Expert: Dan Liddy - 3/2/2009

Question
QUESTION: Endurance racing is getting to be bigger in the US.  What does an engine builder do when they build an engine for enduro racing vs club or sprint type engine?   How are the bearing clearances different etc?

ANSWER: There isn't much difference anymore, though the NASCAR Sprint Cup engines are probably put together a bit looser because they only have to stay together for perhaps 4 or 5 hours.

As far as club racing goes, the rule of thumb is:  .0015" to .0002" at the rods, and about .002" to .0025" at the mains, I've run looser than that by about .0005" or so, with no difficulty.  This also seems to be a fairly common clearance discipline used by many pro teams, both V-8's and smaller engines.

The important thing is to assure that you are getting sufficient oil flow to the crank and cam, and that the oil is at a proper temperature, about 200 to 225 is about right, and ALWAYS use synthetic.  Red Line is good as is Royal Purple, and I use Amzoil which is great .  Most of us avoid Mobil One because it doesn't seem to tolerate heat well, and I had a premature engine failure a while back that I feel was at least partially due to a poor quality lubricant.

There may be some more important differences in cylinder head and valve gear arrangements. An enduro motor might be fitted with a slightly milder cam, especially if the race was to be over 2 or 3 hours. The object is to get to the finish line. If you can do that you'll do alright because others will have crashes, mechanical problems, and even driver fatigue.

I hope this gives you something to think about , write back if you have further questions.

Dan Liddy
Sarasota Florida

---------- FOLLOW-UP ----------

QUESTION: Thanks for your help.

Those are about what I expected, I just like others opinions.  What do you use for ring gap?  I find that I stick with the stock specs.  Tighter doesn't always work well.

Answer
I've been using Total Seal rings for 20 years, and have 3 year old engines with a leak down test figure of 90 to 95%.  Unheard of with conventional rings.  I gap the top ring at .0035 per inch of bore.  In my 3.309 bore that's about .0115 to 012".  But buying from Total Seal, I've never had to file any of them , they're always loose enough without being too loose.   Tight on rings is never right and won't make any power difference anyway.  

With standard rings, follow the manufacturer's recommendation, and maybe add .001 to .0015"

Dan Liddy
Sarasota, Florida