I HAVE AN OLDER MODEL SPEED ENGINE AND WANTED TO KNOW THE THINKNESS OF THE SHIM NEEDED TO RUN 25% NITRO...

For the older OS engines they should have a total of 0.012" / 0.3mm worth of shims. Giving you a NET head clearance of 0.016" / 0.4mm. With this shimming your good for 30% Race Fuel with 7%-11% oil content in the USA.

Same imfo...

I'm running 25% Byrons. Not 30

Your good for 25%

What would be the diff of running 0.010 and 0.015 shim

Just trying to learn a bit.

Thanks

With less shim (thinner shim) you will have more compression ,more pressure on conrod,maybe a little more chance to blow the glow plug ,however 0.05mm diff not too significant...The way how we do here (with 25%-30 nitro) we leave the 2 copper shims with the alu shims during the brake in and about 1 gallon we remove one of the copper shim....

If you can please be more specific about your question.