Hitchcocks Forum

My 14K 500 iron barrel needs a rebore. The still-crosshatched cylinder is like new, the piston is excellent but the clearance is .018 inch. I imagine the bore was too big from birth. Anyway, I intend to use the high-comp US-made piston and an 040 over bore with a 2mm barrel spacer to achieve near-stock compression. Would that suffice or am I missing something as a beginner? I intend to do a valve job. Is it wise to upgrade the exhaust valve with our host's oversized unit?
Thanks.

Collapsed pistons skirts are VERY common; measure the bore to confirm wear! I doubt very much if your barrel needs a rebore. 3.305" nominal std bore dia. 0.0035 - 0.0045 nominal piston/bore clearance with OEM piston.


Advertised compression ratio of 6.5-1 is usually well short of that; 6-1 is not uncommon...


Why waste time and money fitting a High comp piston only to pack out the barrel to reduce compression???

I planned to measure the bore tomorrow with the proper tool, but to answer your question, I intend to replace the piston with a forged one for reliability and the 6.5:1 forged pistons cost a lot.
Thanks.

Hi Guys-------- just to add a bit more to Tims' input if I may Re}--- Why pack out the barrel ??.....Don't forget that when you raise the barrel then anything that's associated with it will be affected in one way or another..For example the pushrod / valve stem geometry will be altered, ..... The carb / fuel tank clearance will be reduced, ..... the alignment of the exhaust pipe into the cyl.head will have been altered, ----- the external oil pipe banjos unions will need to be re-aligned, etc....

When you take a bore reading do 3 in one plane ( Top---centre---bottom ) the 3 more at 90 degrees to the first......making a total of 6 in all, and do this only on the area swept by the piston..Remember that at the lower end the bore will have been opened out into a slight taper....This is to enable you to capture and compress the piston rings when re-fitting the cylinder over the piston.........Make a list to show the readings and from them you will be able to visualise such things as ovality and bell-mouthing........Just in case you didn't quite catch what Tim was getting at, those nominal figures read}--- Three and a half to four and a half thou. clearance, bore / new oe piston...Are you sure you have 18 thou clearance ???............Incidentally as you are removing the head then the formula for Compression Ratio ( CR ) = Swept Vol. plus Clearance Vol. divided by Clearance Vol..

Personally I'd forget about the compression plate....Others have said in the past that the advertised 9:1 is a bit optimistic.....Fit the piston, advance the ignition by a few degrees and use best petrol....oh, and fit a "plus 1 tooth" output sprocket to make use of that slight upgrade in performance..........

Hi Alan R, as one of those 'others', I can confirm I have used many of those so-called 9:1 American pistons, they are well made, have a smooth crown shape and don't have that stupid slot around the oil ring groove, so the crown will stay on, but, they are more like 7.75:1 C.R., unless you SHORTEN the cylinder barrel by around 2mm, when you will achieve a true 9:1. I even suggested to our hosts that they may sell more of these pistons, if advertised at the lower C.R. figure, as, no doubt 9:1 will probably put many potential customers off! Regards, Paul.

Hi Felix, the oversize exhaust valve will make better contact with the valve seat, rather than almost pulling through it like the standard ones try to do. You will reduce the risk of valve seat recession and burning by using one of these. Regards, Paul.

Come the New Year, as this current bike is new to me I'll need to remove my Cyl Head & barrel to see exactly what piston I have fitted and to assess the amount of wear etc.....In any event I'll be fitting one of those "9-to-1" pistons for the very reason Paul has just given...And at £88 that seems good value as well..........I'll give you guys a photographic report back at the time and let you know how it's going....

It really helps to have some advice on this.
I understand Tim regarding the bore and will measure it. I never saw a piston diameter shrink along its full length. To install a better piston, I either spend more or even more. A bore job is $40 here. I'll be ordering the piston an exhaust valve as soon as I get the head apart and check things further. A little extra comp might be a good thing.
Thanks, gentlemen!

Hi Felix----- I know that Mr H. will slap my wrists for this next suggestion but why not see if you can source that "American-made Piston" locally ( ie in the states ) ??.........It might be cheaper AND.. you'll be doing your bit to help lower the Greenhouse Emissions from those Jumbo Jet cargo planes as well !!

I'm unable to find one here. I've never found RE parts here that aren't way overpriced.
Mr. Hitchcock is the man!