US Model B Neracar Restoration: Engine Measurements and Checks

Con Rod and Crankshaft

Con Rod

Connecting Rod Dimensions

Crank Rollers and Crank Pin Dimensions

I first checked if the conrod was twisted or bent. I machined a mandrel that was a slide fit in the big end eye.

I tested the mandrel by setting it in V blocks on a granite block and indicating the left and right end of the mandrel.

Once sure the mandrel was true I indicated it on the right side and then moved the indicator to the left. It was lower by 0.001". The big end is very narrow relative to the diameter and so I clamped the big end to the face of the mandrel. Unless the mandrel was a sliding locational fit in the big end the only way to get a relatively good reading is by clamping the big end to the mandrel. I was getting around 0.001" play just by twisting it - I should have machined the mandrel to a tighter tolerance!

I next moved to the small end and measured both the left and right ends of the piston pin. The left side was lower by 0.001". Not too bad for an old motor....

This takes care of twist. To see if the rod was bent relative to the centerplane of the rod, I measured the distance from the piston pin to the mandrel on the left and right side (no photos taken!) using a height gage. Both were equal.

Big End Rollers

There are 11 cylindrical rollers that measured as follows:

Neracar Model B 2.625" B x 2.875" S

Big - end rollers Part # 628 A-D. NRB 5/16 x 5/16 needle rollers uncaged
https://simplybearings.co.uk/shop/Loose-Balls-&-Rollers-Loose-Steel-Rollers/c23_5043/index.html

Roller #	Diameter (in.)	Length (in.)
1	0.312	0.310
2	0.312	0.310
3	0.312	0.310
4	0.312	0.311
5	0.312	0.311
6	0.312	0.311
7	0.312	0.310
8	0.313	0.312
9	0.312	0.310
10	0.312	0.310
11	0.313	0.311
Median	0.312	0.310
Size	5/16	5/16

Crankshaft

Crank Cheek

Mainshaft

Gland, Spring and Pin

The inner main bearing and the outboard flywheel main bearing are both 6305 roller bearings (25mm x 62mm x 17mm)

The magneto Woodruff key and the starter gear Woodruff key are both key #9, 3/16" wide.

My magneto flywheel bearing housing was not a tight fit on the bearing. I machined out the housing, machined an aluminium sleeve and then pressed this sleeve into the crankcase. The new 6305 bearing was then pressed in.

Machining out the bearing housing in the crankcase:

Machining the new insert:

Insert and machined bearing housing ready for press fit:

Pressing in the insert into the machined bearing housing:

The old bearing housing was machined out to 66.00mm. I machined the OD of the insert to 66.13mm for an interference fit of 0.13mm. The rule of thumb was 0.002"/1.00" of OD for 6061 aluminium.

The insert's ID was first machined to 58mm. Once pressed into the crankcase, I machined it out to 61.85mm for a 0.15mm interference fit for the 62.00mm bearing. The length of the insert was 17.00mm to match the width of the 6305 bearing.

Piston and Bore

US Model B Engine
	Inch	Metric
Capacity	15.6	255
Bore =	2.625	66.675
Stroke	2.875	73.025

Piston Measurements and Piston Bore Clearance

I honed the cylinder to 2.629" to clean up rust spots. It was difficult finding someone who was willing to hone the cylinder on their Sunnen as there the cylinder is made in one piece with the head, and the bore must be run from the bottom with no easy way to align the cylinder. The operator mounted an appropriate hone, and held the cylinder in his hand and ran it up and down the hone by feel.

The Ford Model T from 1925 has a bore of 3.750" and the recommended clearance is 0.003" to 0.004" for new pistons, with max clearance of 0.006".

http://www.cimorelli.com/mtdl/servicemanual/1925smcolor.pdf

This works out to 0.0008 " to 0.001" of clearance per inch of bore, with a max allowable clearance of 0.0016" per inch of bore. For the 2.625" Neracar cast iron cylinder and piston that works out to a new (!) clearance of 0.003", and a max allowance of 0.004".

However, the Model T is a four stroke. For an early two stroke with marginal materials and lubrication a clearance 2x to 3x that of a four stroke is recommended. This works out to 2.50 x 0.003=0.0075". After honing my cylinder, the clearance is 0.0077". After running it for a while I inspected the piston and see no signs of scoring, seizing, or heat build up under the crown.

Piston Pin

The Neracar piston pin is retained by a brass plug pressed into one end of the piston prin. This plug, in addition to be pressed into the piston pin bore, is also retained by a press fit pin that goes through the piston pin and the piston boss.

My piston pin was scored, and too loose for the piston boss bore. The previous owner had filled the clearance with brass sheets.

The diameter of the OE pin was 0.625" (15.88mm). I found a Yamaha piston pin (can't find the damn part number) that measured 0.630" (15.99mm) in diameter. I had to bore holes in the hardened surface of the pin to fit the retainer plug pin.

However, I just found that the Scott two stroke had the same system and the pin is exactly 5/8" (0.625" in dia), and best of all, the retaining plug holes are already drilled.