Base gasket required?

[Rbranst]

Guys- on the parts sites I see an aluminum base gasket. Is this needed or only in certain scenarios? I have a full new Athena gasket kit, but I am wondering if I need the aluminum gasket spacer or not ? 68 250.

[Duccout]

I'd be interested in the answer to this question. AFAIK the aluminium gaskets come in three sizes, but I don't know if it is to correct variations in the machining of the barrel and head, and thus adjusting the squish clearance, or to adjust the compression ratio for different models or markets where petrol was lower octane.

Cheers,

Colin

[Rbranst]

Great additional questions. Since the head and barrel were off of mine, when I got it, I’m not even sure if I’m supposed to have a bass gasket at all let alone what size and since I have an MC supply forge piston that is oversized I too have questions about squish..

[Duccout]

All five of my Ducati Singles have had the aluminium base gasket, so it appears that it is necessary, or maybe it was just a way of increasing performance if the owner wished?


Cheers,

Colin

[blethermaskite]

As far as I am aware it's to adjust the compression ratio, I have only found these on sebrings and monzas. Cheers, George

[Jordan]

It's good to have this easy way to alter compression ratio if desired.
It takes advantage of the OHC engine's feature of a vertical shaft with sliding coupling.

I would probably not ever fit an aluminium spacer, as my bike doesn't have a mild cam profile and I'd like it to be as quick as possible.
So, a paper gasket is all I'd use at the cylinder base.
If a metal spacer were used, this would need two paper gaskets, above and below it.
A non-hardening liquid gasket compound could be used instead, at the risk of blocking the oil passage.

[HMBAtrail]

I've always used the aluminum spacers to adjust for squish band clearance.

[blethermaskite]

After I rebuilt my 75 250 desmo (with a std spec?? ) borgo forged piston.... At tdc the piston was hitting the plug points and closing the gap a bit (now using two solid copper plug washers) I had an alloy barrel base spacer plate (from a monza) in my spares stash but having the motor just built I couldn't be bothered to pull it apart again..... However next time I will fit it. Cheers George
.

[Duccout]

I wonder if the alloy gaskets were simply insurance against incorrect clearance between squish band and piston, due to poor machining tolerances? Otherwise workers would have to check the clearance on each engine.

Interestingly, Velocettes also used a steel compression plate under the barrel, which most owners removed in the hunt for higher performance.

Cheers,

Colin

[George]

Somewhere on this forum is a post by Nigel Lacey explaining how and why to set correct squish band clearance. It would be a good idea to use this post to check if you need Alli gasket, as you have so many unknowns,