Piston clearance?

[rd400canuck]

Hi guys,

How exactly and where do I check piston / cyl gap?  I have a feeler gauge but am not sure where to check.

ty

[The Red Scourge]

I have to imagine you'd be measuring the cylinder bore with a bore gauge and then measuring the piston and finding the gap that way.  I'm barely learning the top end work though.  I doubt feeler gauges would give you an accurate enough reading for those tolerances.  Curious to see what the engine builders say about it though.

[rodneya]

Put the piston without rings in the bore and slide the feeler gauge in the skirt area on the wrist pin side. Pistons are smaller near the crown and exhaust side, so don't measure the top
Not very accurate but can be used as a check. You also need a long feeler gauge to do it properly.
Should fit snug with some resistance.

[rd400canuck]

Thanks guys,

I know a bore gauge and outer diameter gauge is best but I'm just messing about really... I bought cyls and pistons used that were freshly bored and I just wanted to make sure everything was good before the bike runs. I was getting readings of .07 mm until i learned where to check. Everything looks good now at .03  :)  Minty fresh

[SUPERTUNE]

As you know most likely from reading my posts, a dial bore gauge is very precise.
But a feeler gauge when used properly can help when you don't have a dial gauge.
I have a set of snap-on bore feeler gages that work well. They are 8" inches long and can be layed in the cylinders all the way through and then push through the piston at the skirt side through the bore.
When speaking of .03mm (.0012in) that's really tight for today's fuels!
.05mm (.0019) would be what your looking for in a stock engine.
Even more with modified engines. .06mm (.0023in)

My Opinion Is...with today's fuels burn hotter on ethanol based pump fuels than the much better leaded and non-ethanol fuels made when stock bike were produced.
You need to run richer jetting to keep them from seizing anyways now, then add in a little too tight piston to wall clearance, and doesn't take much to hurt the pistons.

Timing also should retarded a little as well. Stock RD400's used 2.3mm BTDC (except the bastard U.S. 2V0 Daytona at 2.4mm)
2.0mm works well on stockers and 1.9mm for modifieds. If on fixed ignition curves.
Road racers are quite happy at 1.7-1.8mm as they are always at redline every shift.

Chuck

[rd400canuck]

Thanks for the knowledge, Supertune :)

Just to make sure I was doing it right...   I should place the piston in the cyl about 1 inch up from the bottom then check with feeler between the wrist pin side of the skirt and the cyl wall.

I have the cyls out so I can just check from the bottom.

Sound right?

[rd400canuck]

ok.. I went to the clymer manual and it say to place the piston upside down and inch up then measure  between the long part of the skirt and wall.

Now I can get a .0015 in there decently and also the .002 but it's very snug but I can get it in there without feeling like I'm going to damage anything. So.. I'm thinking it's closer to .002 since I can indeed get that gauge in there.

If I am measuring the correct way then I think i'm ok.

As for ignition timing I bought the HPI but I suspect I will put the stock points system on and set timing and run it that way for a few weeks or so and to get jetting dialed in. I'm so curious what effect the HPI and its timing curve and light flywheel will have on the motor that I want to get used to what the stock bike feels like first.

[Czakky]

I'll be interested in your report.

[SUPERTUNE]

When you get the HPI on you will have to get aggressive with some jetting for the timing banging in right above idle! 
Chuck

[rd400canuck]

As in more fuel because of the ign curve?

[SUPERTUNE]

Yes. Use bigger pilot jet, then run the airscrew out further to keep a good idle.
C

[rd400canuck]

ok... I ordered a bunch... from 20's to 30's

[SUPERTUNE]

Start with a 30 pilot on a stocker with airbox and Uni replacement filter, bigger from there.
Maybe a 27.5 with hot weather and the stock restrictive paper air filter in the all stock airbox.
C

[rd400canuck]

Thanks again!

Out of curiosity was effect does the advance have on the idle and low rpms and what does giving it more fuel do to help?

[edgefinder]

The hpi will have a small amount of advance like stock for starting. It ramps up to a lot more advance at about 2500rpm then retards down to near stock at 6000. It gives the combustion more time to burn and improves driveability at around town speeds. It allows the motor to run better at lower speeds enough that it can use more gas and burn that too