Cylinder Head Fit

[Doug D.]

Ok, so I finally get a set of heads for the 1835. 1600 heads cut for the 1835 and cc'd at 50 even. I'm getting things fitted up and I notice there's what I consider to be a lot of slop between the jugs and heads. They slid right over the jugs with no resistance at all. I measured the outside diameter of the jugs, and then the inside of the cuts in the heads. I found there's about a 3 mm or .118" difference in the two. I measured the old heads, and found the difference to be about 1.5 mm.

The shop where I bought the heads says this fit is acceptable, that the seal is achieved between the edge of the jug and the mating surface in the head. Things have happened in the recent past that have tested my trust of this shop, but they're the only game in town, so I'm kind of stuck unless I want to roll the dice on line.

So being the skeptic I am, I ask, how critical is this fitment and what is an acceptable tolerance?

PEACE and ride safe!

 

[DeathBySnuSnu]

Errr....uhmmmm.

Well yeah it will work I suppose.

He is right it will work, but far from ideal.

I have about. 005" on mine. Some like them tighter and some looser.

The old school drill powered hobby shop type cutter is fairly sloppy. It is not unusual to see one .040 (1mm) loose fit.

 

[DeathBySnuSnu]

I am at work.

Gimme a while. I have the numbers somewhere.

 

[Doug D.]

Thanks Rex. Sorry you hafta work on a Sunday...

 

[DeathBySnuSnu]

Yeah...

I typically work the 7-7-7 schedule.

7am to 7pm 7 days a week.

I take about 1 day a month off.

 

[DeathBySnuSnu]

Well.....

It don't look like I have the info in a chart form.

I think I intended to do that and never completed it.

I have screen shots of various info nuggets.

I must have been building up the info to create a chart for the tech tech tech thread.

I will see if I have enough to make a short list of the info specific to you.

 

[DeathBySnuSnu]

Old style 88 machine in. Head 95.5mm

New style 88 and old style thin 90.5 Head 97.1mm

Thick 90.5 and thin 92 Head 98.1mm

92 thick and 94 Head 101.1

 

[Doug D.]

Thanks Rex!

It never occurred to me to measure the wall thickness, but I'm sure my jugs are the thin 92's, in which case they cut the heads about 2 mm too big. I got the thing to run, but it's rough, which may be attributed to a host of other problems. Aside from the poor fit, I also have to wonder if they bothered to lap the valves after they did the machine work.

I appreciate all your help! If you ever get that chart together, I'd like to have a copy. Don't know if I'll ever build another bug motor, but if I do, this would be good info to have if I take another set of heads to a machine shop. I'll let you know how it all turns out!

Peace and ride safe!

 

[stinger608]

I've seen many that were "sloppy" in the past. It can be an issue, however, if you "lap" the cylinders to the heads it's usually not a problem.

You can use the little tube of valve grinding grit that you can find at most auto parts stores. Just squeeze a thin line of it on the edge of the cylinder, slide the cylinder into the head, and then start spinning the cylinder while "grinding" a mating surface into each head surface.

It usually takes about 5 minutes per cylinder to finish this. Be sure to label each cylinder so that you know which head hole each goes into.

 

[Doug D.]

Looks like I'm headed for another tear down if I'm gonna get this thing right.

Thanks guys, I appreciate all the advice and info! If I ever do this again, I'll know all the steps.

PEACE and ride safe!

 

[DeathBySnuSnu]

If it is together and not leaking there is no real need to tear it back down.

I don't lap mine, but I do my own machine work and know that I have a good surface to seal on.

With sloppy machine work the sealing area can be uneven or more likely tilted out of square.

Lapping does not take much metal off.

But just like on a valve you can see the contact area and know it is flat.