Piston/cylinder clearance when boring cylinder

[dirty_rat]

A couple of quick questions.

1. Back in the 70's the Hodaka Manual recommend a piston to cylinder clearance of .003" to .004" when boring a cylinder for a new piston. In todays day and age, with much better oils, etc., is that still what everyone is doing or are they reducing the clearances down to .002" or something like that?

2. In looking over the stock ignition timing recommendations for the model 93 Super Rat, the model 98 Super Rat and the model 97 Super Combat and later published timing recommendations for these bikes (from Pabatco Hot Tips, etc.) there seems to be a very wide difference. Example, the model 98 Super Rat's stock setting is .114" BTDC, but with a PVL it list a setting of .074" BTDC and a Hot Tip list timing at .135" BTDC. On the model 97 Super Combat it list a stock setting of .155" BTDC and then two separate settings from Hot Tips of .114" BTDC and .098" BTDC. Harry Taylor also suggested other timing settings. My question is, are there any settings that have been somewhat agreed upon as giving good performance and engine longevity?

Thanks

[viclioce]

From everything I’ve read here for the past 5 years, the old standards are still the current standards. But that’s just my 2 cents. Victor

[Bullfrog]

My response will just be a quick overview.

Piston clearance: With OEM (Art) pistons, the original specifications apply. For Iron cylinder Hodie engines, .004" is recommended. For alloy cylinder engines, .001" (if you REALLY know what you are doing and are willing to open up the top end to check things before the first race after bore job) or .002" (drop it in, do a couple of heat cycles and go racing). Others will have to confirm whether or not those are the correct specifications for clearance with the modern Wiseco pistons.

Ignition timing: A part of your confusion comes from throwing magneto/points ignitions in the same bucket as CDI systems and sort of assuming that timing-is-timing. There are qualitative differences between points type ignition systems and CDI systems which are important in setting "safe" timing specifications. Note that point systems have NO advance/retard curve and have (by comparison) relatively weak spark. Whereas CDI systems DO have an advance/retard curve and have (by comparison) relatively strong spark. That qualitative difference (advance/NO advance) alone means that timing specs for points systems can NOT be transferred over to CDI systems.

Further, a CDI system often contains an automatic adjustment to the timing which "retards" (perhaps 19 degrees or perhaps 5 degrees or some other number) as engine speed climbs from idle to maximum RPM. So the timing specification to be used on a particular engine with a particular brand of CDI MUST be properly selected for exactly that configuration. Note that the "retard curve" is at maximum "advance" at idle, and that is the speed commonly used for checking/setting timing with a timing light -- so the timing specification for a CDI system is virtually always "more advanced" than a standard points system. But that MAY mean that both systems have the same timing at (say) 6,000 RPM.

Ed

[dirty_rat]

Ed, Thanks for the information on cylinder clearances.

As for the ignition timing, the biggest discrepancies I noted were with the Super Combat, where the timing went from .155” BTDC (standard from the manual) to .114” BTDC (Harry Taylor suggestion and a Hot Tip suggestion) to .098” BTDC (a later Hot Tip suggestion), all with the standard Hodaka CDI ignition. With the Super Rat’s (both the 93 and 98) they both listed a timing spec of .114” BTDC, with the only change being a Hot Tip for a heavily modified 98, so my Rats will be timed at .114” BTDC. So, for my Super Combat, I will probably go conservative with the timing on it.