasket or making my own before anyone suggests that. Getting the barrels off is an engine out job in my dart and that’s a major pain in the arse with the exhaust system which I would rather not have to do for another 10 years after this rebuild.

Ta,
Tom

Tom,

How much are you hoping to gain? I have two standard ones in their packets, I tried to measure one, but not easy. I would say it is less than 2mm thick. I presume the thin ones are very thin and easily bent!

Can I ask how a cam affects the compression ratio? I thought CR was based on volume at tdc and bdc. I know on the compression stroke the valves are open for a while to get the old charge out and the new charge in, but unsure how this affect the CR. Thanks.

Hi Tom, have measured new gaskets both Morini/Cagiva original and NLM pattern
Head 1.2 mm
Base 1.01 mm

Also old original gaskets removed from engines, some measure as above and others are
Head 0.9/1.0 mm
Base 0.9/ 1.02/1.24

NB I have had some NLM pattern gaskets in the past with stud and Allen bolt holes that were too small, so always worth checking

Have fun, Ian

I raised this with Stuart Mayhew once and got the "well we've not had any problems" mantra.

Thanks for the replies so far. The new base gaskets I have are 1mm, but hoping to source some a little thinner than that (0.5mm probably). May make no perceptible difference but probably do no harm either.

There's actually 2 reasons I'm keen to try this. Firstly to get a 2+2 onto a donor cam the base circle diameter is reduced by around 1mm, which means you have to compensate for the 'shorter' pushrods at the rockers. There is enough adjustment to do so, and my engine has been running fine like this for years, but losing that half mm in a thinner base gasket would return the rockers to a more nominal adjustment point.

Secondly, as mentioned it'll recover a little compression ratio.

In answer to your question Mike, yes, as you say greater valve overlap helps with gas exchange, using the outrushing exhaust gas to pull fresh charge in. A later intake valve closing also maximises the amount of fresh charge that can enter the cylinder because even though the piston is already starting to travel back up on the compression stroke, the inertia of the air rushing into the cylinder means that you can get a little bit more in. But of course you then have lost some of the available stroke for compression. Dynamic compression ratio is calculated from the point of intake valve closing, so the later you close the valve the lower your dynamic compression ratio.

If you took it to the extreme and never closed your intake valve you would get something approaching a compression ratio of 1 (the volume of air in the entire earth's atmosphere at bottom dead centre and also at top dead centre). But if you calculated the 'static' compression ratio from measuring bore, stroke and combustion chamber volume, you would still get the 11:1 or so that our morinis are quoted at.

So, there is a balance to be had with improved volumetric efficiency by closing the intake valve later to get the benefit of all that intake air inertia (free ram air if you like), and closing it too late and losing some of the mechanical compression available. The point at which that balance happens will depend on amongst other things the intake port geometry, runner length etc, so just fitting the cam isn't really the end of the story, and I am quite sure that when Morini developed the 2+2 profile they will have changed other things in the intake/exhaust system to maximise the gains.

Also, bear in mind that the inertial filling is really only happening at high speeds and wide throttle openings. At part throttle and low speed you aren't getting that benefit, but you are still bound by the late intake valve closing (oh to have variable cam timing!)

From my perspective, I have quite short intake runners on the dart so I suspect the engine doesn't achieve as much cylinder filling as it could, the bike is used in UK climate (i.e. not that hot) on good quality fuel, so it's probably a fairly safe distance from knock (have certainly never experienced any). Similarly I've never seen any mechanical signs of detonation when my engine has been rebuilt, and I don't ride it everywhere flat out so on this basis increasing the compression ratio is probably quite low risk, and (and we're talking only perhaps 0.5CR here).

For info the 2+2 intake valve closes about 12 degrees later than the J cam (standard cam on the road 500/501 engines), about 8 degrees later than a sport cam, and about 5 degrees later even than the L5.

Thank you, Tom. I knew there would be an answer as you know about the dark arts and limitations of the infernal combustion engine. To please Greta the Cult, you should have said it allows a more efficient burn and thus reduces the emissions... I am making the assumption that the Heron head is also a limiting factor in the overlap phase?

I'll stick with an S cam and standard base gaskets, my job for this winter when I source some head gaskets. But first, the new crankshaft seal. I can then rename the bugger from Torrey Canyon...

That reminds me everyone called my T150 the Amoco Cadiz.